PHYSIOLOGICAL AND HORMONAL RESPONSES OF LAMBS REPEATEDLY IMPLANTED WITH ZERANOL AND PROVIDED 2 LEVELS OF FEED-INTAKE

Forty crossbred wethers (average weight 30 kg) were implanted with zer anol (12 mg) at 30-d intervals and fed at two levels of intake in a 2 x 2 factorial arrangement of treatments to determine performance, carc ass and bone characteristics, blood metabolites, and hormones. Restric ted lambs were fed to gain one-half the BW gained by lambs with ad lib itum feed access. Lambs with ad libitum and restricted access to feed were slaughtered after 98 and 154 d, respectively. Zeranol increased A DG (P = .047; 20%), gain to feed (P = .023; 17%), metacarpal length (P = .004; 6%) and weight (P = .013; 13%), and tended to increase carcas s crude protein gain (P = .106; 63%) while reducing kidney pelvic fat (P = .001; 33%) and dressing percentage (P = .038; 3%). Restricted fee d intake increased the percentage of carcass ash and metacarpal length and weight by 27% (P = .048), 5% (P = .006), and 10% (P = .045), resp ectively, while reducing quality grade scores (P = .022; 5%), gain to feed (P = .001; 49%), longissimus muscle area (P = .001;, 28%), the pe rcentage of kidney pelvic fat (P = .033; 13%), and daily fat gain (P = .001; 54%). Zeranol increased pituitary weight (P = .001; 166%), plas ma glucose (P = .036; 13%), mean serum growth hormone (GH; P = .011; 5 2%), baseline GH (P = .048; 34%), GH pulse amplitude (P = .003; 59%), and IGF-I (P = .001; 53%) concentrations. The results indicate that co ntinuous administration of zeranol from 60 d of age to slaughter incre ases GH release, which directs nutrient utilization such that a carcas s with more desirable lean and fat deposition patterns is obtained whe n nutrient availability is adequate.