EFFECTS OF EXPOSURE TO BULLS AND BODY WEIGHT ON REPRODUCTIVE PERFORMANCE IN BEEF HEIFERS

Calving records of 174 2-yr-old heifers from four breed groups; Hereford (HE), Beef Synthetic (SY), Dairy Synthetic (SD) and Crossbred Hereford (HX) were used in this study. Half of the heifers were exposed to three epididymectomized bulls for 6 wk before the start of the breeding season. While exposure to bulls had little influence (P = 0.51) on pregnancy rate, it advanced the calving date by 5.5 days compared with the control group (P < 0.05). Calving season in the exposed heifers was 2 wk shorter than that in the control group. Effect of heifer body weight at the start of breeding season on pregnancy rate differed (P < 0.05) among the four breed groups. Low body weight at mating was associated with lowered fertility in SY, SD and HE while very heavy body weight resulted in lowered fertility in SY and SD breed groups. Increase in body weight advanced calving date in the HX breed group only. Key words: Beef cattle, heifer reproduction, bull exposure, body weight