GnRH-induced ovulation and luteinizing hormone release in beef heifers: Effect of day of the cycle

¹ Division of Animal Sciences, University of Missouri, Columbia, MO 65211

^* To whom correspondence should be addressed. E-mail: Smithmf{at}missouri.edu.

	Abstract

The Co-Synch protocol has been used to synchronize ovulation and facilitate fixed-time AI in beef cattle. Establishment and maintenance of pregnancy was negatively affected, in previous studies, by GnRH-induced ovulation of small dominant follicles ( 11 mm). The reason for the presence of small follicles at the second GnRH (GnRH 2) is not clear. The objectives of this study were: 1) to determine the effect of ovulatory response at the first GnRH (GnRH 1) on diameter and variation in diameter of the largest follicle at GnRH 2; and 2) to determine the effect of day of the cycle (stage of a follicular wave) on GnRH-induced luteinizing hormone (LH) release, and the resulting ovulatory response after GnRH 1 and 2. Two experiments used pubertal beef heifers synchronized to be on different days of the estrous cycle (d 2, 5, 10, 15, and 18 after estrus) in which a dominant follicle would or would not respond to GnRH 1. Ovulatory response to GnRH 1 did not affect size or variation in diameter of the largest follicle at GnRH 2 in Exp.1 or 2. In Exp. 1, ovulatory response after GnRH 1 (0/14^a, 12/13^b, 4/13^ac, 9/13^bc, and 2/10^a in the d 2, 5, 10, 15, and 18 groups; ^abcP < 0.05) and GnRH 2 (13/14^a, 12/13^a, 12/13^a, 2/13^b, and 2/10b in the d 2, 5, 10, 15, and 18 groups, respectively; ^abP < 0.05) was affected by day of the cycle. In Exp. 2, day of the cycle also affected the proportion of heifers ovulating after GnRH 1 (0/7^a, 8/8^b, 0/6^a 5/8^ab, and 5/8^ab of the d 2, 5, 10, 15, and 18 heifers, respectively; ^abcP < 0.05) and GnRH 2 (3/7^ab, 8/8^b, 5/6^b, 1/8^a, and 2/8^a of the d 2, 5, 10, 15, and 18 heifers, respectively; ^abP < 0.05). In both experiments, heifers receiving GnRH 1 on d 15 and 18 had a greater (P < .05) occurrence of luteolysis before PG injection and expression of estrus than heifers treated on d 2, 5, and 10. The GnRH-induced LH surge was of greatest magnitude in heifers receiving GnRH 1 on d 18 of the cycle followed by d 5, 15, 10, and 2 (9,054^b, 5,774^bc, 4,672^c, 2,548^c, and 915^d arbitrary units; respectively; ^abcdP < 0.05). In summary, ovulatory response to GnRH 1 did not affect size of the dominant follicle at GnRH 2. Day of the cycle when GnRH 1 was delivered affected dominant follicle size at GnRH 2. Treatment with GnRH 1 in the earlier part of the estrous cycle (on or before d 10) increased the proportion of dominant follicles that were large enough to respond to GnRH 2 ( 10 mm) and increased ovulatory response after GnRH 2.

Key Words: beef heifers, follicle size, GnRH, luteinizing hormone, ovulation