PROTEIN, ACETATE AND PROPIONATE FOR ROUGHAGE-FED LAMBS .1. BODY AND BLOOD COMPOSITION

Sixty castrated male lambs (ca. 37 kg) were allocated to an initial sl aughter group (no. = 12) or to one of six treatment groups (no. = 8). Chopped oaten hay (sprayed with 10 g urea per kg) was offered ad libit um with 57 g/day chopped lucerne hay. Additional supplements were 0 or 57 g/day formaldehyde-treated casein (protected casein) and volatile fatty acids (VFAs) at ca. 1.45 MJ gross energy per day. The VFAs given were acetate or propionate or a mixture of these (molar ratio 4 : 1). Daily food intake and weekly live-weight (LW) gain were measured and rumen fluid and blood were collected. The lambs were shorn, slaughtere d and body composition was determined. Food intake (g/kg LW) was not a ffected by treatments. Supplementation with protected casein increased LW gain and wool growth. Supplementation with propionate reduced LW g ain, but not when given with protected casein. Plasma concentrations o f urea-nitrogen and insulin were increased, and plasma somatotropin de creased in lambs given protected casein. Final body content of water, fat, protein and gross energy, adjusted to the mean fleece/digesta-fre e body weight of 36.5 kg, was not affected by the treatments. The part itioning of water, fat and energy between the carcass and the rest of the body was affected by the treatments. Excretion of VFAs in urine wa s measured in two sheep in experiment 2, and was negligible compared w ith the quantity of VFAs given. It is concluded that energy from salts of VFAs, in particular propionate, is used inefficiently for body-wei ght gain in lambs given low-protein roughage diets. The nutritional tr eatments had only marginal effects on the composition of body gain.