Mertens, 1994 REGULATION OF FORAGE INTAKE capitulo 11

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Chapter 11 
REGULATION OF FORAGE INTAKE 
D. R. Mertens 
INTRODUCTION 
Forage quality can be assessed in terms of the animal performance that is 
elicited when the forage is offered to the animal. Animal performance is the 
product of supply, nutrient and energy concentration, intake, digestibility, and 
metabolism. Typically, forage quality is measured assuming that the forage is freely 
available and that deficient nutrients other than protein and energy are provided in 
a supplement. Of the remaining factors affecting forage quality, intake is the most 
important one affecting animal performance. Reid (1961) listed the amount of 
forage eaten as the first factor limiting the usefulness of forage-testing schemes. His 
list also included the effect of concentrates on the amount of forage eaten, degree 
of selective feeding, and effect of physical form as important information not 
provided by the forage testing systems used at that time. Unfortunately, intake, 
substitution rates, selection, and physical form still are not a part of routine forage 
evaluation systems in the 1990's. 
Animal performance depends on the intake of digestible and metabolizable 
nutrients. Of the variation in digestible dry matter (DDM) or digestible energy 
(DE) intake among animals and feeds, 60 to 90% is related to differences in intake, 
whereas only 10 to 40% is related to differences in digestibility (Crampton et al., 
1960; Reid, 1961). Intake generally accounts for twice as much variability in DDM 
intake as does digestibility (Milford, 1960; Ingalls et al., 1965). Milford and 
Minson (1966) observed that DDM intake is more closely correlated with dry matter 
intake (DMI) than any other feed or animal characteristic. Similarly, high 
correlations would occur between DMI and metabolizable (ME) or net energy (NE) 
intake, and between DMI and overall animal performance when forages are fed. 
The primary importance of intake in assessing forage quality does not mean that 
digestibility or metabolizability of nutrients are invariant or unimportant. 
Metabolizability can vary depending on the amounts and ratios of absorbed 
nutrients, the individual and interacting biochemical pathways of nutrient conversion 
and the type of animal production (MacRae et al., 1985; Blaxter, 1989). However, 
differences in the efficiency of converting DE to ME are small compared to 
differences in intake and digestibility, and the metabolic conversion of DE to ME 
or NE is typically considered to be relatively constant within type of production, 
such as maintenance, growth, pregnancy, or lactation (Moe and Tyrrell, 1976; 
NRC, 1989). 
Digestibility of forages is more variable than metabolizability, and feces typically 
are the greatest loss of ingested nutrients and energy. Extensive research has been 
devoted to measuring digestibility and relating it to feed characteristics because 
digestibility can be accurately measured with relative ease compared to DMI. 
Although intake is more important than digestibility in assessing forage quality, 
progress in understanding the basic factors that affect intake has been hampered by 
our inability to measure it accurately and to separate the influences of animal and 
diet on intake. Thus, we routinely use availability information (DE, ME, andNE) 
USDA-Agricultural Research Service, US Dairy Forage Research Center, Madison, 
WI 53706 
450 
Published 1994
to economically evaluate feeds and formulate rations, but only the INRA (1989) 
system and NDF-Energy Intake System (Mertens, 1985; 1987) attempt to use 
variation in the intake potential of feeds when formulating rations for ruminants. 
One of the greatest difficulties in predicting forage quality or animal responses 
to diets is related to intake. For evaluation, it is most desirable to have intake as 
a response to the forage; yet to predict animal performance, it is easiest to have 
intake as an input. Does intake determine animal performance (intake as an input) 
or does animal performance determine intake (intake as a response)? This 
dichotomy reflects part of the difficulty in measuring and tabulating the intake 
potential of forages. It also indicates the very real difference in predicting intake 
as a response to a diet of known ingredient and chemical composition, compared to 
estimating intake to formulate a ration to meet an optimal or target level of animal 
performance. In the first instance, the diet composition is the known input, and 
animal performance and intake are unknown responses. In the second instance, 
animal performance and intake are the known (assumed) inputs and diet composition 
is the unknown variable to be solved. When mixed rations are fed, animal 
nutritionists can, and probably should, formulate them to optimize performance or 
profit using intake as a known input. However, when forage quality is measured, 
intake is a response. 
When intake is a response, a second dichotomy is raised. Is the intake response 
limited by the potential of the animal or by characteristics of the diet? Much of the 
confusion in the literature about the mechanisms by which forage characteristics 
influence intake can be related to lack of recognition that animal characteristics also 
can limit intake within or among experiments. To clarify the relationships between 
intrinsic forage characteristics and intake, it is important to develop mechanistic 
conceptual models of intake regulation in ruminants that are based on biological 
principles and defensible theoretical concepts. These models can serve as a basis 
for identifying feeding situations in which forage or animal characteristics limit 
intake. The objectives of this chapter are to: 1) review the mechanisms of intake 
regulation, 2) present integrated models of intake concepts that include both animal 
and dietary factors, and 3) discuss variation in intake and factors affecting the 
measurement of forage intake potential. 
DEFINITIONS 
In discussing concepts of intake regulation, several terms are used which 
sometimes are defined differently by various authors. The following definitions will 
be used in this chapter: 
451 
Appetite. The drive to consume feed. In this chapter, appetite will be defined 
as a response to meeting long term nutrient requirements. Forbes (1986) 
defined appetite as the drive to eat a specific nutrient. 
Eating pattern. The amount and spacing of feed consumption during short-term 
intake regulation. 
Eating rate. The amount of dry matter (DM) consumed per unit of time. 
Hunger. The urge or desire to eat in response to short-term stimuli from 
feeding centers in the brain. 
INRA (Institut National de la Recherche Agronomique) ingestibility. A feed 
characteristic that represents the maximum quantity that is eaten when fed 
to a reference animal under standardized ad libitum feeding conditions. 
INRA intake capacity. An animal characteristic that represents the quantity of 
feed that the animal consumes voluntarily when fed a reference feed under 
standardized ad libitum feeding conditions. 
INRA fill value. The ratio of the DMI of the reference forage divided by the 
MERTENS 
DMI of the test forage for each reference animal. Fill values of a forage 
differ for each reference animal; sheep fill units (SFU) = 75/(forage DMI 
for reference sheep), cattle fill units (CFU) = 95/(forage DMI for reference 
cattle), and lactating cow fill units (LFU) = 140/(forage DMI for reference 
lactating cows). 
INRA reference forage. A pasture grass, cut at the grazing stage of first 
growth, containing 150 g crude protein (CP) and 250 g crude fiber per kg 
DM with an organic matter (OM) digestibility of 770 ~ kg-I. The voluntary 
DMI of the reference forage is 75 g DM kg-I BWO·7 for reference sheep, 
95 g DM kg- I BWO·75 for reference cattle, and 140 g DM kg-I BWO·75 for 
reference lactatingcows. 
INRA reference cattle. Friesian heifers, 16 to 18 months old, with an average 
body weight (BW) of 400 kg (BW range of 350 to 450 kg). 
INRA reference lactating cow. An adult lactating dairy cow producing 25 kg of 
4% fat-corrected milk per day in the fourth month of lactation, with a BW 
of 600 kg. 
INRA reference sheep. A 1.5 to 4 year old Texel wether with an average 
weight of 60 kg (BW range of 40 to 75 kg). 
INRA substitution ratio. Proportional reduction in forage intake (kg d-I) per unit 
of concentrate intake (kg d- I) when a forage diet is supplemented with 
concentrates. Typically, total DMI is increased when concentrates are added 
to forage diets, but forage intake is reduced. 
Intake. The absolute amount of DM consumed per unit of time. Because it has 
the units of time, intake is a rate, not a pool. Traditionally, intake is 
measured over a period of 5 to 10 d and expressed as daily amounts 
consumed per unit of BW. 
Intake, relative. Ratio of the intake of a test feed to that of a reference feed 
when fed to similar animals. To be most useful, relative intake should 
adjust for appetite differences among animals by comparing the intake of the 
test forage to the intake of a diet or high-quality forage that measures the 
long-term intake potential of the test animal. 
Intake constraint. The daily amount of neutral detergent fiber (NDF) the animal 
consumes voluntarily when achieving maximal performance (maintenance + 
tissue balance + production). Intake constraint differs from INRA intake 
capacity in that the former is constrained to meet a variable target energy 
demand whereas the latter is defined for a specific energy demand of a 
particular reference animal. 
Intake potential of the animal. Intake by the animal when energy demand of the 
animal and not the filling effects of the diet limit DMI. This differs from 
the potential intake of Forbes (1986) which he defines as the weight of food 
required to fulfill all of the animal's nutrient requirements. 
Intake potential of the forage. Intake of a forage when the filling effects of the 
forage and not animal characteristics limit DMI. 
Intake regulation, long-term. The mechanisms that regulate intake over periods 
of weeks or longer. If feed energy density and supply are not limiting, non-
producing, mature animals usually balance energy intake with energy output 
and maintain a constant BW; whereas producing animals will balance energy 
intake with the energy demands corresponding to their productive potential. 
Intake regulation, short-term. The events and stimuli that regulate meal eating 
patterns within days or a week. 
Maximum DMI. The daily amount of feed eaten when the energy density of the 
forage or diet is the minimum allowed to meet the target production potential 
(maintenance + tissue balance + production) of the animal. 
REGULATION OF FORAGE INTAKE 452 
Palatability. The characteristic of a feed indicating its acceptability, usually 
associated with the gustatory, olfactory, or visual senses. Palatability affects 
the preference for a feed when several are available and the rate of eating 
and intake when a single feed is offered. 
Preference. Relative acceptability of a feed when given the choice among two 
or more feeds that are available in a cafeteria-style feeding situation. 
Preference is a more specific indication of palatability that affects 
acceptability among feeds, but does not measure intake modification when 
no choice among feeds is allowed (a single feed is fed). 
Refusal ratio. The fraction of the amount of feed offered in a stall-feeding 
situation that is not consumed. 
Ruminal contents. Amount of wet material, DM, or volume in the rumen 
measured at any unspecified time when the animal is fed a diet not 
documented to limit intake by fill. 
Ruminalfill. Volume ofruminal contents when measured immediately after the 
cessation of a meal when the animal is provided a diet that does not meet its 
energy demand and intake is limited by the bulk of the diet. 
Satiety. The condition of having the urge to eat satisfied. The satiety signal 
from the brain stops a short-term eating interval or meal. 
Selection. Specifically defined to indicate preferential consumption among feed 
subcomponents, such as leaves vs stems or immature plant tops vs mature 
plant bases. 
THEORIES OF INTAKE REGULATION 
To delineate those aspects of intake that are inherent characteristics of forage 
quality as opposed to those that are functions of animal characteristics, it is pertinent 
to review the mechanisms by which intake is regulated in animals. The focus of 
discussion in this chapter will be on the non-forage factors that influence or regulate 
intake for the purpose of providing background information that will be useful in 
interpreting intake as a component of forage quality. Intake is regulated in both the 
short and long term. Mechanisms for the short term stimulation of meal initiation 
and cessation within a day are quite different from the long term regulation of BW 
and performance over the lifetime of the animal. Failure to be clear about which 
type of intake regulation is being investigated or discussed often leads to 
unnecessary confusion and conflict. 
Factors affecting intake, and the stimuli and mechanisms that regulate it, are 
incompletely known as indicated by the diversity of information in the reviews of 
Conrad (1966), Balch and Campling (1969), Baile and Meyer (1970), Baumgardt 
(1970), Campling (1970), Jones (1972), Baile and Forbes (1974), Journet and 
Remond (1976), Bines (1979), Waldo (1986), Grovum (1987), NRC (1987), and 
Owens et al. (1991). Although models that predict daily intake by summing meal 
events (Forbes, 1977a) may ultimately be most useful in studying intake 
mechanisms, concepts based on mechanisms of long term intake regulation can 
provide the fundamental basis for understanding biological principles affecting the 
intake of forages. Systems control theory can be used to study intake regulation and 
provide a conceptual model for assessing forage qUality. In its simplest terms, 
control theory is used to analyze systems in which output controls input by means 
of a feedback mechanism. 
Both neural (Bell, 1971) and hormonal (Bray, 1974; Booth et al., 1976; de Jong, 
1986) control systems ensure that food intake equals energy output in mature 
animals so they maintain a relatively stable BW when food quality and supply are 
adequate. When energy density of the diet is altered, rats (Adolph, 1947) and 
453 MERTENS 
ruminants (Dinius and Baumgardt, 1970) adjust intake to balance caloric input with 
output. Similarly, when animals are placed in cold environments (Stevenson, 1954) 
or are shorn (Minson and Ternouth, 1971), thereby increasing their energy demand, 
their intake increases in an attempt to match their increased energy expenditure. 
These observations suggest that animals monitor their energy expenditures or deficits 
and respond by consuming the energy that is needed. 
Initially, there was scientific debate about which physiological need was 
regulated. The current consensus, based on numerous studies, is that energy intake 
is regulated rather than the intake of protein, fat, or carbohydrate. Evidence that 
the supply of readily metabolizable energy regulated intake was obtained by Booth 
(1972) who observed that the satiating effects of various nutrients were related to 
their energy yield and not their convertibility to glucose. The central role of energy 
also is consistent with the observation that excess energy cannot be easily dissipated; 
therefore, its intake must be regulated to balance requirements and obey the First 
Law of Thermodynamics (conservation of energy). Recent studies indicate that 
animals have some ability to dissipate excess energy intake by increasing basalmetabolism via futile (substrate) cycles (Newsholme, 1985); however, the ability to 
excrete excess energy intake is limited and the major control of energy balance is 
through intake regulation. Conversely, excess protein can be catabolized easily and 
excreted in the urine, thereby negating the need to regulate its intake. The concept 
that protein intake is not regulated does not mean that protein concentration in feeds 
does not affect intake, only that the effect is indirect. This will be discussed in later 
sections. 
Although animals regulate energy intake, the control system is not simple and 
direct. Animals have no mechanism for measuring energy per se, and caloric intake 
is not metered as it is ingested (Brobeck, 1960). Rather, daily intake is adjusted 
indirectly to maintain energy balance. If intake were regulated by a simple feedback 
mechanism, the delay between the intake of energy and its availability to tissues in 
the body would result in excess intake. Apparently, animals terminate meals before 
energy is digested and metabolized based on signals other than energy density. In 
the short term, intake may be greater or less than current energy expenditure, but 
in the long term, intake will balance expenditures and BW of mature animals is 
maintained when the quality and supply of the diet is adequate. Although some 
species may become fat at maturity when provided an adequate diet, they 
nevertheless maintain a stable, fatty body condition and do not continue to gain 
weight indefinitely. 
Intake regulation is complicated because the body can store and recover energy 
in glycogen and fat reserves. Although meals are periodic, the need for energy by 
the body is continuous. Energy stores provide a buffer that can accommodate both 
short and long term perturbations in intake. Very poor correlations between energy 
intake and expenditure can occur using daily balances, whereas weekly or bi-weekly 
balances show good correlations between energy input and output (Edholm et al., 
1955). Another factor indicating the complexity of short-term regulation of intake 
is related to psychogenic modulation. This will be discussed in more detail in a 
later section, but it is important to indicate at this point that meals can be initiated 
or postponed based on social interactions, management conditions, and feed 
characteristics. 
The nature of the signal(s) that regulate intake is a major research issue that has 
not been resolved (Mogenson and Calaresu, 1978). Changes in glucose or other 
metabolites (chemostatic control), changes in body temperature (thermostatic 
control), and changes in body fat (lipostatic control) are the most commonly 
accepted mechanisms by which short term intake is regulated. In addition, changes 
in amino acids (aminostatic control), the modulating effects of taste (palatability or 
REGULATION OF FORAGE INTAKE 454 
gustatory control), habit, experience, social and emotional factors, and gastric 
distension (fill limitation) have been proposed as mechanisms controlling intake. 
Some scientists accept the view that intake is under the control of multiple factors; 
others contend that a multifactor mechanism is unnecessarily complex and may 
simply reflect our lack of understanding of basic mechanisms. 
Some of the confusion about intake regulation may be related to the differences 
in short and long term mechanisms. Each mechanism may have different signals 
and control systems. Short term intake may be controlled by glucostatic (or 
thermostatic, aminostatic, or psychogenic) control while long term intake may be 
regulated by lipostatic control (or fill limitation) (Mayer and Thomas, 1967). 
Multiple factors probably are involved in the satiety signals that terminate meals, 
but most researchers have concluded that relatively simple models can be used to 
describe long term intake regulation. 
A set point for body fat has been proposed to explain the constant BW of mature 
animals and the normal growth rate of immature animals when fed a diet that is 
adequate in energy, protein, vitamins, and minerals (Baumgardt, 1970; Cabanac et 
al., 1971). No control center or stimuli has been conclusively associated with a set 
point, and the concept may be useful only in describing the outcome of intake 
regulation rather than in defining the mechanism of long term intake control. The 
concept of a set point is based on a static, rather than a dynamic, conceptual model 
of intake regulation. 
Analysis of dynamic control systems indicates that equilibria, steady-states, or 
"set points" can occur from interactions among dynamic processes. These set points 
can be varied by changing anyone of the many components in a control system. 
Variability is inconsistent with the classical definition of "set point" and Russek 
(1978) introduced the concept of "defended body weight" to indicate the variable 
BW animals will attempt to maintain in a specific situation. As Booth et al. (1976) 
indicated, a dynamic system can achieve stability of fat stores, which is 
mathematically equivalent to a set point, without the use of a set point control 
function that would require: 1) a stimulus from the fat stores, 2) a receptor for the 
stimulus, 3) a system for generating a precise reference fat store value, and 4) a 
comparator mechanism to compute the error between current state and the reference 
set point. 
Although the concept of set point may not be useful or necessary for explaining 
the mechanisms of intake regulation, it can be useful in describing the steady state 
result of a dynamic intake control system. The steady state solution of a dynamic 
model is static, and static models offer advantages in terms of describing models 
algebraically and solving them analytically. Because we formulate rations or select 
forages to meet the long term needs of animals, factors associated with long term 
rather than short term intake regulation mechanisms are related more directly to 
forage qUality. Although debate about intake regulation continues, apparently two 
or three mechanisms control long term intake in animals. These mechanisms will 
be discussed in detail. 
Physiological Regulation 
The intake potential of an animal depends on its species, sex, physiological state 
(maintenance, growth, pregnancy, and lactation), size, body shape, and health. In 
addition, certain environmental factors such as ambient temperature and photoperiod 
(Ingvartsen et al., 1992), as well as management treatments such as exogenous 
hormones or growth promoters, can directly influence an animal's energy demand 
and intake potential. Long term intake regulation involves mechanisms that control 
average daily intake over extended periods of time during which BW equilibrium 
455 MERTENS 
is attained and maintained. Intake would be controlled during homeostasis to 
maintain long term physiological equilibrium. During homeorhesis, intake would 
be coordinated with the control of metabolism to support the changes in intake 
associated with physiological states, such as lactation or pregnancy (Bauman and 
Currie, 1980). Monteiro (1972) developed a closed-loop system model that assumes 
that intake is proportional to the energy demand associated with milk production. 
His model has a delay in response between intake and energy demand that could be 
interpreted as a homeorhetic mechanism associated with lactation. 
When animals are fed high energy rations that are palatable, low in fill, and 
readily digested, intake is regulated to meet the energy demands of the animal, 
unless the diet is fermented too rapidly and digestive disorders occur. Physiological 
regulation of intake can be interpreted to mean that the DMI of the animal times the 
energy concentration in diet DM will equal the animal's energy demand. This 
mechanism of intake regulation can be described easily by a simplealgebraic 
equation that can be rearranged to solve for the energy concentration or intake 
needed to meet a specific animal requirement or potential: 
Ie X E = R Equation 1 
E = RIle Equation 2 
Ie = RlE Equation 3 
where Ie is intake (kg d-I) expected when energy demand is regulating intake, E is 
the energy concentration of the diet (Mcal kg-I), and R is the animal's energy 
requirement or output (Mcal d-I). 
Equation 3 indicates that intake is a positive, linear function of the animal's 
energy requirement; therefore, intake will increase with increasing energy demand 
by the animal. Equation 3 also indicates that intake is a reciprocal function of the 
feed characteristic (available energy concentration). As energy concentration in the 
diet increases, intake will decline in a curvilinear manner. Conversely, if energy 
concentration of the diet decreases, intake must increase to meet energy demand. 
However, there is a maximum limit to intake by the animal. It is important to 
recognize that intake is not the only response in equation I that can be varied by the 
animal. If energy concentration is too low and intake cannot be adjusted to 
accommodate a target level of production potential, the animal has the ability to 
reduce energy output by reducing productivity or increasing the use of body 
reserves. Thus, the animal effectively changes its output (R) to match allowable 
energy input. This dichotomy, that the animal can vary either energy intake or 
energy output to achieve equilibrium, indicates the problem in using this theory of 
intake regulation alone to predict intake, evaluate forages, or formulate rations. 
Equations 2 and 3 illustrate, mathematically, the dual nature of intake as it is 
used to formulate rations and (or) evaluate forage quality. In ration formulation, 
intake is an input that is assumed to be known and equation 2 is used to define the 
energy concentration needed in the ration. If intake is assumed to be lower than 
maximal, the energy concentration needed in the diet will be calculated to be greater 
than the minimum necessary to meet energy requirements. To no one's surprise, 
ration formulation systems that underestimate intake apparently work because they 
are self-fulfilling prophesies. Low estimates of intake result in high estimates of 
energy concentration needed in the diet which result in low intakes that match those 
used to formulate the ration. Caution should be exercised in the adoption of intake 
prediction equations to be used to estimate forage quality or formulate rations. If 
the database used to generate them contains diets with higher concentrations of 
energy than necessary to meet the animal's requirements, intakes predicted by these 
equations will be lower than the maximum that can be achieved by animals. These 
low estimates of intake can be used to formulate rations, but the rations may not be 
optimal and will underestimate the potential contribution that forages can make to 
REGULATION OF FORAGE INTAKE 456 
mixed rations because they overestimate the levels of concentrates needed in rations. 
Evaluation of forage quality is dependent on an estimate of intake potential of 
the forage because intake plays such a critical role in determining animal 
performance. Based on the concept that physiological energy demand regulates 
intake, equation 3 could be used to estimate the intake potential of rations that are 
high in energy in relation to animal requirements. Equation 3 illustrates two 
potential difficulties in quantifying the intake potential of high quality forages. 
First, for high quality diets, the animal's requirements and not characteristics of 
feed determine intake. Thus, intakes measured under these conditions do not 
measure forage or diet qUality. Unless the energy demand of the animal is high, the 
true intake potential of the forage cannot be measured. Second, if the animal's 
demand exceeds the intake potential of the forage, something other than energy 
concentration of the feed is limiting intake. 
When available energy concentration of the forage or ration is low relative to 
animal requirements, equation 3 will overestimate intake potential because it 
assumes that the animal's energy requirement (R) will be met. Under actual feeding 
conditions, the animal will reduce performance or lose weight to attain an 
equilibrium between energy intake and output when consuming low quality diets. 
Measuring intake when animals cannot meet their energy requirements will result 
in biased estimates of intake potential of feeds that are not useful in formulating 
rations to meet specific levels of animal production. Although animals will attempt 
to regulate intake to meet energy demand, intake must be limited by factors other 
than energy concentration of the diet when the animal cannot consume enough feed 
to meet its energy needs. 
Physical Limitation 
When animals are fed diets that are palatable, yet high in bulk (fill) and low in 
available energy concentration, intake is limited by some restriction of capacity in 
the digestive tract (Balch and Campling, 1962; Campling, 1970; Bines, 1971; Baile 
and Forbes, 1974). These diets result in intakes of energy that cannot meet the 
animal's potential demand and the animal reduces performance or loses weight to 
accommodate the limits of the diet. Lehman (1941) postulated that ballast associated 
with indigestible OM residues was a factor limiting intake. Conrad et al. (1964) 
and Owens et al. (1991) followed similar logic when suggesting that there is an 
upper limit on the amount of fecal OM that can be excreted per day. They 
hypothesized that intake was restricted by the capacity of the intestines to pass 
undigested residues. However, Mertens (1973) concluded that the capacity of the 
intestines probably does not limit intake based on the variability in fecal output and 
the increases in intake observed when pelleted diets were fed. When low quality, 
high fiber forages are pelleted, intake often increases, typically with decreases in 
digestibility. Thus, fecal output increases when pelleted feeds are fed which would 
be impossible if fecal output was limiting intake of unpelleted forage. Thus, 
capacity of the intestines probably is not the factor limiting the intake oflow quality, 
high fiber forages. Grovum and Phillips (1978) confirmed the conclusion logically 
derived by Mertens (1973) by infusing methylcellulose in the intestines of sheep and 
observing that intake was not inhibited when fecal output was significantly 
increased. 
Physical distension of the reticulorumen generally has been accepted as the 
major factor limiting intake of many forages and high fiber diets (Balch and 
Campling, 1962; Baile and Forbes, 1974). In long term regulation, stomach 
capacity is modified (within limits) by hypertrophy of organs or reduced 
constrictions associated with internal adipose deposits to achieve a balance between 
457 MERTENS 
distension stimuli and animal performance. The level of distension required to elicit 
satiety probably varies with the physiological state (performance potential) of the 
animal. In short term regulation of intake, stretch receptors in the reticulorumen 
provide the stimulus to the brain satiety center that triggers the end of a meal. 
Some ambiguity in the role of distension in intake regulation can be expected 
because brain centers which control intake integrate a variety of inhibitory or 
stimulatory inputs (Forbes, 1986). 
The physiological stimuli that result in physical control of intake provides a 
conceptual basis for defining "fill". Too often, fill is used to describe any or all 
measurements of the weight of ruminal contents. However, the term "fill" is useful 
in discussions of intake regulation only when it is used to indicate the occupied 
volume of the rumen when intake is controlled by distension.If stretch receptors 
are involved, it is contended that volume rather than weight of ruminal contents 
should be used to measure "fill". Fill can only be measured when the rumen is full, 
immediately after the end of a meal in which intake is limited by bulk. All other 
measurements of ruminal contents should be referred to as "ruminal contents" and 
not as "ruminal fill". Perhaps "ruminal capacity" would be a better term than 
"ruminal fill" because it implies an upper limit to occupied ruminal volume. 
However, "fill" has been used traditionally to describe ruminal contents when 
distension of bulky diets limits intake. Some confusion occurs in the research 
literature because fill has been used to describe ruminal contents when high energy, 
low fill diets were fed. It is inappropriate to call these ruminal contents "fill" 
because this implies that the limit to intake in these instances was due to the filling 
effect of the diet rather than the energy demand of the animal. 
The relationship between ruminal contents and intake of feeds was shown by 
Blaxter et al. (1961) who observed that sheep offered poor, medium, or good hay 
had similar amounts of DM in the reticulorumen. Campling and Balch (1961) also 
provided evidence that mass in the rumen affects intake. Removing swallowed hay 
from the rumen increased the eating time of cows whereas adding digesta to the 
rumen during a meal decreased intake. Weston (1966) inserted coarsely ground 
roughage, sawdust, or finely ground polyvinyl chloride into rumens of sheep offered 
chopped forages. Intake decreased in relation to the amount of materials given 
intraruminally. The observation that ruminants eat to a constant ruminal fill 
(capacity) also was confirmed by the work of Ulyatt et al. (1967) and Thornton and 
Minson (1972). 
The fill limitation concept of intake regulation indicates that when animals are 
provided an adequate supply of palatable rations that are high in fill, intake is 
limited by the intake capacity or constraint of the animal. Physical limitations to 
intake can be interpreted to mean that the intake of the animal times the diet's filling 
effect equals the animal's intake constraint. This mechanism of intake limitation can 
be easily described by a simple algebraic equation that can be rearranged to solve 
for the filling effect or intake allowed by a specific animal intake constraint: 
If X F = C Equation 4 
F = C/I f Equation 5 
If = C/F Equation 6 
where ~f is intake (kg d-l~ expected :-vhen fill is .limiting i.ntake, ~ is the volu~e of 
the fillIng effect (L kg- ) of the dIet, and C IS the anImal's mtake capaCIty or 
constraint (L d-1). 
Equation 6 indicates that intake is a linear function of the intake constraint of 
the animal and will increase as the animal's intake constraint increases. Equation 
6 also indicates the potential difficulty facing an animal that has access to a diet that 
is high in fill. Because intake is a reciprocal function of the feed's filling effect, it 
will decrease in a curvilinear manner as the fill of the forage or diet increases. 
REGULATION OF FORAGE INTAKE 458 
However, intake can only go so low and allow the animal to obtain enough energy 
to survive or attain its performance potential. Equation 4 indicates that if appetite 
is large due to a high energy demand, the animal can accommodate a diet high in 
fill by increasing its intake constraint (C). 
The possibility that the animal can vary either intake or capacity in response to 
a diet of a given filling effect illustrates the confusion associated with defining C. 
Although the animal can increase its constraint, it is obvious there is a maximum 
beyond which the gut cannot stretch and a passage rate it cannot exceed. This 
maximum intake capacity when animals are fed forages or rations that are so high 
in fill that performance or even maintenance of life cannot be sustained is of little 
importance in the practical evaluation of forage quality. It seems more logical to 
define C in relation to the intake that is needed to meet a target requirement. This 
capacity is more appropriately termed an intake constraint because the upper limit 
of intake capacity that is acceptable is constrained by energy requirements needed 
to meet the animal's performance potential. When energy requirements are not met, 
but intake capacity is not at its absolute maximum, an ingestive capacity of the 
animal is being measured that is between the intake constraint and the absolute 
maximum intake capacity of the animal. 
Equations 5 and 6 indicate the dual nature of intake in formulating rations or 
estimating forage qUality. In ration formulation, intake is assumed to be known 
and equation 5 can be used to calculate the filling effect of the ration that is allowed 
under the constraint of a given production target. If intake is estimated to be too 
low, the acceptable filling effect of the diet will be estimated to be too high. This 
will result in a low intake that does not meet the animal's energy needs. The safest 
strategy when formulating rations is to use equation 2 with a slightly low estimate 
of intake because it will at least guarantee that the energy concentration in the diet 
will be adequate to meet animal requirements. Conversely, using only equation 5 
to formulate rations will not guarantee that the animal's energy needs are met but 
only indicates that its rumen will be full. 
Equation 6 is probably the most important equation for the prediction of intake 
as a component of forage quality. When forages are offered as the sole diet and 
animal performance potential is high, it is often the filling effect of the forage that 
limits intake. When the filling effect of a forage or diet is high, equation 6 will 
adequately describe the intake potential of the feed regardless of whether the animal 
has a high or low energy demand. Although animals with higher energy demand 
may try to accommodate a high fill diet by increasing C, the filling effect of the diet 
will still be the factor limiting intake. However, when the filling effect of the 
forage is low, such as for high quality legumes, the energy demand of the animal 
used to evaluate forage quality can have a dramatic effect on the measurement of 
the intake potential of the forage. The intake potential (maximum intake) of a 
forage can be measured only when the animal's energy demand is high enough to 
ensure that the animal's energy demand is not limiting intake. This important 
criteria for measuring forage quality will be illustrated more thoroughly in the 
section on measuring forage intake potential. 
Psychogenic Modulation 
In humans and other animal species, taste, smell, texture, and visual appeal can 
affect both short and long term intake. In addition, emotional states, social 
interactions, and learning can modify the intake of foods. Mertens (1985) postulated 
that these same or similar factors affect the feed intake of animals and suggested 
that they be aggregated into a class of psychogenic modifiers or modulators of 
intake. The psychogenic regulation of food intake involves the animal's behavioral 
459 MERTENS 
response to inhibitory or stimulatory factors in the feed or feeding environment that 
are not related to the feed's energy value or filling effect. Baumont et al. (1989) 
observed that the duration and rate of eating of forages were influenced by behavior 
and palatability. 
The most commonly recognized feed characteristic that impacts psychogenic 
modulation of feed intake is palatability. Differences in forage palatability have 
been reported (Minson and Bray, 1986; Burns et al., 1988; Black et al., 1989). 
The definition of palatability used in this chapter is more broad than that used by 
Marten (1970) in his extensive review of the subject at the first National Conference 
on Forage Quality Evaluation and Utilization.He offered the following conceptual 
definition of Relative Forage Palatability: "A plant characteristic(s) eliciting a 
proportional choice among two or more forages conditioned by plant, animal, and 
environmental factors which stimulate a selective intake response by the animal; this 
characteristic(s) may also be described in terms of acceptability, preference, 
selective grazing, and relish conditioned by sensory impulse, and while it may 
influence voluntary intake, it is never synonymous with voluntary intake properly 
measured." His definition of palatability is more restrictive than that used in this 
chapter and is closer to my definition of preference. In this discussion, palatability 
will include any feed characteristic that stimulates or inhibits the intake of a feed 
whether it is fed alone or is a choice among alternative feeds. 
The major reason that palatability has been defined in terms of preference among 
feeds is because intake differences among freely chosen feeds can be used to 
quantify preference (as opposed to palatability). When two feeds are provided to 
animals simultaneously, the amount of each feed consumed can be a quantitative 
measure of preference. Unfortunately, this measure of preference may not be 
related to any modulation of intake that occurs due to palatability when only one 
feed is offered. Sometimes we scientists define accurately what we can measure 
rather than measure what is needed for understanding. There is a need to define 
palatability and other psychogenic modulations of intake in a way that can be 
measured and investigated. 
The psychogenic modulation of intake can be interpreted to mean that the 
expected or predicted intake potential based on the physiological or physical 
mechanisms is modified by some proportional factor. This mechanism of intake 
modulation can be described by a simple algebraic equation that can be rearranged 
to measure the psychogenic effect: 
Ia = ~ X M, Equation 7 
M = IaI~, Equation 8 
where Ia IS the actual or observed intake (kg d-1) of the animal, Ip is the predicted 
intake potential (kg d-1) of the animal and diet based on physiological or physical 
control mechanisms, and M is the proportional psychogenic modulation factor 
(dimensionless ratio). 
It is proposed that psychogenic modulation is multiplicative rather than additive 
based on the logic that animals with greater intake potential will have a larger 
absolute change in intake. Equation 8 offers the potential for quantifying the 
modulating effects of management, disease, social interactions, and palatability on 
long term intake regulation. It is speculated that M will be less than 1.0 in most 
circumstances because most psychogenic effects inhibit intake. However, analyses 
of dynamic control systems of intake regulation indicate that diets with high 
palatability can result in increases in the dynamic set point or defended body weight 
(Russek, 1978) of the animal. 
REGULATION OF FORAGE INTAKE 460 
INTEGRATION OF INTAKE MECHANISMS 
Static or Steady-State Models 
The physiological, physical, and psychogenic mechanisms of intake regulation 
each establish independent controls on intake which need to be integrated into a 
common equation. The physical and physiological mechanisms of intake control 
provide limits at the opposite extremes of forage quality. When high energy, low 
fill rations or forages are fed, physiological energy demand regulates intake; 
whereas when high fill, low energy rations or forages are offered, physical fill 
limits intake. In any situation, the lesser of the two intake limits will predict the 
intake potential of a specified animal-feed combination. This system of controls can 
be described by an equation that can be combined with equation 7 to predict actual 
intake under conditions when feed availability is not limiting: 
Ip = min (Ie' If), 
Ia = min (Ie' If) X M, 
where all terms are as previously defined. 
Equation 9 
Equation 10 
Equation 9 implies that intake regulation is a discontinuous function of diet or 
forage characteristics because, in any specific situation, either energy demand or fill 
limits intake. In general, the energy value and filling effect of diets or forages are 
inversely related. This allows equations 3,6, and 9 to be described by a graph with 
inversely related X-axes (Figure 1). The psychogenic modifier (M) in equation 10 
would modify Figure 1 by a simple scalar adjustment to the Y-axis. 
One of the characteristics of this simple static model of intake regulation based 
on dual control mechanisms (Figure 1) is the occurrence of a unique solution. 
Because fill and energy concentrations in feeds are inversely and curvilinearly 
4 
11 3 
c 
::;) 
GI 
.li: 
1\1 
... 
.5 
~ 
-i; 
a; 
a: 
1 
I ax 
O+---'---~--'---~--~--r---r---r--'r-~ 
o 20 80 100 
100 80 60 40 20 o 
Available Energy Units (kg-1 0M) 
Figure 1. Illustration of the bi-phasic, discontinuous nature of intake regulation based on simple 
algebraic equations describing expected intakes when limited by physiological energy demand (Ie) 
or physical fill (If). Maximum intake (Imax) occurs at the intersection of the two theories of intake 
regulation and defines the diet having maximum fill (Fillmax) and minimum energy concentration 
(EnergYmin) which meets the animal's energy requirement and maximizes ruminal fill. 
461 MERTENS 
related to intake potential, the intersection of equations 3 and 6, when Ie = Ir, is the 
maximum intake that can both meet the animal's energy demand and fill the rumen. 
Solving for the maximum constrained intake yields the following equations: 
Imax = ~, when Ie = Ir, Equation 11 
RlE = C/Fmax, Equation 12 
F maxIE = C/R, Equation 13 
Fmax = (E X C)/R, Equation 14 
where Imax is the maximum intake (kg dol) that will provide the energy needed to 
meet the animal's requirements, Fmax is the maximum filling effect (L dol) of a diet 
that can meet the animal's needs, and all other variables as previously defined. 
If C and R are held constant and E (energy concentration) is increased, the 
filling effect of the diet (F max) can be increased and still meet animal requirements. 
Increasing C has a similar impact on Fmax' However, if the animal's energy 
requirement (R) is increased, the maximum fillirg effect of the diet (Fmax) must be 
reduced to allow enough intake to meet requirements. If the goal is to formulate 
rations that maximize forage content or to select the lower limit of forage quality 
that meets the performance needs of the animal, equation 14 provides a quantitative 
approach that is based on sound biological principles. Remember that C in equation 
14 is not the absolute maximum intake capacity of the animal because this fails to 
allow the animal to meet its needs; rather, C in equation 14 is the intake capacity 
that is constrained to meet the animal's requirements. 
Equation 10 essentially summarizes the most commonly accepted theory of 
intake regulation in ruminants. This comprehensive framework of intake regulation 
can be summarized in the following statements: 
1. The basic drive to eat (appetite) of an animal is determined by its genetic 
potential and physiological state which corresponds to the maximum rate of 
nutrient utilization by its tissues. 
2. When the diet contains adequate concentrations of available energy, protein, 
vitamins, and minerals, the animal consumes feed at a level that matches its 
appetite, and animal potential is the limit for intake. 
3. When diets of low nutritive value are offered, the animal consumes feed at a 
level that matches its gut capacity, and intake is restricted by the filling effect 
of the diet and the intake constraint of the animal. The tolerance of the animal 
for ruminal fill increases to accommodate increased nutrient demands, but 
animal performance alsois compromised. 
4. Modifying the dominant roles of physical limitation and physiological regulation 
on intake is the psychogenic stimuli associated with palatability, social 
interactions, disease, and feeding management. 
This conceptual framework of intake regulation and the equations that can be used 
to describe it will be used in the remaining discussion to illustrate the difficulties in 
measuring intake as a component of forage quality. 
NDF-Energy Intake System 
The accepted static theories of intake regulation have practical utility in forage 
evaluation and ration formulation only when they can be related to specific feed and 
animal characteristics that are routinely measured. In addition, the theory can be 
tested only when the vague concepts of the feed's filling effect and available energy 
and the animal's intake constraint and energy requirements are quantitatively 
defined. Mertens (1985, 1987, 1992) developed and refined the concept that NDF 
and net energy of lactation (NET) can serve as proxies for the filling effect and 
available energy in the accepted theories of intake regulation. They can be used as 
REGULATION OF FORAGE INTAKE 462 
starting points for relating mechanisms of intake regulation to feed and animal (dairy 
cow) characteristics simultaneously and can serve as reference points for relating 
intake mechanisms to a routinely measured characteristic of feeds (NDF). As more 
information is gained and digestion kinetics, specific gravity, volume, and rate of 
particle size reduction and passage can be routinely measured or estimated, NDF 
concentration can be refined to more accurately estimate the true filling effect of the 
forage or diet. 
Using NDF to represent the filling effect of the diet requires that the intake 
capacity or constraint of the animal also be expressed in units of NDF. Based on 
experiments designed to detect the NDF concentration of diets that optimized intake 
and dairy cow performance, Mertens (1985) observed that DMI was maximized 
when NDF intake was 12.5±1.0 g kg-l BW dol. Using this value for C and 
predicting NEL from forage NDF concentration allows the accepted theory of intake 
regulation to be illustrated using a routinely measured characteristic of feeds (Figure 
2). Preliminary evaluation suggests that the NDF intake constraint of mature beef 
cattle (Mertens, unpublished) and sheep (Mertens, 1973) is also approximately 12.5 
g kg-l BW dol. 
Recent literature contains numerous reports (Briceno et al., 1987; Reid et al., 
1988; Harlan et al., 1991; Llamas-lamas and Combs, 1991; Weiss and Shockey, 
1991) that NDF is not related to intake for use in ration formulation or forage 
evaluation. These discrepancies can be explained by comparing their results to the 
accepted mechanism of intake regulation using the NDF-Energy Intake System. 
Assuming that psychogenic effects in most research experiments are minimized (M 
= 1), equation 10 indicates that intake is a function of either fill or available 
energy. Because the relationship between NDF and fill is the opposite of that 
between NDF and available energy, the relationship of NDF to intake will be 
different depending upon whether intake constraint or energy demand is limiting 
intake in any particular instance. When energy is limiting intake, intake will be 
positively correlated with NDF concentration in the feed whereas, when fill is 
50~----------------------------------------~ 
-
"C 
:: 40 
co 
CII 
.:.= 30 
S 
CD 
.:.= 
:! 
.5 20 
... 
CD 
i; 
:E 10 
~ 
o 
........... 
". 
" . 
........ 
............ 
". 
" 
' . 
.•.•.•.•••.•••.•.•.•...•.... 
. ....................................................... . 
Fill Limitation 
O+------.------.------.------.-------.-----~ 
200 400 600 800 
Neutral Detergent Fiber (g kg -, OM) 
Figure 2. Expected relationship between the neutral detergent fiber (NDF) concentration of the diet 
and intake when regulated by energy demand or fill limitation. It is assumed that NDF is positively 
correlated with the filling effect and negatively correlated with the energy availability of the diet. 
463 MERTENS 
limiting intake, intake will be negatively correlated with NDF concentration (Figure 
2). Indeed, when measured over a wide range of diet NDF concentrations by 
animals with low energy demands, expected intake (indicated by the solid 
discontinuous lines in Figure 2) will have essentially a zero linear correlation with 
NDF concentration in the forage or diet. This does not mean that intake is not 
related to NDF, only that the relationship is not a simple linear one. 
The summarization by Jung and Linn (1988) provides an excellent data base for 
evaluating the NDF-Energy Intake System. They compiled experiments published 
in the Journal of Dairy Science from 1986 through 1988 and obtained about 100 
cow-treatment combinations in which both NDF and intake were measured. Some 
of the NDF values published were high in comparison to those commonly observed 
(NRC, 1989; Mertens, 1992), probably due to starch contamination when amylase 
is not used in the NDF procedure. Even after the data base is edited by replacing 
atypical NDF values with values adjusted to correspond with the acid detergent fiber 
(ADF) values that were published, a poor overall correlation is observed (Figure 3). 
Jung and Linn observed a correlation of -0.12 and concluded that NDF was not 
useful in predicting intake. However, if the data are compared to the theoretical 
lines derived for the NDF-Energy Intake System, the opposite conclusion is reached 
and the explanation for the apparent discrepancy is evident. 
As illustrated by Mertens (1992, Figure 25.2), any intake below and to the left 
of the fill limitation line is feasible. The relationship between intake and NDF 
would be proven to be invalid only if intake observations occurred above and to the 
right of the fill limitation line because all other observations will satisfy either 
equation 3 or 6. Only a few intakes occur in this region indicating that the accepted 
theory of intake using NDF is consistent with observations. The exceptions are 
related to diets containing finely ground, wet corn gluten feed which probably d~s 
not have the same filling effect per unit of NDF as long forages. The data baSe 
compiled by Jung and Linn (1988) indicates that most of the diets reported in the 
Journal of Dairy Science contain higher than needed levels of energy and that 
energy demand of the cows and not the filling effect of the diets limited intake. 
46.0~------------------------------------, 
-
"C ;l: 42.0 
a:l 
'7 C) 38.0 
~ 
C) 
-; 34.0 
~ 
:l 
.5 30.0 
.. 
CD 
t: 
~ 26.0 
~ 
o 22.0 
c = Corn gluten feed diets 
e = Energy limited diets 
f = RII limited diets 
c c 
200 240 280 320 360 400 440 480 
Dietary Neutral Detergent Fiber (g kg -10M) 
Figure 3. Comparison of the NDF-Energy Intake System predictions to the observed intakes of dairy 
cows reported in the Journal of Dairy Science as compiled by Jung and Linn (1988). 
REGULATION OF FORAGE INTAKE 464 
Diets were classified as energy (e) or fill (f) limited in Figure 3 by using equations 
3 and 6 to calculate the expected intakes for each mechanism and labeling each diet 
by which intake was minimum as indicated by equation 9. The positive relationship 
between intake and NDF indicated by the energy demand lines for various levels of 
milk production (Figure 3) corresponded well with observed productions. The 
negative correlation between NDF and intake will be high only when the filling 
effect of the diet is high in relation to the cow's energy demand as indicated by the 
diets labeled with an uf' in Figure 3. 
The prediction of intake and its use in forage evaluation is hampered by two 
important factors: 1) intake is influencedby characteristics of the animal, diet, and 
feeding situation and 2) prediction of intake is affected by the type of information 
that is known. Equations that attempt to predict intake solely as a function of 
animal characteristics (BW, production level, BW change, physiological state, etc.) 
or diet properties (fiber, bulk, energy density, chewing requirement, nutrient 
balance, etc.) will not have universal applicability and are doomed to failure. These 
equations fail because they implicitly assume that only one component of the intake 
regulation system (Le., animal, diet, or feeding situation) limits intake in all 
instances. The NDF-Energy Intake System overcomes this deficiency by integrating 
animal characteristics, diet properties, and feeding situation attributes into a simple 
system of mathematical equations. The aggregation of information into concepts of 
intake constraint and energy demand of the animal, and filling effects and energy 
availability of the diet, does not prove they exist or suggest that they are not 
complex concepts that require additional evaluation and characterization. Mertens 
(1992) indicated the limitations of using a chemical entity (NDF) to describe a 
physio-biochemical entity (filling effect) and suggested that NDF must be adjusted 
for particle size differences to accurately serve as a proxy for the filling effect of 
a feed. Likewise, it can be speculated that kinetic characteristics, such as rates of 
digestion and passage, should be used to modify NDF to more correctly reflect the 
filling effect. 
Intake prediction also is affected by the information that is known and the 
purpose for estimating intake. When a ration is formulated, an intake prediction is 
needed under the assumption that the animal's requirements are known and will be 
met. Diet properties are unknown, but it is implicitly assumed that they will be 
optimiZed for some function (profit, cost, production, etc.) under some set of known 
constraints. The NDF-Energy Intake System was designed to predict intake under 
these circumstances. Therefore, C in equations 6 and 14 was defined as a NDF 
intake constraint that was determined when 4% fat-corrected milk production was 
maximized (Figure 4). It is speculated that the system will predict intake most 
accurately when animals are near their maximal production. The NDF intake 
constraint is useful because it provides an upper limit to DMI and maximum forage 
in the ration that will allow the animal to maximize performance. Any intake less 
than this prediction is acceptable as long as the ration is formulated to have the 
increased energy density needed to meet the energy demands of the animal 
(Mertens, 1992). 
When the diet is known and the performance of the animal is unknown, intake 
prediction is much more difficult because the animal can accommodate the diet by 
varying intake and (or) performance. Figure 4 indicates that NDF intake can 
exceed the NDF intake constraint for maximal performance when animals are fed 
suboptimal diets. This indicates that the NDF intake constraint is not a constant and 
suggests that additional research is needed to refine the NDF-Energy Intake System 
for predicting intake under suboptimal conditions. The adjustment of the intake 
constraint by the animal may even suggest to some that fill never limits intake. 
465 MERTENS 
20 
~ 
25 -
~ 
~ ~ 16 
a::I NDF intake constraint that maximizes production 
- ~ 
20 Cl ~ 
~ 12 
~ 
15 :!: 
FCM U u.. 
CD 
~ 8 ca 
.... 
.5 
" 
, 
10 
*' 0 
5 <ci 
0 
u.. 
C 4 
2 
0 
" 
" 
, 
" " Ration NDF concentration 
" 
" that maximizes production , 
0 200 400 600 
Neutral Detergent Fiber (g kg-1 OM) 
Figure 4. Relationship of neutral detergent fiber (NDF) concentration in the ration to NDF intake 
and 4% fat-corrected milk (FCM) production (Mertens, unpublished). The NDF intake constraint 
is determined experimentally as the NDF intake that maximizes FCM production. 
However, if the animal cannot achieve its productive potential after it increases its 
intake constraint, the possibility that fill ultimately limited intake cannot be 
precluded. 
The value of the NDF-Energy Intake System for predicting intake is that it not 
only provides a specific, quantitative system for estimating intake when formulating 
rations that maximize DMI and forage use, but also it serves as a framework for 
quantitatively understanding intake regulation. Although animals integrate 
information from a variety of sources and compromise between performance and 
intake for any specific situation, it seems logical that the simplest approach to 
developing a system for predicting intake would attempt to identify the most limiting 
factor and use a quantitative measure of it to estimate intake. The NDF-Energy 
Intake System assumes that energy demand by the animal or filling effect of the diet 
are first limiting factors. This simple model may not predict intake in all situations, 
but it provides a quantitative way of identifying when intake is limited by fill (when 
NDF intake is > 1l.0 to 13.0 g BW-l dol) or by energy demand (when NE intake 
equals the potential energy demand of the animal). 
Optimization of Oxygen Utilization 
The accepted static theories of intake regulation are based on the concept that 
it is beneficial for animals to increase the consumption of feed up to the level 
needed to match their maximum genetic and physiological potential. This assumes 
that animals seek to obtain a maximal growth and production rate, resulting in 
maximal feed intake unless limited by fill. Ketelaars and Tolkamp (1991) proposed 
an alternative theory of intake regulation that is based on the concept that animals 
optimize oxygen utilization. They speculate that feed intake is regulated to 
maximize the benefits (input of NE) relative to the costs (oxygen consumption) to 
the animal. They contend that oxygen free radicals, which are a byproduct of 
metabolic reactions, are toxins that damage cells and shorten the life span. They 
REGULATION OF FORAGE INTAKE 466 
theorize that animals may regulate intake to optimize the amount of productive 
energy input per unit of oxygen consumed. Thus, the amounts and ratios of 
absorbed nutrients control intake according to their theory. Their thesis presents an 
extensive critique of the accepted theory of intake regulation, especially the concept 
that fill limits intake. Their proposal provides an alternative model of intake 
regulation and many of the objections they raise to the currently accepted theory of 
intake regulation will be discussed in the remaining sections. 
Ketelaars and Tolkamp (1991) use the data from ARC (1980) as the primary 
validation set for their theory. They observed that the voluntary intake equation 
used by ARC to predict the intake of long forages agrees very closely with the 
intake that optimizes the ratio of net energy intake to oxygen consumption. 
Although their model is internally consistent with the data of ARC (1980), it is not 
evident from their presentation how ARC intake predictions or their model 
predictions compare with the accepted theory of intake regulation. The theory of 
Ketelaars and Tolkamp (1991) is based on the metabolizability (q-value) of feeds 
which is used to determine the NE output and oxygen input of the animal. 
Information about feeds grown in the UK (Ministry of Agriculture, Fisheries, and 
Food, 1990) were used to develop the relationship between NDF and q for grasses 
(q = 1.023 - . oo82*NDF , R2 = .78). 
Intakes estimated by the equations of ARC (1980) for coarse diets and the theory 
of Ketelaars and Tolkamp (1991) were plotted on a common graph (Figure 5) with 
the fill limitation line proposed by Mertens (1987). The agreement between ARC 
and the fill limitation line (Mertens, 1987) is exceptionallyhigh in the range of 500 
to 800 g NDF kg-1 DM, considering the difference in animal species (sheep versus 
dairy cattle), diets (forages versus mixed rations), and feed characteristics used to 
predict intake (q versus NDF) between the two data sets. The difference between 
the ARC (1980) and Mertens' (1987) predictions when NDF is less than 500 g kg-1 
DM probably is related to the few observations in the ARC data set in this range 
and the likelihood that the intake of sheep would be limited by energy demand in 
";' 
"D 
;: 
In 
";' 
Q 
.¥ 
~ 
CD 
.¥ 
IV 
... 
.5 
"-CD 
~ 
::= 
~ 
a 
32 
30 
28 , 
26 
.. ,~, 
24 
-;\:" 
',~ .... 
,-
ARC (1980)', Ketelaars & Tolkamp (1991) 
""':' _ _ Oxygen OPt~imization 22 
20 
18 
16 
400 
.... -- ~"""'"IIr __ 
...... ----
fit .. 4t -----_ 
500 600 700 800 
Neutral Detergent Fiber (g kg -10M) 
Figure 5. Differences in predicted intakes estimated by the empirical equations of ARC (1980), the 
NDF-Energy Intake System (Mertens, 1987), or the optimization of oxygen utilization proposed by 
Ketelaars and Tolkamp (1991). 
467 MERTENS 
that range of NDF (Figure 2). The theory of Ketelaars and Tolkamp predicts 
substantially higher intakes of high-fiber grass forages than that indicated by theories 
based on fill limitation (Mertens, 1987) or empirical equations based on diet 
metabolizability (ARC, 1980). This suggests it would over-predict the intake of low 
quality forages which often result in intakes that are lower than those predicted by 
fill limitation (see final section of this chapter). Their theory also over-predicts 
intake for high energy, low fiber diets because the fill limitation line (Mertens, 
1987) in Figure 5 would normally be truncated by the energy demand line as shown 
in Figure 2. Figure 5 illustrates the value of modeling in providing the basis for 
designing experiments that would discriminate among the theories. Although 
additional research is needed, it appears that the concept of optimizing oxygen 
utilization may be more useful in describing the point of maximal intake for good 
to medium quality forages rather than be a substitute for the fill limitation of intake 
as Ketelaars and Tolkamp (1991) contend. 
Dynamic Models of Intake Regulation 
Dynamic models are well suited to the study of intake regulation because they 
offer the opportunity to evaluate concepts rigorously as they change over time. 
Steady-state solutions to dynamic models, although technically not dynamic, often 
provide quantitative insight into the operations of conceptual models. Waldo (1970) 
presented the key principle that resulted in our current understanding and use of 
digestion kinetics at the first National Conference on Forage Quality Evaluation and 
Utilization when he postulated that the curvilinear nature of digestion might be due 
to the presence of an indigestible fraction. With co-workers (Waldo et al., 1972), 
he developed the first-order model of digestion and combined it with rates of 
passage to calculate the filling effect of diets based on cellulose. Mertens (1973) 
used this model to relate digestion kinetics of NDF in forages to their intake. Ellis 
(1978) used the steady-state solution for digestibility proposed by Waldo et al. 
(1972) to estimate intake from fecal output as envisioned by Conrad et al. (1964) 
using rates of passage and digestion and the pool of undigested DM in the rumen. 
Pienaar et al. (1980) developed a more complex steady-state model for predicting 
intake from the flows of fermentable and unfermentable OM. Poppi et al. (1981b) 
investigated the relationship of intake to the ruminal retention time of DM using a 
steady-state model that described the flow of large and small particles through the 
rumen. 
Baldwin et al. (1977) proposed a dynamic model of digestion (second-order) and 
passage (first-order) for evaluating nutritive value; however, intake was an input to 
this model that was not calculated as a model response. Mertens and Ely (1979, 
1982) developed a dynamic model that included digestion as a first-order process as 
proposed by Waldo et al. (1972) and divided passage into first-order rates of particle 
size reduction and escape from the rumen. Intake was predicted by iteratively 
solving the model to estimate the intake that resulted in a ruminal fill of 15.8 g NDF 
kg-1 BW d-1. Illius and Gordon (1991) modified the model of Mertens and Ely 
(1979) to scale rate of passage and particle size reduction to BWO·27 and observed 
that model simulations agreed with intake observations for sheep and cattle. 
Several dynamic models (Forbes, 1977b, 1983; Black et al., 1981; Bywater, 
1984; Kahn and Spedding, 1984; Fisher et al., 1987) have attempted to integrate 
both physical and physiological mechanisms of intake regulation in a single model. 
Most of these models have attempted to use an either (or) approach for deciding 
whether intake is controlled by fill or energy demand, similar to that proposed for 
the static model (equation 9). Dynamic models, however, calculate the change in 
fill and energy supplied over time within a day to regulate intake and incorporate 
REGULATION OF FORAGE INTAKE 468 
kinetic characteristics such as rates of digestion and passage into intake control 
mechanisms. Fisher et al. (1987) assumed that intake is restricted by ruminal 
distension and nutrient absorption, but incorporated a feedback function that adjusted 
intake for both mechanisms simultaneously. Intake rate was adjusted based on the 
ratio of maximal ruminal contents to current ruminal contents and optimal nutrient 
absorption flow to current nutrient absorption flow. Their function modulates the 
chemostatic effect based on the level of distention. 
INTAKE VARIATION AND REGULATION MECHANISMS 
Appreciation and knowledge of the mechanisms controlling intake in animals 
indicates that intake is not simply a feed attribute, but is a function of characteristics 
of the feed, animal, and feeding situation. To obtain valid estimates of forage 
quality, the extrinsic factors affecting intake must be controlled so the intrinsic 
characteristics of the forage that determine intake are measured. Raymond (1969) 
stressed the need for rigid standardization of the conditions under which intake is 
measured. This is necessary not only to reduce variation in intake measurements, 
but also to improve the comparability of measurements among experiment stations 
and to ensure that forage characteristics and not animal or feeding situation factors 
are being measured. 
The multitude of factors that influence intake is indicated in Figure 6 which 
attempts to present a relatively complete listing of variables in a structured format. 
It is clear that many of these factors do not impact intake through a singular and 
distinct path. Numerous interactions occur that would make the figure extremely 
complicated if they were indicated. For example, rate of passage is described in 
Figure 6 as an animal characteristic, yet it is clear that rate of particle size 
degradation, rate of digestion, and plant morphology all affect rate of passage. 
Likewise, rate of digestion probably is affected by animal characteristics that 
influence the ruminal environment and its microbial population. Many of these 
factors have been discussed, but space does not allow the elucidation of all factors 
that influence intake regulation. Three factors will be discussed in this section 
because they have important impacts on the measurement, regulation, and 
description of intake as a component of forage qUality. 
Refusal Allowance and Selection 
The intake of forages can be measured only if they are available ad libitum. 
This implies that animals are allowed to eat as much as they desire, i.e., the 
amount of feed offered is greater than that which the animalconsumes. 
Unfortunately, ad libitum intake is complicated by the observation that animals eat 
forages selectively. This suggests that animals may have greater intake if more feed 
is offered (greater refusal allowed), thereby providing greater opportunity for 
selection. Greater opportunity for selection also could increase digestibility, 
assuming that animals select the less fibrous and more digestible components of the 
feed. 
Blaxter et al. (1961) indicated that, to avoid bias in measuring intake, the same 
excess of feed should be offered to all animals and suggested that animals be fed at 
least 1.15 times their consumption on previous days. However, the amount of 
refusal allowed by investigators can vary from 0 to 30% of that consumed, with a 
typical range of 5 to 15% (Brown et al., 1968; Colovos et al., 1970; Jones et al., 
1972; Dulphy and Demarquilly, 1983). Some researchers (Heaney et al., 1963; 
Minson and Milford, 1967) restricted intake to maintain a constant amount rather 
than proportion of refusals. When the amount of refusal is held constant, the 
469 MERTENS 
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proportion of refusal is higher for forages with low intakes which allows a greater 
degree of selection for low quality forages. 
More recently, Zemmelink (1980) extensively reviewed the literature and 
concluded that variation in techniques among investigators resulted in measurements 
of intake that were not comparable. He observed that the ranking of intakes of 
tropical forages varied with the level of excess that was offered. This implies that 
selection of components within a forage provides the animal a wide latitude in 
varying intake and probably digestibility. It also indicates that the level of refusal 
that is observed in any feed intake studies should be reported with the results of the 
experiment to provide important information for the interpretation of intake and 
performance responses by the animal. When large refusal allowances are permitted, 
caution must be exercised in relating chemical composition or any other feed 
characteristic to intake measurements because the characteristics of the feed offered 
usually differs from that of the feed consumed. Differences between the feed 
consumed and that offered may explain some of the discrepancies that exist in the 
literature when feed characteristics are related to intake. 
Nutrient Deficiencies or Imbalances 
The effect of nutrient deficiencies on intake has been discussed by Forbes 
(1986r Intake often is de~ressed when t~e CP conte~t of diets is belO\\;' 60 to 80 
g kg- DM (Blaxter and WIlson, 1962; Mmson and MIlford, 1967). This level of 
CP is near the requirement for protein and ammonia needed by bacteria in the 
rumen. If each kilogram of fermentable OM yields 150 to 250 g of microbial CP 
(NRC, 1985), this suggests that one gram of CP is needed for each 6 to 7 g of OM 
fermented in the rumen. If 85 % of the total tract digestibility occurs in the rumen, 
and forage protein is 25 % undegradable, the range in CP needed for effective 
digestion (ignoring recycling of N to the rumen and microbial inefficiencl of N 
utilization) would range from 70 g CP kg- I DM for forages with 400 g kg- DDM 
to 140 g CP kg-1 DM for forages with 800 g kg-I DDM. The need for CP and 
other nutrients, such as minerals, vitamins, and microbial growth factors, suggests 
that some or all of the effects of nutrient deficiencies on intake could be mediated 
via physical limitation. If microbial fermentation is inhibited, feed will not 
disappear from the rumen as quickly, the apparent filling of the diet would be 
increased, and intake will decrease. 
Egan (1965) infused casein into the intestines of sheep fed low N roughages and 
observed that intake and ruminal contents increased. Thornton and Minson (1973) 
reported that forages with low CP resulted in reduced intake and gut contents. 
These observations suggest that intake was not limited by fill; rather, some 
metabolic mechanism related to protein characteristics or the protein to energy ratio 
was limiting the intake of diets low in CP. It is difficult to envision a mechanism 
whereby increasing protein concentration would result in increased intake without 
including energy regulation in the theory. If protein is deficient in the diet relative 
to energy and animals were consuming feed in response to protein concentration, 
it would seem that they would increase intake of a low protein diet, not consume 
more when protein was supplemented. If it is energy intake that is actually 
regulated, however, the increase in intake with increasing protein supplementation 
can be resolved. 
Because animals have limited ability to dissipate excess energy, diets that are 
inadequately balanced for nutrients in relation to energy pose a dilemma for the 
animal. If intake is increased to obtain more of the deficient nutrient, excess energy 
is absorbed as well and must be accumulated by the animal. Eventually, the excess 
energy accumulation results in reduced intake in an attempt to balance energy input 
471 MERTENS 
and output. Thus, when diets low in protein relative to energy are fed to animals 
with high protein requirements (rapidly growing young animals, wool growth, or 
lactating