Utilization of ileal digestible amino acids by growing pigs: isoleucine

Two experiments were conducted to determine the utilization of ileal digestible isoleucine by growing pigs. In the first, the apparent ileal digestibility of amino acids in cottonseed meal, lupin-seed meal and soya-bean meal was determined in pigs fitted with 'T'-shaped cannulas. In the second, three isoleucine-deficient diets were formulated to 0.23 g ileal digestible isoleucine/MJ digestible energy (DE) with the three protein concentrates contributing the only source of isoleucine in sucrose-based diets. An additional three diets were formulated with supplements of isoleucine to confirm that isoleucine was limiting in the first three diets. The growth performance and retention of isoleucine by pigs given the six diets over the 20-45 kg growth phase were then determined. The apparent ileal digestibility of isoleucine in the three protein concentrates (proportion of total) was: cottonseed meal 0.68, lupin-seed meal 0.86, soya-bean meal 0.86. There were no significant differences (P > 0.05) in growth rates (g/d) and crude protein deposition rates (g/d) of the pigs given the three diets formulated to 0.23 g ileal digestible isoleucine/MJ DE: cottonseed meal 590, 84; lupin-seed meal 613, 87; soya-bean meal 594, 91 (SEM 13.0, 2.9) respectively. The response of pigs to the addition of isoleucine confirmed that isoleucine was limiting in these diets. The proportion of ileal digestible isoleucine retained by pigs given the cottonseed meal (0.65) was slightly lower than that retained by pigs given soya-bean meal (0.73; P < 0.05). These results indicate that values for the ileal digestibility of isoleucine in protein concentrates more closely reflect the proportion of isoleucine that can be utilized by the pig than occurs for other amino acids such as lysine, threonine and methionine.