Doctoral Dissertations

Date of Award

8-1993

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Animal Science

Major Professor

Karl M. Barth

Committee Members

Bert H. Erickson, James K. Miller, John T. Smith, Fred L. Allen

Abstract

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.

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