Utilization of ileal digestible amino acids by growing pigs: threonine

An experiment was conducted to determine the utilization of ileal digestible threonine by growing pigs. Three threonine-deficient diets (0.22 g ileal digestible threonine/MJ digestible energy (DE] were formulated using cottonseed meal, meat-and-bone meal and soya-bean meal respectively, as the only source of threonine in the diet. An additional three diets were formulated with supplements of threonine to confirm that threonine was limiting in the first three diets. The growth performance and retention of threonine by pigs given the six diets over the 20-45 kg growth phase was then determined. Growth rates (g/d) of the pigs given the three diets formulated to 0.22 g ileal digestible threonine/MJ DE were significantly different (P less than 0.001): cottonseed meal 417, meat-and-bone meal 452, soya-bean meal 524 (SED 13.6). The response of pigs to the addition of threonine confirmed that threonine was limiting in these diets. Crude protein (nitrogen x 6.25) deposited by the pigs (g/d) was significantly higher (P less than 0.001) for those given soya-bean meal (75), relative to meat-and-bone meal (62) and cottonseed meal (47) (SED 3.3). The proportion of ileal digestible threonine retained by pigs given the three protein concentrates was: cottonseed meal 0.44, meat-and-bone meal 0.59, soya-bean meal 0.64 (SED 0.024). These results indicate that values for the ileal digestibility of threonine in protein concentrates are unsuitable in dietary formulations as the assay does not reflect the proportion of threonine that can be utilized by the pig. It appears that, with heat-processed meals, a considerable proportion of the threonine is absorbed in a form(s) that is (are) inefficiently utilized.