Economic values for changes in carcass lean and fat weights at a fixed age for terminal sire breeds of sheep in the UK

Economic values for increases in the weights of carcass lean and fat for crossbred lambs at a fixed age, under UK commercial conditions, were estimated using a bio-economic model. Based around a biological model describing growth in lean and fat tissues over time, the model accounted for variation between lambs in rates of growth and fattening, seasonal fluctuations in market prices, feed requirements and seasonal variation in feed costs. Experimental data for female and castrated male Suffolk cross Mule lambs were used to adapt the model to represent typical commercial conditions. Lambs were sold for slaughter on reaching a chosen fatness level or when their carcass weight exceeded 21 kg. Carcass value was determined from its weight, fatness and conformation. Weekly market prices from six consecutive years were used in turn to investigate the effect of differences in prices between years. The economic values, calculated by comparing profit per animal before and after genetic increases in each trait, varied between sexes and among years. Economic values for lean weight ranged from 1.04 to 1.91£ kg^-1 for females and from 1.16 to 1.93£ kg^-1 for castrates. Values for fat weight ranged from -0.04 to -1.81£ kg^-1 for females and from 0.69 to -2.60£ kg^-1 for castrates. These values are specific to the breed-cross and production system considered. However, the model could be adapted to estimate economic values under other drafting strategies, or with appropriate data, other production systems and other breed-crosses of lambs.