Relationships between energy status and reproductive efficiency in first-calf beef heifers

Study 1. Effects of increasing energy intake at breeding on reproductive performance was investigated with 26 first-calf Angus heifers. In separate but comparable pastures, half (lower energy group) were supplemented with 7 kg corn silage*heifer^-1*d^-1; half (higher energy group) were given 3.5 kg corn grain*heifer^-1*d^-1 in addition to silage. Supplementation period extended from 22 d before to 22 d into breeding. Body weight (BW) and backfat were taken on d 1 and d 44. Blood samples were collected on d 2, 11, 22, 33 and 44 of supplementation. Basal blood samples were analyzed for non-esterified fatty acids (NEFA), progesterone (P₄), luteinizing hormone (LH) and follicle stimulating hormone (FSH). Blood samples collected on d 11 at 0 and 2 h after 10 μg of gonadotropin-releasing hormone (GnRH) were analyzed for LH, FSH and P₄. Postpartum interval (PPI), conception rate, subsequent calving and weaning rates were determined. There were no significant effects of diet on BW, NEFA, P₄, LH, FSH, response to GnRH, reproductive performance, or growth of the current calf. Heifers fed the higher energy diet accumulated more (P < .05) fat (1.5 vs .4 mm). Based on these results increasing energy intake during breeding of heifers fed pasture and 7.0kg*heifer^-1*d^-1 corn silage does not improve and may impair reproductive performance.

Study 2. First-calf Angus heifers (n = 83) were utilized to characterize postpartum nutritional and hormonal status of those that are reproductively efficient or inefficient. A short period (< 17 d) from exposure to the bull to conception (PPI) and delivery of a live calf represented reproductive efficiency. Body condition scores (BCS), BW, BW to hip height (BW:HH) ratio, NEFA, LH, FSH, P₄, E₂ and androstenedione were determined. Hormones released in response to GnRH and estradiol (E₂) were determined. Prolactin (PRL) was determined for indicating stress. Blood collection for hormonal assays was made between 35 d and 63 d postpartum. Heifers with a short PPI had higher (P < .05) BW:HH ratio between 35 d and 63 d postpartum, less (P < .05) fat and change in BW from beginning to 60 d into breeding, higher (P < .05) basal LH, lower (P < .05) FSH, lower (P < .05) androstenedione, lower (P < .01) release of FSH in response to GnRH and higher (P < .05) secretion of PRL. These results suggest that a fat body condition may have contributed to a longer PPI, a first-calf Angus heifer gaining less fat and BW during breeding and having more LH and PRL and less FSH and androstenedione is likely to be reproductively efficient.