Effect of true ileal digestible dietary methionine to lysine ratios on growth performance and carcass merit of boars, gilts and barrows selected for low backfat

A total of 288 Landrace x Yorkshire boars, gilts and barrows selected for low backfat were used in two trials to determine the effect of an increased dietary ratio of true ileal digestible methionine to lysine on growth performance, carcass merit and nutrient utilization. The three experimental grower and finisher diets were: (1) barley-soybean meal diets with no supplemental amino acids as control containing true ileal digestible lysine at 0.45 and 0.36 g MJ(-1) DE in grower and finisher diets, respectively, (2) the same as diet 1 with supplemental lysine to provide true ileal digestible lysine at 0.65 and 0.49 g MJ(-1) DE, and supplemental threonine and methionine to give ratios of 0.7 and 0.3, respectively, to lysine on a true ileal digestible amino acid basis, and (3) the same as diet 2 with an increased true ileal digestible methionine to lysine ratio of 0.4. All diets were fed as pellets ad libitum in self-feeders, and with. free access to drinking water. The ADG and FCE during the grower, finisher and overall periods were higher (P < 0.05) for diets 2 and 3 than for diet 1, but they were not different between diets 2 and 3 in boars, gilts and barrows. The lean growth rate and whole body protein accretion were also higher (P < 0.05) for diets 2 and 3 than for diet 1, but they were not different between diets 2 and 3 in boars, gilts and barrows. The apparent faecal digestibilities of dry matter and energy, as well as nitrogen retention and excretion, were similar (P > 0.05) for all the diets. The results indicated that boars and gilts in the present study may not have reached the maximum genetic potential for lean growth rate due to an inadequate feed and amino acid intake. A lack of improvement in pig performance with a higher dietary ratio of true ileal digestible methionine to lysine (0.40 vs. 0.3) indicated that the current NRC recommendations may be adequate.