Influence of premature induction of an LH surge with GnRH on ovulation, luteal function and fertility in cattle

¹ Department of Animal Sciences, The Ohio State University, Columbus, Ohio, USA

^* To whom correspondence should be addressed. E-mail: day.5{at}osu.edu.

	Abstract

We tested the hypothesis that luteal function and fertility would be reduced in cattle induced to ovulate prematurely compared with spontaneous ovulation. Estrus was synchronized in 56 beef cows (24 non-lactating and 32 nursing calves). At 6.4 ± 0.1 d after estrus all follicles 5 mm were aspirated (day of aspiration = d 0) with a 17 G needle using the ultrasound-guided, transvaginal approach. On d 1.5 and 2, cows were administered two luteolytic doses of PGF₂. Ovarian structures were monitored by transrectal ultrasonography from d -2 to D 12, or ovulation. Emergence of a new follicular wave occurred on d 1.7 ± 0.1. When the largest follicle (F1) of the newly emerged wave was 10 mm in diameter (d 4.8 ± 0.1), animals were assigned on an alternating basis to receive 100 µg GnRH (GnRH-10; n = 29) to induce ovulation, or upon detection of spontaneous estrus, to the spontaneous (SPON) treatment (n = 24). Cows were bred by AI at 12 h after GnRH (GnRH-10) or 12 h after the onset of estrus (SPON) as detected using an electronic surveillance system (HeatWatch, CowChips LLC, Denver, CO). Blood samples were collected every other dy beginning 2 d after ovulation until pregnancy diagnosis 30 d after AI. Ovulation and AI occurred in 29/29 cows in the GnRH-10 and 24/24 cows in the SPON treatment. Ovulation occurred later (P < 0.05) in the SPON (d 7.7 ± 0.1) than GnRH-10 (d 6.8 ± 0.1) treatment. Double ovulations were detected in 47% of cows resulting in 1.5 ± 0.1 ovulations/cow. Diameter of the ovulatory F1 and the second ovulatory follicle (F2; in cows with 2 ovulations) were greater (P < 0.05) in the SPON (12.0 ± 0.3 mm and 10.5 ± 0.4 mm, respectively) than GnRH-10 (10.7 ± 0.1 mm and 9.2 ± 0.3 mm) treatment. Cross sectional area of luteal tissue and plasma concentrations of progesterone during the mid-luteal phase were greater (P < 0.05) in the SPON (3.62 ± 0.2 cm² and 6.4 ± 0.3 ng/ml) than GnRH-10 (3.0 ± 0.2 cm² and 5.4 ± 0.2 ng/ml) treatment. Conception rate to AI in the SPON (100%) was greater (P < 0.05) than in the GnRH-10 (76%) treatment. The animal model used in this study resulted in unusually high conception rates and double ovulations. In conclusion, premature induction of the LH surge reduced diameter of the ovulatory follicle(s), luteal function and conception rate to AI.

Key Words: Cattle, Corpus luteum, Fertility, Ovarian follicle