AVERAGE DAILY GAIN, BLOOD METABOLITES, AND BODY-COMPOSITION AT FIRST CONCEPTION IN HEREFORD, SENEPOL, AND RECIPROCAL CROSSBRED HEIFERS ON 2LEVELS OF WINTER NUTRITION AND 2 SUMMER GRAZING TREATMENTS

Hereford (n = 48), Senepol (n = 42), and reciprocal crossbred (n = 34) heifers from two consecutive calf crops were stratified by breed, age , and BW to receive bahiagrass (Paspalum notatum) hay offered free cho ice and 150 mg monensin.heifer(-1).d(-1) in addition to either 2.27 kg .heifer(-1).d(-1) of a 75% cracked corn and 25% soybean meal mixture ( CS) or .91 kg.heifer(-1).d(-1) of soybean meal (SBM). Heifers in each treatment were divided into two winter pasture replicates and exposed to fertile bulls. In spring of each year, one-half of the heifers from each winter treatment were allotted to either a continuous (CONT) or rotational (ROTA) grazing system on bahiagrass pastures for the summer phase of the study. Heifers supplemented with CS had higher ADG from the beginning of the study to first conception than heifers fed SBM (. 39 vs .31 +/- .02 kg/d; P < .01). Heifers fed CS were younger at first conception than SBM heifers (500 vs 563 +/- 32 d of age; P < .05) but had similar BW (312 vs 317 +/- 7 kg; P > .10). During the 2-yr study, a subset of Hereford (n = 12), Senepol (n = 15),: and reciprocal cros sbred (n = 14) heifers were subjected to urea space measurements to de termine body composition at first conception. Change in body compositi on over time was analyzed by regression and body composition at first conception was predicted from; these regressions. At first conception, percentage of empty body fat was not affected by treatment or year; h owever, percentage of empty body fat tended to be higher in crossbred than in Hereford and Senepol heifers (16.6 vs 14.3 and 14.4 +/- .94%; P < .10). Stepwise regression of BW, body condition score (BCS), fat t hickness (determined by ultrasound), and body composition at first con ception on age at first conception revealed that BCS and BW accounted for 55% of the variation in age at first conception (P < .01).