PERFORMANCE AND CARCASS COMPOSITION OF LARGE WHITE TOMS AS AFFECTED BY DIETARY CRUDE PROTEIN AND THREONINE SUPPLEMENTS

This experiment evaluates the effects of decreasing dietary CP, in add ition to the effects of dietary supplements of L-Thr to low CP diets, in Large White Nicholas toms from 0 to 18 wk of age. Toms were fed die tary treatments consisting of four levels of dietary CP as a percentag e of NRC (1994) recommendations (100, 92, 84, and 76% of NRC recommend ations). Additional treatments consisted of supplements of L-Thr (0.1 and 0.2% of diet) added to the 92 and 84% NRC CP treatments. All eight dietary treatments were formulated to meet a minimum of 105% of NRC ( 1994) recommendations for Met, TSAA, Lys, Thr, and Trp. Body weight, f eed conversion, mortality, and carcass composition responses were meas ured. Decreasing CP to 84% of NRC resulted in 18-wk BW lower than that (P less than or equal to 0.001) of toms fed diets containing 100 or 9 2% of NRC CP; however, toms fed 84% of NRC CP diet supplemented with 0 .1% L-Thr had 18-wk BW equal to (P less than or equal to 0.001) that o f the 100 and 92% NRC CP treatments. Toms fed diets containing 76% of NRC CP had depressed BW and feed:gain in comparison to all other treat ments. No adverse effects in cumulative feed:gain (0 to 18 wk) were no ted by decreasing CP from 100 to 84% of the NRC recommendations. Morta lity did not differ among treatments. Treatments had no effect on carc ass fat expressed as a percentage of hot carcass weight. Breast meat y ield (deboned Pectoralis major and Pectoralis minor) was highest (P le ss than or equal to 0.001) in toms fed the 100 and 92% NRC CP treatmen ts. The 84 and 76% NRC CP treatments resulted in decreased breast meat yield regardless of L-Thr supplements. These results indicate that di ets containing Met, TSAA, Lys, Thr, and Trp at a minimum of 105% NRC r ecommendations may support favorable breast meat yield when CP is decr eased to 92% of the NRC (1994) recommendation. If growth and feed conv ersion are the desirable traits, rather than breast meat yield, CP lev els below 92% of the NRC (1994) recommendation may support favorable r esponses.