Effect of Gonadotropin-Releasing Hormone and Calf Removal on Pituitary-Ovarian Function and Reproductive Performance in Postpartum Beef Cows2

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Abstract
Twenty-four postpartum cows were used to determine the effects of suckling and exogenous gonadotropin-releasing hormone (GnRH) on pituitary-ovarian function and reproductive performance. Twelve cows were suckled, and calves were removed from the other 12 within 24 hr after birth. Six cows in the suckled group (S) and six in the nonsuckled group (NS) were injected with 250 µg GnRH on day 5 postpartum (parturition = day 0). The remaining animals in each group were injected with saline. Pituitary response to GnRH was estimated by measuring serum luteinizing hormone (LH) concentrations in blood collected hourly beginning 2 hr before and continuing until 9 hr after injection. Ovarian structures were observed by laparoscopy on days 5 and 8 postpartum. Injection of GnRH caused an increase (P<.05) in circulating LH concentrations in all animals. LH concentrations were highest 2 hr after injection. The area under the LH response curve was greater (P<.05) for NS than for S cows. Average daily LH concentrations during the first 25 days postpartum were higher in NS (2.4 ± .4 ng/ml) than in S cows (1.5 ± .3 ng/ml) because no ovulations occurred in the S group during this period. Ovaries of NS cows had greater (P<.01) follicular volume on day 5 than did ovaries of S cows (1,194 mm3 vs 195 mm3) because of a greater number of medium and large follicles, but estrogen concentrations were not significantly different on day 5 (NS = 6.7 ± .9 pg/ml; vs S= 7.4 + 1.8 pg/ml). On day 8, follicular volume was increased in GnRH cows and S cows (GnRH = 2,338 mm3 vs no GnRH = 588 mm3 and NS = 2290 mm3 vs S = 637 mm3). Estrogen concentrations on day 8 were higher in S + GnRH cows (9.3 ± 2.1 pg/ml) than in S + saline (4.9 ± .6 pg/ml), NS + saline (5.7 ± .9 pg/ml) and NS + GnRH (6.3 ± .7 pg/ml) cows. Day to day repeatability of estrogen concentrations within animals, within treatment groups was estimated by intraclass (intracow) correlations. Correlations of estrogen values determined from days 6 to 25 were .38, .69, .56 and −.03 for the S + saline, S + GnRH, NS + saline and NS + GnRH groups, respectively. Correlation coefficients between LH and estrogen were .06, .08, .63 and .36 for the NS + GnRH, NS + saline, S + GnRH and S + saline groups, respectively. Intervals from parturition to first estrus and first ovulation were shorter (P<.01) for NS cows (14.9 and 9.6 days) than for S cows (57.6 and 36.8 days). Progesterone concentrations after first ovulations indicated the occurrence of short luteal phases in all NS cows. They had shorter intervals from calving to conception than S cows but required more inseminations per conception. GnRH treatment of NS cows had no significant effect on any intervals. However, five of the six S + GnRH-treated cows had shorter (P<.05) intervals to conception than dis S + saline-treated cows. It was concluded that calf removal hastens reestablishment of postpartum reproductive activity by eliminating a suppressive effect on pituitary gonadotropin release caused by suckling, which in turn causes hastened ovarian follicular development. Copyright © 1980. American Society of Animal Science . Copyright 1980 by American Society of Animal Science.