| Three bioassays and two digestibility trials were conducted to determine the true digestible isoleucine (Ile) requirement for maximal weight gain and minimal plasma urea-nitrogen (PUN) of growing (25 to 45 kg) pigs. An Ile-deficient basal diet (0.30% true digestible Ile) utilizing red blood cells (RBC) was developed and validated. Digestibility of Ile in the RBC product was determined in both cecectomized roosters and ileal-cannulated pigs and found to be 97%. Broken-line analysis of dose titrations of Ile (0.38 to 0.58% true digestible Ile) suggested a true digestible Ile requirement of 0.50% for growing pigs. Minimal plasma urea-nitrogen levels in the blood agreed with results of the growth assay, suggesting that growing pigs between 25 and 45 kg require an estimated 0.50% true digestible Ile in the diet, or 1.46 g/Mcal ME, for maximal weight gain and minimal PUN. A second project examined the true digestible Ile requirement of high-lean-gain, late-finishing pigs, utilizing maximal weight gain and minimal PUN as response parameters. True digestibility of Ile in the basal diet was calculated to be 88%, based on the 97% true digestibility value obtained for Ile in RBC, together with true Ile digestibility values for corn and soybean meal. Maximal weight gain and minimal PUN responses to incremental Ile suggested a true digestible Ile requirement estimate of 0.31%, or 0.89 g/Mcal ME for high-lean-gain, late-finishing pigs. The third project evaluated the ideal ratios of Ile and valine (Val) relative to lysine (Lys) in chicks during the second and third week of life. A basal diet deficient in Lys, Ile and Val was formulated utilizing corn-gluten meal (CGM). Six dose levels of each AA were fed in three different experiments. Quadratic responses to each AA occurred when graded levels of digestible Lys (0.68 to 1.28%), Ile (0.45 to 0.95%) or Val (0.51 to 1.06%) were fed. Breakpoint requirement estimates for gain and gain:feed, respectively, were: Lys (0.85, 0.96%), Ile (0.59, 0.58%), and Val (0.74, 0.74%). Using the higher of broken-line requirement estimates for gain and gain:feed, ideal ratios (as a % of Lys) were: Lys (100), Ile (61.6) and Val (77.8). |
