Self-selection of lysine by growing pigs: choice combinations between deficient or suboptimal and adequate or superoptimal dietary levels according to sex.

Self-selection of dietary lysine was studied in one 42-d experiment involving 100 Large White growing pigs, with an equal number of females and castrated males, initially weighing 17.7 kg, within 5 treatments of 20 animals in each. A common basal diet (17% CP, 13.8 MJ DE/kg) based on maize, soybean meal and peanut meal, and containing a deficient level of lysine (0.61%), was used. Supplementary L-lysine was provided to obtain suboptimal (0.74%), optimal (0.85%) and superoptimal (1.21%) levels of total lysine for growth, as assumed from usual recommendations. In addition to treatment 1, a control where pigs were fed ad libitum a single diet with 0.85% lysine, 4 treatments with free choice of lysine within paired diets were compared. These included deficient or suboptimal levels of lysine on the one hand and optimal or superoptimal levels on the other according to a 2 x 2 factorial plan: 0.61 vs 0.85% (treatment 2), 0.74 vs 0.85% (treatment 3), 0.61 vs 1.21% (treatment 4), 0.74 vs 1.21% (treatment 5). The results showed sex difference in diet selection according to lysine content. Females had a distinct preference for the superoptimal level (1.21%) compared to the optimal level (0.85%), especially during the initial period of the trial, their requirement being higher than the presupposed 0.85% optimal level. For castrated males, the preference for lysine was restricted to the lower 0.85% level. The ability of females to self-select a greater proportion of the high lysine feed as opposed to castrated males was related to a higher potential for lean tissue growth. These results confirm that the growing pig is able to differentiate between diets differing only in their lysine contents. Complementary observations on plasma-free amino acids suggested the use of diets adequately balanced for amino acids when offered for free choice.