EFFECT OF GROUP COMPOSITION AND PEN SIZE ON BEHAVIOR, PRODUCTIVITY AND IMMUNE-RESPONSE OF GROWING PIGS

Levels of aggression, injuries, activity, performance and immune response were determined in 288 growing pigs in a 2 X 2 factorial experiment; the factors being group composition and pen size. Pip were classified as small (SM) when allotted and then reclassified as medium (MED), large (LG) and extra-large (XL) at 3 week intervals. Static groups were initiated by 12 SM pigs and they remained together for 12 weeks. Dynamic groups consisted of three pip of each size class. Pigs were introduced into dynamic groups as SM pigs and remained there for 12 weeks, progressing up through the size classes. At 3 week intervals the three XL pigs in dynamic groups were removed and replaced with three SM pigs. Pen sizes were 9.5 m2 and 7.6 m2. Pigs were weighed weekly and gains determined. Aggression during the 4 h period after regrouping was determined by 10 min of continuous observations at 20 min intervals. Ear and shoulder injuries were evaluated at 6 h, 24 h, 48 h and 144 h post-regrouping and each week thereafter. Intradermal response to phytohemagglutinin (PHA) as an indicator of in vivo cellular immunity was assessed in dynamic, static and control pigs (remaining in nursery pens and not regrouped), as was plasma cortisol concentration and neutrophil-to-lymphocyte ratio (N/L). PHA was injected 1 h prior to regrouping and the response was measured at 8 h and 24 h post-regrouping. Weight gains and activity budgets over the entire trial did not differ between treatments (P > 0.10). However, SM and MED pigs in static groups and XL pigs in dynamic groups gained more than their contemporaries in the other grouping treatment (P < 0.05). A greater proportion (P < 0.05) of pigs in dynamic groups (13.5%) were removed from test owing to poor health than in static groups (6.3%). The SM pigs in dynamic groups spent less time fighting (72.3 s per pig) during the initial 2 h after regrouping than SM pigs in static groups (196.5 s per pig) (P < 0.05). In conclusion, levels of aggression following regrouping can be decreased by the use of dynamic grouping, however, this practice reduces the overall well-being of the pigs and should be avoided.