Induction of estrus in ovariectomized cows and heifers: effects of estradiol benzoate and gonadotropin releasing hormone.

Three experiments were conducted with ovariectomized (OVX) cows and heifers to investigate potential neuroendocrine mechanisms controlling estrous behavior. In Exp. 1, 10 OVX cows were treated with either 125 micrograms estradiol benzoate and 10 cc saline (125 micrograms EB + SAL), 125 micrograms EB and 500 micrograms gonadotropin releasing hormone (125 micrograms EB + GnRH), 250 micrograms EB and 10 cc SAL (250 micrograms EB + SAL), 250 micrograms EB and 500 micrograms GnRH (250 micrograms EB + GnRH) or 500 micrograms EB and 10 cc SAL (500 micrograms EB + SAL) in a replicated 5 X 5 Latin-square design. During the 48 h following EB injection, 2-h observation blocks were alternated with 2-h non-observation blocks. During each 2-h observation block, 14 behavioral interactions were monitored. The percentage of cows in estrus was lower for cows receiving 125 micrograms EB as compared with those given the higher doses. However, the cows receiving 125 micrograms EB + SAL did not differ in their estrous response from those receiving 125 micrograms EB + GnRH. The interval from injection to the onset of estrus and the duration of estrus were similar for all treatments. In Exp. 2, 10 OVX heifers were subjected to the same treatments and observation procedures utilized in Exp. 1. The results of Exp. 2 were similar to those of Exp. 1. In Exp. 3, 10 OVX cows were treated with either 300, 600, 1,200, 2,400 or 4,800 micrograms EB in a replicated 5 X 5 Latin-square design.(ABSTRACT TRUNCATED AT 250 WORDS)