Milk replacers containing isolated soybean protein for preruminant lambs: influence of experimental design on estimates of requirements for supplementary methionine

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Abstract
Preruminant male crossbred lambs, aged 2–5 days at the start of the experiment, were used in two experiments with low protein milk replacers (0.10 of total energy as protein) containing isolated soybean protein as the sole source of protein. Experiment 1. Groups of three lambs were given the soybean diet supplemented with DL-methionine (seven levels) in increasing (experiment la) or decreasing amounts (experiment 1b). Dietary periods were each of 3 days. Estimates of minimum intakes of methionine plus cystine, coincident with maximum animal response, based on a 1 day nitrogen balance or on plasma urea nitrogen concentrations, indicated that there was a carry-over effect when methionine was given in decreasing amounts. Experiment 2. An 8 x 8 change-over design was used to minimize carry-over effects. Each lamb was given each diet (seven levels of methionine) for 4 days, in such an order that in the final design each diet was preceded once by every other diet. No carry-over effects were observed. It was concluded that the change-over design could be used to estimate amino acid requirements with fewer animals and with shorter dietary periods than in the conventional balance experiment.