A growth trial using 240 growing-finishing pigs (22 to 109 kg) was conducted to determine whether pigs offered a choice of low- and high-lysine sorghum-soybean meal diets can select the correct proportion of each to optimize performance and carcass leanness. Pigs on the choice treatments could select from two diets offered simultaneously in identical feeders. In two of the choice treatments, pigs had access only to the low-lysine diet for the first 2 d of each 21-d period to determine whether forced adaptation to the low-lysine diet would alter the proportion of diets selected. These were termed the adjusted treatments. The choice treatments were 1) .50 or 1.10% lysine, 2) .50 or 1.60% lysine, 3) same as 1 but adjusted, and 4) same as 2 but adjusted. Five additional treatments were arranged as a titration study to determine the lysine requirement of the pigs. The dietary lysine levels needed in sorghum-soybean meal diets to optimize performance and carcass leanness of barrows and gilts were .95, .80, and .70% lysine for the 22 to 52, 52 to 78, and 78 to 109 kg weight intervals, respectively. Lysine intake of pigs on the choice treatments exceeded the amounts needed to maximize performance and carcass leanness. The adjustment practice lowered lysine intake, but intake was still excessive. Percentage of lean and gain/feed were less desirable for the choice treatments than for pigs fed the .95-.80-.70% lysine treatment. These reductions, plus the higher lysine intakes, indicate that the choice treatments used in this research are not feasible for commercial swine production.

Growth trials with starter (n = 120, 6.8 kg initially, 28 d of age, Exp. 1) and finisher (n = 70, 59 kg initially, Exp. 2) pigs were conducted to compare quality protein maize (QPM, .40% lysine) and normal corn (.31% lysine) in simple corn-based diets containing the same levels of soybean meal. In Exp. 1, pig performance was similar (P greater than .10) on all diets, regardless of the level of soybean meal, suggesting that QPM and normal corn have similar feeding value in lysine-adequate (.99 to 1.11%) diets. In Exp. 2, less soybean meal was needed in QPM than in normal corn diets to maximize performance; increasing soybean meal from 10.8 to 13.8% improved rate (P less than .05) and efficiency (P less than .01) of gain of pigs fed normal corn diets but had no effect on performance of pigs fed QPM diets. A QPM-based diet containing 6% soybean meal and supplemental lysine and tryptophan failed to maximize feed efficiency, but growth rate was equal to that obtained on the normal corn diet with 13.8% soybean meal. The apparent fecal digestibility of GE and ileal digestibility of N were similar for QPM and normal corn, but apparent ileal digestibility of most essential amino acids was slightly higher for QPM (Exp. 3). Experiment 4 compared apparent digestibilities of QPM, conventional opaque-2 corn and two high-protein corns. Digestibilities differed (P less than .05) among the corns, but the absolute differences were small and were likely due to differences in amino acid content of the corns.(ABSTRACT TRUNCATED AT 250 WORDS)