Optimum ratio to lysine of threonine, tryptophan, and sulfur amino acids for finishing swine.

Forty-eight crossbred (PIC line 26 x Camborough 15) pigs were used in two finishing trials to compare the ideal ratios of threonine (Thr), tryptophan (Trp), and sulfur amino acids (SAA) to lysine (Lys) determined for young pigs to a proposed ratio of these amino acids for finishing pigs. Trial 1 involved 20 barrows and 20 gilts that were self-fed in sex groups of two. Trial 2 was a Latin square design that used four barrows and four gilts that were individually fed in metabolism cages. Separate diets were used for the early (EF = 56 to 90 kg) and late (LF = 90 to 112 kg) finishing periods. Diets were formulated from a corn-soybean meal mixture and contained 11% CP and .55% digestible lysine for EF pigs and 10% CP and .50% digestible lysine for LF pigs. Negative-control diets in both the EF and LF periods were designed to be slightly deficient in lysine and to contain digestible Thr (65%), Trp (18%), and SAA (60%) at the ideal ratio to digestible Lys determined for 10- to 20-kg pigs. The experimental diet in both the EF and LF periods was formulated to contain digestible Thr (70%), Trp (20%), and SAA (65%) at the proposed ideal ratio to digestible Lys for finishing pigs. In Trial 1, increased ratios of Thr, Trp, and SAA improved gain:feed ratio, whole-body and carcass protein concentration, and whole-body and carcass protein accretion. In Trial 2, LF pigs responded to the increased ratios of Thr, Trp, and SAA with decreased urinary nitrogen excretion and increased N retention.(ABSTRACT TRUNCATED AT 250 WORDS)