EFFECTS OF PRENATAL ANDROGENIZATION, MELENGESTROL ACETATE, AND SYNOVEX-H(R) ON FEEDLOT PERFORMANCE, CARCASS, AND SENSORY TRAITS OF ONCE-CALVED HEIFERS

Three studies were designed using a 2 x 2 factorial arrangement of tre atments within each experiment to evaluate the effects of various mana gement techniques on growth and carcass traits of once-calved heifers. In Exp. 1, 33 yearling (16 control [C] and 17 prenatally androgenized [PA]) and 39 once-calved (20 C and 19 PA) nonpregnant heifers were us ed to evaluate the effects of age and PA. Once-calved heifers were app roximately 31 kg heavier (P < .03) on-test, initially consumed 11% mor e (P < .01) DM, and gained 11% faster (P < .08), but had similar (P = .44) gain to feed ratios (.138 vs .131) as yearlings. Growth performan ce was not affected by PA. The yearling and once-calved heifers were t hen slaughtered at approximately 707 and 1,080 d of age, respectively. Despite maturity differences (P < .01), marbling scores (P = .26) and percentage of choice carcasses (P = .27) were similar. However, on th e basis of newly accepted grading changes, percentage of once-calved h eifer carcasses grading choice decreased from 57.3 to 34.4%. Organolep tic properties did not differ. Experiments 2 and 3 were then conducted to evaluate effects of commercially available products (melengestrol acetate [MGA] and Synovex-H(R)) on feedlot characteristics of heifers following weaning of their first calf. In Exp. 2, 48 once-calved Simme ntal-based heifers were fed during the summer to a constant compositio nal fat thickness of .8 cm. Experiment 3 was conducted in the late fal l and early winter, during which 52 moderately sized heifers were fed for 116 d postweaning and slaughtered. Performance of heifers in Exp. 2 was not affected by MCA. However, heifers fed MGA in Exp. 3 gained 9 % faster (P < .05) than controls. In Exp. 2, Synovex-H tended to impro ve ADG (P < .09) and feed efficiency (P < .07), but did not affect per formance in Exp. 3. Most carcass traits were not affected by either tr eatment. Nonpregnant, once-calved heifers seem to have a large capacit y for efficient weight gain as compared with heifers. However, hormona l compounds, testosterone propionate provided pre- or postnatally (Syn ovex-H), and MGA do not appear to provide the consistent, positive res ponses often seen with younger, more typical feedlot heifers.