Eighty castrated male lambs were allocated to either an initial slaughter group (no. = 16) or one of eight treatment groups (no. = 8). Ammoniated barley straw was offered ad libitum. This was supplemented with sodium (Na) propionate (0 or 20 g/day), formaldehyde-treated casein, (protected casein, 0 or 50 g/day) and/or calcium (Ca) salts of long-chain fatty acids (Ca soap, 0 or 30 g/day), as a factorial design. Measurements were made of food intake, live-weight gain and wool growth, rumen fluid parameters and changes in body composition over a period of 155 days.

Food intake (g dry matter per kg live weight) was lower in the groups supplemented with Ca soap and Na propionate. Fleece-free live-weight gain was increased by supplementation with protected casein and Ca soap, but was not affected by Na propionate. Supplementation with protected casein increased wool yield. Adjusted means for water, protein and ash content of the body (adjusted to mean fleece/digesta-free body weight at slaughter of 27·1 kg) were affected significantly by the second-order interaction between the supplements. Adjusted body water and protein mass were lower when Na propionate was given alone or with Ca soap and protected casein. Fat mass tended to be higher under those conditions. Supplementation with Ca soap increased adjusted fat and gross energy contents and decreased adjusted protein and water mass. Supplementation with protected casein decreased total body fat and increased adjusted protein mass in the body. The partitioning of water, protein, fat, ash and energy between the carcass and non-carcass residues was affected to a small extent also. These results demonstrate interactive effects of the supplements used, on nutrient partitioning in lambs offered straw-based diets.