Controlling First Service and Calving Interval by Prostaglandin F2α, Gonadotropin-Releasing Hormone, and Timed Insemination

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Abstract
Our objective was to determine if calving intervals could be shortened and made less variable by using postaglandin F2.alpha. to control the occurrence of first services. Holstein cows (n=348) were assigned at calving to four treatment groups. Control cows (n=88) were inseminated at their first observed estrus after 40 d postpartum. Estrous cycles of the remaining cows were synchronized with prostaglandin F2.alpha. to allow insemination (first services) 80 h after the second injection (n=86), insemination at 80 h preceded by gonadotropin-releasing hormone at 72 h (n=86), or insemination at 72 and 96 h (n=88) after the second injection (51 to 57 d postpartum). By design, interval to first service was reduced to 57 d for treated cows (63 d for controls) and was less variable (12% of that for controls). Conception rate at first service was lower after timed inseminations than that of controls. Intervals to conception and subsequent calving were similar in all treated cows and controls. Of cows sampled, 23 of 176 (13%) failed to respond with luteolysis when progesterone in serum exceeded 1 ng/ml, and 26 of 176 (15%) had low concentrations of progesterone in serum and could not respond to prostaglandin F2.alpha.. Poor response to timed inseminations may have occurred because only 72% of 176 cows sampled responded with luteolysis. When only cows observed in estrus were considered, conception rate approached that of controls (51%). Methods of timed insemination rates prevented a true test of the utility of estrous synchronization to reduce duration and variability of calving intervals, even though variability of intervals to first service was reduced.