The interaction of liveweight and the degree of competition on drinking behaviour in growing pigs at different group sizes.

The pattern of drinking behaviour in pigs, and the influence of social factors such as competition on this behaviour, have been poorly documented. If competition for access to a limited resource is substantial, access to that resource may be allocated disproportionately to different members of a group. Priority may be based on a dominance hierarchy, or on physical characteristics, such as weight. The pattern of drinking behaviour was examined and the effect of pig liveweight on drinking behaviour under conditions likely to produce different levels of competition was investigated. Four replicates of a 2x2 factorial design of two group sizes (20 vs. 60) and two drinker:pig ratios (1:10 vs. 1:20) used a total of 640 growing pigs (start weight 36+/-5.0 kg). The drinking behaviour, skin lesion score and performance of nine focal pigs (three each of heavy (41.9 S.E.M. 0.57 kg), medium (35.7 S.E.M. 0.51 kg) and light weight (30.9 S.E.M. 0.63 kg)) per pen were recorded. For all focal pigs, the maximum time spent at the drinkers occurred between the 1800 and 1900 h and the minimum time between 0700 and 0800 h. The mean number of drinking bouts per pig per day was 30.9 S.E.M. 1.41, the median bout length was 21 s and the mean daily duration spent at the drinkers was 832 s. The frequency of visits to the drinkers, drinking bout duration and daily drinking time were affected by group size and drinker allocation, but not by weight or the interaction of treatments and weight. The diurnal spread of drinking was similar for each of the four treatment combinations and each weight 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.