| Four experiments were conducted to investigate the effects of ractopamine on amino acid (AA) requirements for finishing pigs. The specific AA that were investigated were lysine and threonine. Exp. 1 investigated the effects of increasing lysine concentrations for barrows fed ractopamine diets during the late finishing phase in order to estimate the appropriate requirement for optimal growth response. Exp. 2, 3, and 4 investigated the effects of increasing the threonine to lysine ratio of diets with ractopamine on growth, carcass data, and plasma urea N concentrations and estimated the optimal threonine to lysine ratio for maximum growth rate. All experiments used individually fed barrows and corn-soybean meal based diets. In Exp. 1, 36 pigs were allotted to 6 dietary treatments with 0 or 10 ppm ractopamine and increasing levels of lysine. In Exp. 2 and 3, 63 pigs were allotted to 7 dietary treatments with 0 or 10 ppm ractopamine, different lysine, and increasing threonine concentrations. For Exp. 4, 35 pigs were allotted to similar diets as Exp. 2 and 3 that were limit fed 3 kg/d. In Exp. 1, a total lysine intake of 24.7 g/d was adequate for obtaining maximal growth for barrows fed ractopamine. In Exp. 2, 3, and 4, there were effects on intakes and growth rates due to increasing threonine to lysine ratios (P < 0.05). It was estimated from Exp. 4 results that 68% threonine to lysine concentration, on a standardized ileal digestible basis would result in maximum growth rate for pigs consuming diets containing ractopamine. |
