Influence of dietary total sulfur amino acids and methionine on growth performance and carcass characteristics of finishing gilts.

We conducted three experiments to determine the sulfur amino acid (SAA) and methionine requirements of finishing gilts. Gilts (PIC Line 326 x C-15, Exp. 1; Line 326 x C-22, Exp. 2 and 3) were blocked by initial weight in randomized complete block designs. In Exp. 1, 64 gilts (initially 54 kg) were fed diets containing either .56 or .44% apparent digestible lysine with increasing SAA levels (63, 70, and 77% of apparent digestible lysine) in a 2 x 3 factorial. A lysine x SAA interaction (P < .10) was observed for ADG and ADFI. Increasing SAA:lysine ratios from 63 to 70% in diets containing .56% apparent digestible lysine increased ADG and ADFI; however, increasing the SAA:lysine ratio in diets containing .44% apparent digestible lysine decreased ADG and ADFI. Pigs fed .56% apparent digestible lysine had higher (P < .05) ADG and gain:feed ratio (G/F) and lower 10th rib fat depth than pigs fed .44% apparent digestible lysine. Increasing the SAA:lysine ratio had no effect on G/F or carcass characteristics. In Exp. 2, 80 gilts (initially 74 kg) were fed diets containing .225, .25, .275, .30, or .325% apparent digestible SAA (45, 50, 55, 60, or 65% of .50% apparent digestible lysine, respectively). Increasing SAA concentrations decreased ADG and G/F (linear, P < .06). In Exp. 3, 105 gilts (initially 72 kg) were fed diets containing .20% apparent digestible cystine and .10, .125, or .15% apparent digestible methionine (20, 25, or 30% of .50% apparent digestible lysine). Increasing digestible methionine increased ADG, ADFI, plasma methionine concentrations (linear, P < .01), and G/F (quadratic, P < .03). The greatest increases in ADG and G/F were observed when apparent digestible methionine was increased from .10 to .125%. Based on these results, the apparent digestible methionine requirement is no greater than 25% of apparent digestible lysine, in diets containing excess cystine. This equates to an apparent digestible SAA:lysine ratio that is no greater than 50%.