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), t ryptophan (Trp), and sulfur amino acids (SAA) to lysine (Lys) determin ed for young pigs to a proposed ratio of these amino acids for finishi ng pigs. Trial 1 involved 20 barrows and 20 gilts that were self-fed i n 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 (L F = 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. Negativ e-control diets in both the EF and LF periods were designed to be slig htly deficient in lysine and to contain digestible Thr (65%), Trp (18% ), and SAA (60%) at the ideal ratio to digestible Lys determined for 1 0- 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. I n Trial 1, increased ratios of Thr, Trp, and SAA improved gain:feed ra tio, whole-body and carcass protein concentration, and whole-body and carcass protein accretion. In Trial 2, LF pigs responded to the increa sed ratios of Thr, Trp, and SAA. with decreased urinary nitrogen excre tion and increased N retention. The results suggest that Thr, Trp, and SAA ratios to Lys used for young pigs are too low for use in finishin g pigs.