Development and reproductive performance of Hereford heifers of different frame sizes up to mating at 14-15 months of age

ABSTRACT Body development and reproductive performance of a hundred forty-two 14 to 15-month-old heifers, classified at weaning according to frame size as small, medium, and large, were evaluated. The parameters evaluated were: body weight, hip height, body condition score, weight gain, ovarian activity, and pregnancy rate. At weaning, body weight and hip height were significantly different among frame scores, (small – 133.0 kg, 92.2 cm; medium – 158.5 kg, 96.6 cm; and large – 185.2 kg; 100.2 cm). After weaning, heifers grazed together on natural pastures during the autumn and on ryegrass (Lolium multiflorum La.) during the winter and spring. Frame score differences remained until the beginning of the breeding season (BS), starting on average at 14 months of age. Weight gain between weaning and the beginning of BS was not different among frame scores (0.740 kg/day, on average). Body weights at the beginning of the BS were significantly different, of 255.7 kg (53.3% of the mature weight) for small heifers, 285.0 kg (59.4%) for medium heifers, and 307.6 kg (64.1%) for large heifers. Ovarian activity at the beginning of the BS was not different among the three groups. The average weight gain values during the BS of 0.492, 0.472, and 0.421 kg/day for small, medium, and large heifers, respectively, were significantly different. Pregnancy rates were not different among groups (small, 71.4%; medium, 76.4%; and large, 76.5%). Frame score did not influence the reproductive performance of heifers, but the small and medium heifers conceived 29 and 20 days earlier, respectively, than the large heifers.

摘要 本研究对142头14~15月龄的小母牛进行了评估，这些个体在断奶时按体尺框架（frame size）分为小型、中型与大型三组。评估指标包括体重、体高、体况评分（body condition score）、日增重、卵巢活动情况以及受胎率。断奶时，不同体尺评分组的体重与体高均存在显著差异：小型组为133.0 kg、92.2 cm，中型组为158.5 kg、96.6 cm，大型组为185.2 kg、100.2 cm。断奶后，小母牛于秋季共同放牧于天然草场，冬、春季则放牧于多花黑麦草（Lolium multiflorum Lam.）草地。体尺评分的差异持续至配种期（breeding season, BS）初期，该配种期平均始于14月龄。断奶至配种期初期的日增重，在不同体尺评分组间无显著差异（平均为0.740 kg/天）。配种期初期的体重存在显著差异：小型组为255.7 kg（达到成年体重的53.3%），中型组为285.0 kg（59.4%），大型组为307.6 kg（64.1%）。配种期初期的卵巢活动情况在三组间无显著差异。配种期内的平均日增重分别为：小型组0.492 kg/天、中型组0.472 kg/天、大型组0.421 kg/天，组间差异显著。各组的受胎率无显著差异：小型组71.4%、中型组76.4%、大型组76.5%。体尺评分并未对小母牛的繁殖性能产生影响，但小型与中型组小母牛的配种受孕时间分别比大型组提前29天与20天。