EFFECTS OF A BETA-ADRENERGIC AGONIST (L-644,969) AND MALE SEX CONDITION ON MUSCLE GROWTH AND MEAT QUALITY OF CALLIPYGE LAMBS

The objective of this study was to determine the effects of dietary ad ministration of a beta-adrenergic agonist (BAA; L-644,969) and male se x condition (ram vs wether) on muscle growth and meat quality of Dorse t x Romonov lambs believed to be heterozygous for the callipyge gene. At approximately 17 wk of age, lambs were blocked by weight within eac h sex condition and randomly assigned to BAA treatment group. The inte raction of BAA and male sex condition was not significant for any of t he traits measured. Rams had greater initial and final live weights, a verage daily gain, and hot carcass weight (P < .01). Rams did not diff er (P > .05) from wethers with respect to any of the carcass traits, p ossibly because the wethers were so lean and heavily-muscled that ther e was little room for improvement. Kidney-pelvic fat weight was reduce d 26% by BAA (P < .05). Knife separable lean weight and whole carcass proximate composition were not affected (P > .05) by BAA or male sex c ondition. Administration of BAA increased calpastatin activity at 20 d (1.1 vs 1.5 units/g), but not at 0 h (3.9 vs 4.8 units/g) postmortem, decreased myofibril fragmentation index (60.7 vs 44.9), and increased shear force (8.2 vs 10.9 kg) at 20 d postmortem (P < .05). These data suggest that muscle growth rates are near maximum in lambs expressing the callipyge gene, regardless of male sex condition or BAA treatment . Therefore, it seems that the callipyge gene exerts most, but not all , of its effect through intracellular events similar to those initiate d by administering BAA.