UTILIZATION OF ILEAL DIGESTIBLE AMINO-ACIDS BY GROWING PIGS - ISOLEUCINE

Two experiments were conducted to determine the utilization of ileal d igestible 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 th ree protein concentrates contributing the only source of isoleucine in sucrose-based diets. An additional three diets were formulated with s upplements of isoleucine to confirm that isoleucine was limiting in th e first three diets. The growth performance and retention of isoleucin e by pigs given the six diets over the 20-45 kg growth phase were then determined. The apparent ileal digestibility of isoleucine in the thr ee protein concentrates (proportion of total) was: cottonseed meal 0.6 8, lupin-seed meal 0.86, soya-bean meal 0.86, There were no significan t differences (P > 0.05) in growth rates (g/d) and crude protein depos ition rates (g/d) of the pigs given the three diets formulated to 0.23 g heal digestible isoleucine/MJ DE: cottonseed meal 590, 84; lupin-se ed 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 isol eucine was limiting in these diets. The proportion of ileal digestible isoleucine retained by pigs given the cottonseed meal (0.65) was slig htly 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 proport ion of isoleucine that can be utilized by the pig than occurs for othe r amino acids such as lysine, threonine and methionine.