The effect of stocking density on the responses of growing pigs to dietary lysine

An experiment was conducted to measure the effects of stocking density (increased number of pigs per pen) on lysine requirements of pigs grown from 25 to 60 kg live weight. Two hundred and sixty-four female Large White X Landrace pigs were assigned at 25 kg to one of four dietary lysine treatments (13.3 (L1); 11.4 (L2); 9.5 (L3) and 7.6 (L4) g/kg) and either seven or 13 pigs per pen (or 1.0 and 0.5 m2 per pig, respectively). An additional treatment of one pig per pen (2.0 m2 per pig) was included to compare the responses of solitary- versus group-penned pigs. Animals were given ad libitum access to dietary treatments from a mean pen starting weight of 26.1 (s.e. 0.35) kg to a mean pen finishing weight of 63.4 (s.e. 0.61) kg live weight. There were no significant interactions between dietary lysine content and floor space per pig on food intake (FI), average daily growth rate (ADG), the amount of food per unit of gain (FCR) and the rate of protein retention (PR). Significant interactions were evident for body composition and the rate of lipid retention (LR). Over the weight range 25 to 40 kg there were significant differences in FI (P < 0.05) and FCR (P < 0.001) between dietary lysine treatments but most of these differences had disappeared over the 40 to 60 kg live weight. Individually penned animals had significantly higher (P < 0.05) FI and ADG than group-penned animals. However, there were no differences between seven and 13 pig per pen treatments. Stocking density had no effect on LR or body protein content but did cause a significant reduction in PR (P < 0.001) and an increase in body lipid content (P < 0.05) as the number of pigs per pen increased from seven to 13. Lysine requirements (expressed in g/day) therefore could be seen to be reduced with increasing stocking density. However, as lysine intake was reduced in group-penned animals, the reduced daily requirement does not necessarily warrant a reduction in the lysine content of the food. Feeding according to the requirements for maximum PR will still produce the best carcass and growth performance irrespective of the group size. The improvement in PR associated with higher dietary nutrient levels did not completely offset the adverse physiological effects of higher stocking density but may partly counteract the effect of reduced lysine intake. However, there were indications that feeding crowded pigs a lower dietary lysine concentration may not further reduce the already diminished protein (lysine) growth rate. An additional experiment was performed to test whether the number of feeder bins may have constrained food intake and therefore growth in group-penned animals. The results of this experiment showed that the number of bins had no significant effect on FI, ADG and FCR in group-penned pigs, and therefore a single feeder bin was not considered a constraining factor in pigs housed with limited floor space.