Effects of food quality on growth and carcass composition in lambs of two breeds and their cross

The effects of food quality, breed type and sex (ram and ewe) on lamb growth and carcass composition, and their changes throughout growth, were measured. The three breed types were Scottish Blackface (B; no. = 24), Suffolk (S; no. = 28) and their reciprocal crosses (X; no. 33). The lambs had free access to a nutritionally non-limiting food, H, or a bulky food, L. Each lamb was scanned using X-ray computed tomography to measure the weights of fat, lean and bone in the carcass at three degrees of maturity (0·30, 0·45 and 0·65) in live weight. Live weight and food intake data were recorded weekly. Average daily gains in live weight (ADG) and carcass tissues, intake (ADI) and efficiency (EFF = ADG/ADI) were calculated for each lamb between degrees of maturity. Gompertz and Spillman functions were used to investigate relationships between weight and both time and cumulative food intake. There was a breed by food interaction for fat and lean proportions ( P < 0·05). Only on H was there a breed difference ( P < 0·05) with S having less fat and more lean than either B or X, which did not differ from each other ( P > 0·1). On food L there were no breed effects ( P > 0·1). Across breeds, sexes and stages of maturity, food L caused lambs to have 0·810 as much fat and 1·063 as much lean compared with H ( P < 0·001). There were breed by food interactions for ADG ( P < 0·05) and EFF ( P < 0·01). ADG on L was 0·72 of that on H for S, as compared with 0·79 for B and X. EFF on L was 0·463 of that on H for S, as compared with 0·586 for B and X. These were such that S was more sensitive to food effects on growth. The Gompertz and Spillman functions described growth well.