Ž category. Heavy pigs had the greatest number of lesions, and light weight pigs the least 13.2 vs.
. 8.8 S.E.D. 1.18, P - 0.005 , but this was not affected by the four treatment combinations. Lesion
score correlated poorly with the parameters of drinking behaviour. The treatments, in isolation, or in interaction with weight, did not affect performance. No treatment encouraged sufficient
competition to compromise the drinking behaviour, social behaviour or performance of the lightest animals in the pen. q 2000 Elsevier Science B.V. All rights reserved.
Keywords: Resource access; Competition; Group size; Drinking behaviour; Pig-feeding and nutrition
1. Introduction
The factors which influence the water balance within the pig have been reviewed by Ž
. Ž
. Brooks and Carpenter 1993 , and Barber et al. 1988 have described the consequences
of restricting the water delivery rate to growing pigs. Despite this, very little is known about the water requirements of the pig. Less still is understood about the pattern of
ingestion of that water, such as the amount of time spent at the drinking points each day, the number of visits within that time, the average drinking bout length and the diurnal
pattern of ingestion. The influence of social factors, such as competition for access to a small number of drinking points, on drinking behaviour has not been described.
Competition between individuals arises due to the limitation, either in quantity, spatial distribution or temporal availability, of a resource such as food, water, space, or a
mate. Competition for resources accounts for the greater part of animal aggression Ž
. Ž
. Fraser and Rushen, 1987 . Meese and Ewbank 1973 identified prioritising access to
limited resources as a principal reason for the emergence of dominance hierarchies, Ž
. although this suggestion has been disputed by others e.g., McGlone, 1986 . If social
rank is correlated with resource access, the pattern of allocation to different ranked individuals is likely to be dependent on the availability of the resource itself. In cases of
severe restriction the highest-ranking individual would obtain a substantial amount of the resource, while all lesser ranks would obtain a lower level. In cases of only moderate
restriction only the very lowest ranking individual or individuals would be penalised,
Ž .
while all others would acquire a higher amount of resource Craig, 1986 . Furthermore, the nature and density of the resource affects the ease with which it can be defended. A
concentrated resource, which is easily defended, promotes frequent and intense aggres- Ž
. sion Schnebel and Griswold, 1983 . During periods of competition, therefore, the
ability of certain individuals to access resources may be compromised, as a result of their lower social rank or their poorer competitive ability. Consequently, under commer-
cial conditions, provision of resources to a group of pigs must be adequate to allow every individual sufficient access, regardless of social status or competitive advantage or
disadvantage.
Competition for access to resources may be elevated in large group housing condi- tions. The sight of an individual engaged in feeding or drinking behaviour may
encourage similar behaviour in another individual; a process know as social facilitation. Ž
Furthermore, individuals have been described by Gonyou unpublished data, cited in .
Chapple, 1993 as expressing preferences for certain sections of certain feeders. The
existence of a greater number of pigs feeding at any moment, and thereby encouraging social facilitation, and the possibility of an excessive number of animals preferring the
Ž .
same feeding point in large groups have been suggested by Spoolder et al. 1999 as potential sources of increased feeding competition in large group systems. Without
evidence to the contrary, it is possible that such mechanisms may operate in large groups to elevate the demand for drinker access also.
Although the volume of drinking water offered to growing pigs is generally not restricted, the ability of the pigs to gain access to the drinking points themselves has
received little attention. The current Welfare Codes have adopted caution in advising the provision of one nipple drinker per 10 growing pigs fed a dry diet. Producers have
frequently extended this ratio to one drinker per 20 animals without encountering problems. In view of this discrepancy and the increased popularity of large group
housing systems, a reassessment of the current drinker provision recommendation is required.
The consequences for drinking behaviour, overt aggression and performance of the pen as a whole resulting from the restriction of drinker allocation from one drinker per
10 pigs to one drinker per 20 pigs for animals housed in groups of 20 and 60 have Ž
. recently been reported by Turner et al. 1999 . These findings indicate that, under the
environmental conditions and feeding regime used in the experiment, the water intake, behaviour and performance of the pen as a whole were not compromised by the more
restricted drinker allocation, large group size or the interaction of these two factors. However, the consequences for the detailed drinking behaviour of specific individuals
within the pen have not yet been reported.
Thus, the purpose of this investigation was, firstly, to describe in detail the drinking behaviour of growing pigs. The second objective was to examine how the drinking
behaviour, aggression and performance of pigs of different weight categories, selected as examples of individuals likely to be different in their competitive ability, was influenced
by changes to drinker allocation, group size and the interaction of these two factors.
2. Materials and methods
