Milk replacers for preruminant lambs: limiting amino acids in two soybean protein isolates determined with a change-over design

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Abstract
Preruminant male crossbred lambs, aged about 2 weeks at the start of the experiment, were used. Low protein milk replacers (0.10 of total energy as protein) containing isolated soybean protein (ISP-A or ISP-B) as the sole source of protein were supplemented with mixtures of amino acids. Experiment 1a. Six lambs were each given six isonitrogenous diets in a 6 x 6 change-over design with 4-day dietary periods. The diets contained ISP-B and were supplemented with (1) methionine alone, or methionine plus (2) lysine, (3) isoleucine, (4) threonine, (5) valine or (6) tryptophan. Experiment 1b. Groups of six lambs were given diets containing either ISP-A or ISP-B. Each lamb within a group was given six diets in a 6 x 6 change-over design with 4-day dietary periods. One diet was supplemented with six amino acids (Met, Lys, Isol, Thr, Val and Try) whilst the other five diets had one amino acid (not Met) omitted. Nitrogen balances were determined in experiments 1a and 1b on the last 2 days of each dietary period. Experiment 2. Two groups of four lambs were used in a conventional 12-day nitrogen balance experiment. One group was given the milk replacer containing ISP-B supplemented with the six amino acids used in experiment lb, and the other group the same basal diet supplemented only with Met, Lys and Thr. The addition of single amino acids in addition to methionine did not improve nitrogen balances. Lambs given diets supplemented with Met plus Thr, Val or Try had significantly lower nitrogen balances than those given a supplement of Met alone (experiment 1a). Lambs given diets in which Lys or Thr were omitted from the mixture of six amino acids had Lower nitrogen baIances than any other lambs (experiment 1b). It was concluded that Lys and Thr were equally second limiting in ISP-A and ISP-B at the 0.10 protein energy concentration. The retention of apparently digested nitrogen by lambs given ISP-B supplemented with Met, Lys and Thr (0.669) was significantly higher than that of lambs given ISP-B supplemented with six amino acids (0.590; experiment 2). The mean digestibility coefficient of nitrogen in ISP-B (0.842 ± 0.009) was significantly higher than that in ISP-A (0.764 ± 0.014) and the diets containing ISP-B were more readily accepted by the lambs.