Isoleucine Requirement of the Growing Pig and Leucine-Isoleucine Interrelationship

Two trials involving 72 growing Large White pigs were conducted to reevaluate the isoleucine requirement and leucine-isoleucine interrelationship. Graded Levels of L-isoleucine and/or L-leucine were added to a 13.5% protein semi-purified diet based on herring meal and amino acid mixture, and containing 3,300 kcal digestible energy per kilogram. In the first trial, four groups of 12 pigs (half barrows and gilts) averaging 15.8 kg live weight initially were fed .38, .46, .54 and .62% dietary isoleucine, respectively, up to 51 kg final weight. There was a trend towards decreased daily gain in both sexes with increasing isoleucine content of the diet, whereas no significant change was observed in feed intake, feed efficiency or carcass characteristics. But levels of blood free isoleucine at 51 kg final weight increased sharply with increased dietary isoleucine. Thus, the estimated requirement for dietary isoleucine would not be higher than .38% of the diet. In the second trial of 35 day duration, four groups of six female pigs averaging 19.8 kg live weight were fed diets providing .38, .48% isoleucine and .64, 1.14% leucine in a 2 × 2 factorial arrangement of treatments, corresponding to a leucine-isoleucine ratio range of 1.7 to 3.0. No significant improvement of daily gain and feed efficiency was obtained with increasing dietary isoleucine, and no leucine-isoleucine interaction was observed on these criteria. Level of dietary leuclne exerted a sharp decrease in the concentration of the other free branched amino acids (isoleucine and valine) in the blood, as well as of some essential amino acids, especially lysine and methionine, without interaction between leucine and isoleucine. It was concluded that doubling the leucine-isoleucine ratio in the young pig had no deleterious effect either on growth performance or on isoleucine requirement.