Sequence of limiting amino acids in groundnut protein isolate for the preruminant lamb

Abstract

Preruminant male crossbred lambs, aged 2–5 days at the start of the experiment, were used in five experiments to determine the sequence of limiting amino acids in milk replacers containing groundnut protein isolate (GPI) as the sole source of protein. Experiment 1. Eight lambs were each given eight diets in an 8 x 8 change-over design. The diets contained GPI, unsupplemented with amino acids, to provide from 0.10 to 0.45 of the total energy as protein. Experiment 2. Eight lambs were used in two 4 x 4 change-over designs. The protein energy concentration of the diets after supplementation with amino acids was 0.22. In both experimental designs the GPI was supplemented with Met, Lys and Thr as a group in one diet. In experiment 2a each of these amino acids was added singly in successive diets, while in experiment 26 one of the amino acids was omitted in turn from the mixture. Experiment 3. Six lambs were each given six diets based on GPI, supplemented with Met and Lys, at six levels of protein energy (0,100-0.225) in a 6 x 6 change-over design. Experiment 4. Eight lambs were used in an 8 x 8 change-over design. The protein energy concentration of the eight diets was 0.12 after supplementation with a group of amino acids that included Met, Lys, Thr, Val, Leu, Isol, Try and Phe. In successive diets Thr, Val, Leu, Isol, Try and Phe were omitted singly and, in the final diet, these six amino acids were omitted as a group. In experiments 1-4 the dietary periods were of 3 or 4 days. Nitrogen balances were estimated and plasma urea nitrogen concentrations (PUN) measured on the last day of each dietary period. Experiment 5. Six lambs, in two equal groups, were used in a conventional balance experiment, with a 5-day preliminary and a 6 day collection period. One group was given an unsupplemented low protein diet based on GPI, the other group had a similar diet supplemented with Met, Lys and Thr. The protein energy concentration in both diets was 0.10. The sequence of limiting amino acids in GPI (based on a 1 day nitrogen balance) was Met, Lys (equally first limiting at a protein energy concentration of 0.22), Thr, Try (equally third limiting at a protein energy concentration of 0.12). A 1-day nitrogen balance (or urinary urea nitrogen excretion in experiment 4) was a more sensitive indicator of the protein quality of a diet than PUN concentrations. The 1 day collection period gave a similar estimate of nitrogen balance to that determined in the conventional 6 day balance experiment.