Influence of Day of Estrous Cycle at Treatment on Response to Estrous Cycle Regulation by Norethandrolone Implants and Estradiol Valerate Injections

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Abstract
Two experiments were conducted to explore the effect of day of initiation of treatment on estrous cycle regulation by subcutaneous implants of norethandrolone and injection of estradiol valerate (EV). In Experiment I, 24 dairy heifers were assigned in lots of four to be implanted with 250 mg norethandrolone and injected with 5 mg EV on one of 6 days in the estrous cycle; 2, 5, 8, 11, 14 or 17 days after an estrus. The implants were removed 9 days later and the average intervals to estrus were, respectively: 6.6, 5.2, 7.5, 2.1, 3.0 and 2.2. Those implanted prior to 10 days postestrus had longer (P < .01) return intervals than did those implanted after 10 days postestrus. In Experiment II, 35 Holstein heifers were allotted to six groups: (1) implanted 2 days postestrus, injected with 5 mg EV, implant removed 9 days later; (2) implanted 14 days postestrus, injected with 5 mg EV, implant removed 9 days later; (3) implanted 2 days postestrus, no EV, implant removed 9 days later; (4) implanted 14 days postestrus, no EV, implant removed 9 days later; (5) implanted 2 days postestrus, no EV, implant removed 16 days later; and (6) implanted 14 days postestrus, no EV, implant removed 16 days later. The numbers in estrus between 1 and 5 days after implant removal were two of six, five of five, none of six, five of six, six of six and five of six. In comparing those implanted for 9 days, more were in heat when implanted 14 days postestrus than 2 days postestrus (P < .05) and in those injected with EV than those not injected (P < .05). The interaction was not significant. In comparing 9- and 16-day implant periods without EV, more were in heat when implanted for 16 days than nine days (P < .01) and there was an interaction (P < .01) of implant insertion day (2 or 14 days postestrus) and implant period (9 or 16 days) caused by a failure of any heifers implanted for 9 days beginning 2 days postestrus, without EV, to show estrus within 5 days, while nearly all heifers in the other three groups not injected with EV did so. The numbers of heifers conceiving to first service ranged from three of six to four of five for the first five groups, but none of six conceived when implanted 14 days postestrus for 16 days. Copyright © 1974. American Society of Animal Science. Copyright 1974 by American Society of Animal Science.

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