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The effects of group size on grazing in sheep
Article  in  Applied Animal Behaviour Science · July 1993
DOI: 10.1016/0168-1591(93)90103-V
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Applied Animal Behaviour Science, 37 ( 1993 ) 101-109 
0168-1591/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved 
101 
The effects of group size on grazing time in sheep 
P.D. Penning *,a, A.J. Parsons a, J.A. Newman b, R.J. Orr ~, A. H a r v e y a 
aAFRC Institute of Grassland and Environmental Research, North Wyke Research Station, 
Okehampton EX20 2SB, UK 
bAFRC Unit of Ecology and Behaviour, Department of Zoology, University c~lO.xj~rd, 
South Parks Road, Oxford OXI 3PS, UK 
(Accepted 8 March 1993) 
Abstract 
Voluntary intake of herbage by grazing sheep (Ovis aries L. ) is the result of several interacting 
behaviours. In gregarious animals, such as sheep, the social environment may strongly influence some 
of the components of ingestive behaviour that control food intake. This study investigated the effects 
of group size on grazing time, one of the main components of ingestive behaviour governing daily 
herbage intake. Grazing time was recorded for focal sheep within groups, ranging in size from one to 
15 over periods of 24 h. The animals were continuously stocked on a perennial ryegrass pasture ( Lol- 
ium perenne L. cv Parcour) maintained at a sward surface height of 6 cm. Groups were established 1 
week prior to recording the behaviour of a focal animal in each grazing group. Recordings were made 
on three occasions for 24 h, with a period of 24 h between each recording. It was found that sheep in 
smaller groups spent less time grazing than sheep in larger groups. There was no relationship between 
group size and intra-meal intervals, prehension biting rate or number of meals, but animals in groups 
of one and two tended to have shorter meals than those in larger groups. There was indirect evidence. 
from measurements of changes in sward surface height, to suggest that intake of herbage was also 
reduced for animals grazing in groups of less than four. It was concluded that a minimum flock size 
of three sheep, but preferably four, is required for studies of grazing behaviour. 
Key words: Sheep; Grazing; Biting rate; Feeding 
Introduction 
Ef f i c i en t a n i m a l p r o d u c t i o n d e p e n d s o n a d e q u a t e levels o f v o l u n t a r y i n t a k e 
( F o r b e s , 1986 ). T h i s s t a t e m e n t i m p l i e s t h a t a m a j o r l i m i t i n g f a c t o r in s h e e p 
p r o d u c t i o n is t he i r i nges t i ve b e h a v i o u r . T h i s c o m p r i s e s t h r e e s e p a r a t e c o m - 
p o n e n t s : g r a z i n g t i m e , b i t e m a s s a n d p r e h e n s i o n b i t i n g rate . A n i m a l sc ien t i s t s 
h a v e s p e n t m a n y yea r s a s sess ing f a c t o r s t h a t a f fec t t hese c o m p o n e n t s a n d 
h e n c e v o l u n t a r y f eed i n t a k e ( see r e v i e w b y F o r b e s , 1986 ) . A t y p i c a l v o l u n - 
t a r y f eed i n t a k e s t u d y w o u l d be c o n d u c t e d i n d o o r s u s i n g i n d i v i d u a l l y p e n n e d 
a n i m a l s t h a t a re o f f e r e d f eed ad l i b i t u m (e.g. W e s t o n , 1 9 8 9 ) . T h e s e s tud i e s 
a re c o n d u c t e d w i t h t he e x p e c t a t i o n t h a t t he k n o w l e d g e o f f a c to r s t h a t c o n t r o l 
v o l u n t a r y i n t a k e c a n be a p p l i e d genera l ly , so t h a t levels o f p r o d u c t i o n in re- 
*Corresponding author. 
102 P.D. Penning et al./Appl. A nim. Behav. Sci. 37(1993) 101-109 
lation to intake and genetic potential can be predicted and manipulated. 
However, the application of classical theories of intake control, developed in 
this manner, to grazing animals, may be vitiated by many other factors that 
cannot be taken into account in indoor feeding studies. Principal among these 
must be social interactions, because sheep are typically classified as social 
animals. For instance, McClymont (1967) reported social facilitation of 
phagic behaviour in sheep, and Rook and Penning ( 1991 ) found that sheep 
tend to be synchronous in their start of grazing bouts. This study investigated 
the effects of group size on grazing time, one of the major factors controlling 
daily intake. 
Animals, materials and methods 
In September 1991, group sizes of I to 10, 12 and 15 of mature, non-lactat- 
ing, non-pregnant Scottish Half bred ewes, with a mean liveweight of 
84.7 _+ 1.29 kg, were established. The groups were maintained on monocul- 
tures of perennial ryegrass (Lolium perenne L. cv Parcour) at a sward surface 
height (SSH) of 6 cm (Bircham, 1981 ) with a constant area of 0.03 ha per 
animal. None of the plots containing the animals were contiguous and there 
was a m in imum distance between plots of 5 m. The plot boundaries were 
constructed from wire mesh and electric fencing. It was not possible to visu- 
ally isolate the groups of animals. These groups remained together for 1 week 
in order to become accustomed to their group members and group sizes. All 
animals had access to water and no supplementary feed was offered. 
After 1 week, one sheep (at random) from each group was fitted with a 
behaviour recorder. This technique has been described in detail by Penning 
(1983) and Penning et al. (1984). Briefly, gnathometers were used to pro- 
duce analogue electrical signals of the jaw movements that were recorded on 
miniature tape recorders, carried on the ewes, over a 24-h period commenc- 
ing at 10:00 h. These signals were subsequently replayed and the waveforms 
analysed by computer to give time spent grazing, ruminating and idling and 
to count the jaw movements associated with these activities. The data were 
summarised on a minute by minute basis and an animal was deemed to be 
either eating, ruminating or idling if it spent at least 30 s in any 1 min per- 
forming one of these behaviours. To fit or remove grazing behaviour equip- 
ment required 5 min per animal when the animals were restrained in small 
pens adjacent to each plot. Otherwise, the animals were left undisturbed on 
an area of pasture that was isolated from roads and tracks, and were only 
visited by the experimenters. Group size 1 was replicated three times and 
group size 2 was replicated twice. However, lack of land, animals and behav- 
iour recording equipment prevented replication of the larger groups. For each 
group size, the behaviour of one focal animal each day was recorded on Days 
8, 10 and 12 after the groups were formed. A different focal animal was cho- 
P.D. Penning et al./Appl. Anirn. Behav. Sci. 37(1993) 101-109 103 
sen at random each day. The weather was similar on the 3 days when the 
recordings were made. Individual animals within a group could not be con- 
sidered independent as the hypothesis to be tested was that, the number of 
individuals within a group affected the behaviour of the individuals within 
that group. In addition, Rook and Penning ( 1991 ) have demonstrated that 
sheep within a group do not behave independently. Therefore a mean of the 
three measurements made at each group size, was calculated and the data 
were analysed by regression using group size as the independent variate. Ad- 
justed R2 (proportion of the variance accounted for;/~2) was calculated for 
each of the regression models fitted. 
Twenty-five measurements of SSH were recorded on each plot at the start 
and end of the experiment. 
Results 
Fig. 1 shows the asymptotic relationship between grazing time and group 
size. The equation calculated was: 
grazing time (min h - ] ) = 629 - 311" exp [ - 0.46* group size ), 
n = 12, R2=0.48. 
Linear and quadratic regressions were also fitted to the data and/~2 was 
0.28 and 0.43, respectively. The asymptotic relationship is preferred as it ac- 
counts for a greater proportion of the variance than the linear or quadratic 
8 0 0 
7 0 0 - 
# 
-~ 600 
~ 500 
4 0 0 
3 0 0 ~ ' 
0 5 1 0 15 
G r o u p s i z e 
Fig. 1. Grazing time (min 24 h ]) as a function of group size. The points are mean values fbr 
three animals. (- exponential fit; ...... piecewise regression with the break point at group size 3 
and - - piecewise regression with the break point at group size 4 ). 
104 P.D. Penning et aL /Appl. Anita. Behav. Sci. 37(1993) 101-109 
regressions and, in addition, it is biologically meaningful. That is a group 
would be expected to contain some m i n i m u m critical number of animals and, 
above this group size, grazing t ime would remain constant, provided the area 
of pasture per animal also remains constant, as was the case in this experi- 
ment. An asymptotic curve describes this type of response, whereas a qua- 
dratic relationship would predict a decrease in grazing time for large groups 
of animals. 
Piecewise linear regression analysis was also carried out (Draper and Smith, 
1981 ) and break points were systematically fitted for group sizes 2 to 8 to 
determine the m in im um group size at which grazing time was not affected. 
The greatest variance accounted for was achieved when the break point (see 
Fig. 1 ) was set at a group size 3 (/~2 =0.50, n = 12) which corresponded to 
587 min of grazing in 24 h. Animals in groups of one or two grazed for 164 
min and 82 min less 24 h-~, respectively. The slope of the regression up to 
the break point was 82 ( + 31.0) min per animal in the group. The slope of 
the regression from the break point to group size 15 was 5 ( + 5.0) min per 
animal in the group and was not significantly different from zero. Setting the 
break point to four (see Fig. 1 ) did not significantly reduce the variance ac- 
counted for (_g2 = 0.49, n = 12), however, using group size 5 as the break point 
reduced the variance accounted for/~a = 0.40, n = 12 ). 
The/~2 values show that group size only accounted for about 50% of the 
variation in grazing time. The replication in this experiment was achieved by 
recording grazing t ime on three separate days. To examine whether differ- 
ences between days could explain some of the variability in the data, an anal- 
ysis of variance on the mean grazing time, across groups, was carried out con- 
sidering day as the treatment (n = 12 ). No significant difference between days 
was found (P=0 .25 ) and the mean values were: 595, 612 and 514 (s.e.d. 
43.7 ) min 24 h -1. It was not possible to analyse for interactions between day 
and group size as only one focal animal was used within a group on each day, 
as previously discussed. 
Figure 2 shows a comparison of the patterns of grazing between group 
size 1 and group size 15. It shows that in any given hour animals in a group 
of 15 are likely to have grazed for a longer period of time than those kept 
individually. This result is particularly obvious for the period of darkness 
(20:00-06:00 h ). 
The effects of group size on bouts of grazing ( 'meals ') was also analysed 
using the method described by Forbes (1986) and Sibly et al. (1990) to de- 
termine intra- and inter-bout intervals. These authors showed that the distri- 
butionof pauses during eating comprises two populations, those that occur 
within an eating bout (intra-bout intervals) and those that occur between 
eating bouts (inter-bout intervals). The distribution of pauses in grazing was 
analysed to determine the inter-bout interval. Piecewise regression analysis 
was applied to the natural logarithm of the frequency of each pause duration. 
P.D. Penning et al./Appl. Anim. Behav. Sci. 37 (1993) 101-109 [ 05 
60 i 
I 
5O 
=~ 40 
~_ 20 1 
1 0 
0 ~ ~ C~ 
1 0 : 0 0 1 4 : 0 0 1 8 : 0 0 2 2 : 0 0 0 2 : 0 0 
T i m e ( h ) 
0 6 : 0 0 
Fig. 2. Patterns of grazing in group sizes 1 • and g~ 15. This figure shows the number of minutes 
spent grazing mean for three animals) in each hour for each group. 
6 " 
o>, 
5!" 
4 ~ 
3 
' ~ • O O • 0 2 
• 
1 . . . . t • 
• • • o o o 
0 I _ _ u = = ~ . _ $ j 
0 1 0 2 0 3O 4O 
Interval ( m i n ) 
Fig. 3. Log frequency of intervals during grazing plotted against length of interval. The fitted 
regression lines show the change from intra- to inter-bout interval occurred at 6 rain. 
The frequency d i s tr ibut ions o f pause lengths did not vary be tween group sizes 
so all data were p o o l e d ( 2 4 h recordings for 36 a n i m a l s ) and are s h o w n for 
intervals from 1 to 30 m i n in Fig. 3. The result o f this analysis suggested that 
6 min was the length o f the inter-bout interval On(frequency)=5.7-0.6* 
106 P.D. Penning et aL/Appl. Anirn. Behav. Sci. 37 (1993) 101-109 
Table 1 
The effects of group size on prehension biting rates, number of grazing bouts of 6 min length and more 
and duration of bouts (means for three animals) 
Group size Prehension rate Number of Length of 
(number of animals ) (bi tes/min ) grazing bouts bouts (min) 
24 h - l 
1 79 7.3 62 
2 70 6.3 72 
3 68 4.3 148 
4 102 6.3 114 
5 75 9.0 69 
6 77 6.7 105 
7 97 7.0 84 
8 62 6.5 114 
9 84 4.0 169 
l0 76 5.3 116 
12 73 7.0 103 
15 79 7.0 97 
c 
& 
c 
z:: 
© 
-1 r 
i 
2 
0 
L 
5 10 1 5 
Group size 
Fig. 4. The effects o f group size on the change in SSH (cm) . 
(interval min); for interval 6 min; K 2 = 0 . 8 4 ) . 
Using intervals > 6 rain as the inter-bout duration, numbers of bouts and 
bout durations were calculated for each group size (Table 1 ). It can be seen 
from Table 1 that the number o f meals in 24 h was not correlated with group 
size, but the mean length o f each bout tends to be shorter for group size 1 or 
2 than for larger groups. Table 1 also shows that the mean prehension biting 
P.D. Penning et al./AppL Anim. Behav. Sci. 37(1993) 101-109 1 07 
rate (b i tes /min grazing) was not correlated with group size. Since bite mass 
was not measured, it is not possible to estimate intake rate and thus total daily 
intake. However, since the SSH and stocking densities were the same across 
groups, it seems unlikely that there would be marked differences in intake 
rate between groups (Penning et al., 1991 a). Therefore, a decrease in grazing 
time seems likely to have resulted in a lower daily intake of herbage. This is 
supported, indirectly, by the SSH data (Fig. 4), which shows that there was a 
smaller decrease in SSH from group sizes less than four compared with larger 
groups. This implies that less herbage was consumed by animals in the rela- 
tively small groups. 
Discussion 
Although this study has little direct practical application for grazing man- 
agement, it should be important to researchers who study voluntary feed in- 
take or diet choice in sheep. Many researchers use single sheep, individually 
penned as units of replication (e.g. Illius et al., 1992; Newman et al., 1992) 
although Marsden and Wood-Gush (1986) and Done-Currie et al. (1984) 
have questioned the validity of the general extrapolation of results from such 
animals. This study shows that sheep kept individually at pasture may not 
behave in the same way as when they graze as a member of a flock. The results 
suggest that the min im um unit of replication for grazing experiments should 
be groups of, at least, group size 4 or 5, but certainly not less than 3. These 
results are also supported by those of Southcott et al. ( 1962 ), who found that 
animals maintained in smaller group sizes gained less weight than those 
maintained in larger group sizes. 
It is not known why sheep should have longer grazing times in larger flocks 
and it may have been the result of social facilitation. However, Rook and 
Penning ( 1991 ) found that sheep in groups tended to synchronise the start of 
their grazing bouts, but were less synchronised at the end of these bouts. Hence, 
bout duration does not seem to be a result of social facilitation. Also, Benham 
(1984) only identified social facilitation for movement , but not for feeding 
in grazing dairy cows. Another possibility is that it is an ancestral behaviour 
correlated with the level of perceived danger of predation. Many animal spe- 
cies flock for anti-predator benefits and individual sheep may perceive them- 
selves to be under greater threat than they might be as part of a group, owing 
to the 'Dilution effect' (Pulliam and Caraco, 1984). In addition, animals that 
graze in groups may benefit from the increased number of individuals that 
are being vigilant, the so called 'many eyes' hypothesis (Pulliam, 1973 ). Many 
other social animals show this same effect of group size on feeding time in 
studies where the results can be more readily related to apparent danger of 
predation (see Lima and Dill, 1990, for review). 
108 P.D. Penning et aL/Appk Anim. Behav. Sci. 37(1993) 101-109 
This experiment can do tittle to resolve these two possibilities, but it does 
suggest that group size may be a critical variable in determining grazing be- 
haviour and should perhaps be considered in other contexts, e.g. in under- 
standing the mechanisms controlling daily intake. 
In attempts to understand factors controlling daily intake by grazing rumi- 
nants studies using 'sward boards' (Black and Kenney, 1984), grazing cages 
(Burlison et al., 1991 ) and turves (Illius and Gordon, 1990; Newman et al., 
1992) have provided valuable information on the mechanisms and physical 
constraints to intake rate (e.g. the interrelationships between sward density, 
bite mass, intake rate etc.). These experiments, by their nature, last at the 
most for 1 or 2 min and cannot therefore address the mechanisms controlling 
grazing time. Animals in many circumstances control daily intake by adjust- 
ing grazing time, often completely compensating for low intake rates (Pen- 
ning et al., 1991 b ). The understanding of intake rate alone cannot, therefore, 
be extended to understand how total intake is controlled. Ecologists studying 
wild animals have had some success in addressing total daily feeding time by 
considering it as a behavioural decision rather than, necessarily, a constraint. 
They consider that this behaviour may be shaped by natural selection, such 
that the consequences of an increase in grazing time, must be weighed against 
the consequences of engaging in alternative behaviours (e.g. avoiding preda- 
tion). This 'Darwinian fitness' approach can provide a non-mechanistic hy- 
pothesis regarding the 'limitations' on grazing time (i.e. that it is an adaptive 
behaviour rather than a mechanistic constraint). Illustrations of this ap- 
proach are seen in Mangel and Clark (1986), McNamara and Houston 
(1986), and Newman (1990). 
The present paper clearly indicates that, for whatever motivation, sheep do 
use behavioural cues to modify grazing time. 
Acknowledgements 
We thank R.A. Champion, S. Da Costa and R.H. Johnson for assistance in 
the field and Dr A.J. Rook for statistical advice. This research was supported 
by a grant from the Agricultural and Food Research Council's Joint Agricul- 
ture and Environment Programme. The authors also thank Prof. J.R.Kreb- 
sand and Dr A. Lawrence for useful comments on earlier versions of this 
manuscript. 
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