Effects of metaproterenol, a beta-adrenergic agonist, on feedlot performance and body composition of two fat-tailed breeds of sheep

Abstract

Metaproterenol (MP) was injected subcutaneously at 0, 5 (low dose) and 10 (high dose) μg kg −1 W 0.75. Most feedlot parameters were not influenced by MP treatment. Low dose of MP increased weight of cold carcass excluding the fat-tail ( P < 0.05). Carcass moisture, ash, fat and protein percentages were not affected by MP. Mean weight of chemically determined carcass fat was higher in low dose than control lambs ( P < 0.05). Total carcass protein was higher for lambs receiving the low dose of MP for 100 days. Both low ( P < 0.05) and high ( P < 0.01) doses of MP resulted in lower dissected empty carcass fats, if MP was used for 120 days. MP did not affect Longissimus dorsi (LD) area and depth, but LD width was greater at higher dose of MP. The low dose of MP resulted in a greater subcutaneous fat (SCF) depth in Ghezel lambs at 8th rib, whereas the high dose decreased SCF in depth at 12th rib in Mehraban (breed X treatment interaction). A breed X day X treatment interaction was noted for the effect of MP on heart weight but the weights of other internal organs were unaffected by MP treatment. Weight, volume and length of fat-tail was decreased when low dose of MP was used for 100 days; however, fat-tail width was increased at high dose ( P < 0.05). Treatment of lambs for 80 days with low dose of MP decreased weight of carcass bone; however, treatment for longer periods increased amount of bone in the carcass ( P < 0.05). Hap and total carcass meat was increased in lambs receiving low dose of MP ( P < 0.05). Results indicated that the beta-adrenergic agonist MP, affected body composition of fat-tailed sheep in a manner which was dependent on breed, dose and period of treatment, as has been reported for other betaagonists in thin-tailed sheep. MP decreased fat-tail weight but increased carcass fat and protein; however, percent decrease in fat-tail weight was greater than increase in carcass fat. Further experiments are needed to determine the most suitable beta-agonist for use in fat-tailed sheep.