The utilization by preruminant lambs of isolated soya bean protein in low protein milk replacers

Abstract

Preruminant male crossbred lambs, aged 2–5 days at the start of the experiment, were used in five experiments with low protein milk replacers (0.10 of total energy as protein). Experiment 1. A diet with isolated soya bean protein (ISP) as the sole source of protein was supplemented with different amounts of DL-methionine. The ISP contained 1.06 g methionine (M) and 1.04 g cystine (C) per 16 g nitrogen (equivalent to 9.1 mg M + C per 100 kJ diet). Maximum nitrogen balance was achieved when the M + C concentration was 15.3 &plusminus; 3.7 mg per 100 kJ (equivalent to 3.8 g M + C per 16 g nitrogen).

Experiment 2. The ISP diet (plus DL-methionine), was supplemented with L-valine, or L-threonine, or L-isoleucine, or L-lysine monohydrochloride, or a mixture of essential amino acids, to raise their concentration in rsp to those in whole egg proteins (g per 16 g nitrogen). There was no significant improvement in nitrogen balance as aresult of adding amino acids other than DL-methionine.

Experiment 3. The ISP diet was supplemented with either DL-methionine (2.4 and 14.8 mg/100 kJ), or L-methionine (2.4 and 1 4 8 mg/100 kJ), or methionine hydroxy analogue (MHA) (14.8 mg/100 kJ), or L-cystine (14 8 mg/100 kJ). Nitrogen balances were determined. It was concluded that the Dand L-isomers of methionine had similar biological activities when used to supplement ISP, and that MHA was as effective as DL-methionine. There was no significant increase in nitrogen balance with the L-cystine supplement.

Experiment 4. The ISP diet (plus DL-methionine) was given to three lambs continuously for 33 days. There was no significant change in nitrogen balance with increasing age.

Experiment 5. The ISP diet (diet A) and a diet of equivalent protein content based on cows' milk proteins (diet B) were supplemented with DL-methionine and given to groups of lambs over a wide range of intakes of gross energy (GE). The maximum intake of GE by lambs given the ISP diet ad libitum was 1147 kJ/day per kg^0.73, an intake far lower than that which would have been taken by lambs given the cows' milk diet ad libitum. The apparent digestibilities of nitrogen and DM in diet A were significantly lower than those in diet B and, at the mean intake of apparently digested nitrogen of 538 mg/day per kg^0.73, there were significant differences in the nitrogen balances: the values were 362 and 255 mg nitrogen/day per for the cows' milk and ISP diets respectively.