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Induction of Estrus in Ovariectomized Cows and Heifers: Effects of Estradiol Benzoate and Gonadotropin Releasing Hormone^1,2,3,

Purdue University, West Lafayette, IN 47907

Abstract

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 µg estradiol benzoate and 10 cc saline (125 µg EB + SAL), 125 µg EB and 500 µg gonadotropin releasing hormone (125 µg EB + GnRH), 250 µg EB and 10 cc SAL (250 µg EB + SAL), 250 µg EB and 500 µg GnRH (250 µg EB + GnRH) or 500 µg EB and 10 cc SAL (500 µg 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 µg EB as compared with those given the higher doses. However, the cows receiving 125 µg EB + SAL did not differ in their estrous response from those receiving 125 µg 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 µg EB in a replicated 5 x 5 Latin-square design. The cows were observed for behavioral interactions following injection of EB as described in Exp. 1 and 2. The 300-µg EB treatment induced estrus in a lower percentage of cows. The interval from injection to the onset of estrus was similar for all treatments. However, the duration of estrus was longer for cows receiving 4,800 µg EB. Based on these results, we contend that in the OVX cow and heifer, GnRH does not potentiate the actions of estradiol when sub-optimal dosages of EB are administered. In addition, the neuroendocrine mechanisms responsible for estrus in the cow do not display a linear dose-response relationship with exogenous estradiol.

¹ Journal paper No. 10,512, Purdue Univ., Agr. Exp. Sta.

² Research was supported in part by USDA Grant No. 84-CRSR-2-2420.

³ The authors express appreciation to Sign'e Beebe, Joni Bruns, Melissa Haymaker and Jane Wortman for their invaluable technical expertise and to Dr. M. D. Brown (Ceva Lab., Inc.) for the gift of GnRH.

⁴ Dept. of Anim. Sci.

⁵ Dept. of Large Anim. Clinics.

⁶ Address reprint requests to this author.