Effects of allometric space allowance and weight group composition on grower-finisher pigs

The average daily gain (ADG; kg d^-1), pen efficiency (PE; kg ADG m^-2 floor space) and welfare indicators (injury levels, salivary cortisol concentrations and behaviour) of grower-finisher pigs were evaluated in groups of 19 barrows, at four levels of floor space allowances calculated mathematically (area = k × BW^0.667) using a constant k, (with values 0.027, 0.031, 0.034, and 0.037) for a mean final market weight of 116 kg and in two levels of group weight composition (uniform and varying weights - based on uniformity and variation of body weights of pigs within a pen at the beginning of the experiment). The corresponding space allowances (m2pig~') at the market weight of 116 kg were 0.64 (SA0.64), 0.74 (SA0.74), 0.81 (SA0.81), and 0.88 (SA0.88). The data were analyzed using repeated measure ANOVAs and independent sample T tests. The pigs in SA0.64 had a lower ADG (0.859 ± 0.017), spent a lower proportion of time lying in preferred areas (55.22 ± 1.644) and had higher total injury scores (4.581 ± 0.139) and higher number of aggressions (1.94 ± 0.286) (P < 0.05 for all) than those in SA0.88 (0.936 ± 0.020; 61.02 ± 1.203; 3.858 ± 0.208; 1.08 ± 0.212, respectively) and SA0.81 (0.916 ± 0.019; 59.77 ± 1. 417; 3.855 ± 0.112; 1.24 ± 0.238, respectively). Pigs in SA0.64 had higher (P < 0.05) overall PE (1.344 kg ADG m^-2 floor space) than those in SA0.88 (1.063 ± 0.023) SA0.81 (1.131 ± 0.024) and SA0.74 (1.224 ± 0.029) allowance treatments. Pigs in the varying weight group spent a higher (P < 0.05) proportion of time lying in preferred areas (59.73 ± 0.942) than the uniform weight group (57.08 ± 1.078). Pigs in SA0.74 and SA0.64 spent lower proportion of time (P < 0.05) lying isolated (2.76 ± 0.441 and 2.18 ± 0.372, respectively) than pigs in SA0.81 (4.52 ± 0.467). The uniform weight group exhibited more (P < 0.05) exploratory behaviour (8.83 ± 0.448) than the varying weight group (7.25 ± 0.356). On fully slatted floors, space allotted considering the final market weight of barrows corresponding to k values of 0.037 and 0.034 were better (P < 0.05) than 0.027 in terms of growth rate and welfare indicators.