1. Weaned single-suckled calves (castrates) were housed at 260·7 kg live weight. Six genotypes, all two- or three-way crossbreds, were represented. A diet of bruised barley, wet distiller's grains, swedes and grass silage was offered until the required minimum live weight (363 kg) and body composition was reached. The cattle were then slaughtered (treatment A), or retained for 8 (treatment B) or 16 weeks (treatment C). During the period when treatment B and C animals were retained their diet was 53% bruised barley and 47% wet distiller's grains offered ad libitum.
2. Live weights at slaughter were 418·2, 458·6 and 496·1 kg (P < 0·001), carcass weights 240·5, 266·2 and 295·7 kg (P < 0·001), and killing-out percentages 57·5, 58·1 and 59·6 (P<0·01) for treatments A, B and C respectively. Daily live-weight gain did not decline during the 8 or 16 weeks cattle were retained on treatments B and C.
3. Retaining the cattle to slaughter points B and C caused increases in the percentage of low-value joints in the carcass, carcass fat percentage, dry matter and fat contents of the dissected m. longissimus dorsi; and decreases in the percentage of high-value joints, carcass lean and bone percentage, crude protein and ash contents of m. longissimus dorsi. The carcass composition was 55·6, 53·5 and 51·4% lean (P<0·001); 30·7, 33·3 and 36·2% fat (P<0·001); 13·7, 13·2 and 12·4% bone (P< 0·001) for treatments A, B and C respectively.
4. The results are discussed in relation to the choice made by beef producers between slaughter and retention of cattle which have reached suitable live weight and condition for marketing.