The effect of supplemental lysine and threonine on growth and development of yearling horses.

Thirty-nine Thoroughbred and Quarter Horse yearlings were used in two 112-d experiments to determine the effect of lysine and threonine supplementation on growth and development. Yearlings were individually fed three dietary treatments that consisted of a pelleted concentrate containing corn, oats, and soybean meal fed to appetite twice daily and Coastal bermuda grass hay group-fed at a rate of 1 kg/100 kg BW. Three concentrates were tested: (A) basal, (B) basal plus .2% lysine, and (C) basal plus .2% lysine, and .1% threonine. Feed intake, weight, withers height, girth, hip height, body length, and hoof growth (Exp. 1) were recorded every 28 d, and initial and final radiographs taken for estimating bone mineral content. Final croup fat thickness was measured ultrasonically in Exp. 1, and initial and final croup fat measured in Exp. 2. Blood samples were taken every 28 d for determination of serum urea N and protein in Exp. 2. Average daily feed intake (as-fed) was 8.8 +/- .14, 9.0 +/- .13, and 9.2 +/- .13 kg (P < .09), ADG was .57 +/- .02, .64 +/- .02, and .67 +/- .02 kg/d (P < .02), and girth gain was 9.7 +/- .49, 10.1 +/- .46, and 11.3 +/- .47 cm (P < .05) for Treatments A, B, and C, respectively. Gain:feed ratios in Exp. 1 were 70.5, 70.8, and 75.5 g/kg (P > .10) and in Exp. 2 were 61.7, 70.8, and 70.2 g/kg (P < .10) for Treatments A, B, and C, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)