Including dietary valine and tryptophan, but not isoleucine, above the requirement for growing pigs may partly ameliorate negative effects of excess leucine from corn protein on nitrogen balance and growth performance

Abstract

Two experiments were conducted to test the hypothesis that increasing concentrations of dietary L-valine, L-isoleucine, or L-tryptophan in diets containing excess leucine (Leu) from corn protein will alleviate negative effects of excess dietary Leu on N balance and growth performance of growing pigs. In Exp. 1, 72 barrows (body weight: 33.9 ± 2.6 kg) were housed in metabolism crates and randomly allotted to 8 dietary treatments in a 12-day experiment. A basal diet based on corn and a high-protein corn product was formulated (16.9 mega joule/kg gross energy and 12.7 g/kg lysine). Two levels of crystalline L-isoleucine (0 or 1.0 g/kg), two levels of crystalline L-valine (0 or 1.0 g/kg), and two levels of crystalline L-tryptophan (0 or 0.5 g/kg) were added to the basal diet for a total of 8 diets that were used in a 2 × 2 × 2 factorial. Results indicated that fecal N output increased if isoleucine (Ile) was added to diets without adding valine (Val), but that was not the case if Val was added (interaction, P < 0.05). Isoleucine addition to diets reduced N retention, but N retention increased with tryptophan (Trp) addition to diets without valine addition, but not if Trp was added to diets with added Val (interaction, P < 0.05). The biological value of protein increased if Trp was added to diets without addition of Ile, but if Ile was added, Trp addition did not increase the biological value of protein (interaction, P < 0.05). In Exp. 2, a total of 288 growing pigs (body weight: 28.6 ± 2.5 kg) were randomly assigned to 9 dietary treatments in a 28-day growth performance experiment. There were 2 barrows and 2 gilts per pen and 8 replicate pens per treatment. A control diet based on corn and soybean meal (16.5 mega joule/kg gross energy and 11.1 g/kg lysine) was used in Exp. 2 in addition to the 8 diets used in Exp. 1. Results indicated that final body weight and average daily gain of pigs fed the control diet were greater (P < 0.001) than for pigs fed all other diets, except pigs fed the diet with addition of both L-valine and L-tryptophan. Pigs fed the control diet also had greater (P < 0.001) plasma concentrations of Trp and Val than pigs fed the basal diet or diets not containing L-tryptophan or L-valine, respectively. In conclusion, adding Ile alone reduced N retention, but adding Trp alone or in combination with Ile or Val increased N retention. The combination of Val and Trp supplementation may be beneficial for preventing detrimental effects of excess Leu on growth performance of pigs.