Induction of ovulation with gonadotropin-releasing hormone during proestrus in cattle: influence on subsequent follicular growth and luteal function

Induction of ovulation by administration of gonadotropin-releasing hormone (GnRH) is commonly practiced in cattle to treat repeat breeders or cows exh ibiting long estrous periods. This treatment may, however, disturb normal r eproductive functions if timing is incorrect. The objective of the present study was to investigate the effect of exogenous GnRH on estradiol secretio n of the ovulatory follicle, occurrence of ovulation, development and funct ion of the corpus luteum (CL) and growth of a dominant follicle after ovula tion in the bovine, when GnRH treatment was given before the expected physi ological LH-surge. Luteolysis was induced by cloprostenol (PG) in three cow s and six heifers. Every animal was assigned once to each of the following treatment or control manipulations, receiving either a single dose (0.1 mg) of GnRH (gonadorelin) at (1) 24 h (T1), (2) 48 h (T2), or (3) 72 h (T3) af ter PG, or (4) no gonadorelin (control manipulation, C). Ovaries were scann ed by ultrasound and blood samples were collected for progesterone (P-4) an d estradiol-17 beta (E-17 beta) determination. Growth curves of dominant fo llicles between treatment 1 and the control differed significantly (P < 0.0 1). One day after ovulation, the diameter of the dominant follicle was almo st 1 mm larger in T1. This difference remained almost unchanged during the entire follow-up period. The recruitment of a new follicular wave after ovu lation seemed to occur earlier. Development of CL and levels and profiles o f P-4-production remained unaffected. When GnRH was given 1 day after PG in jection, two animals showed significantly different development of CL (P < 0.05) and of P-4-production (both in concentrations [P < 0.05] and profile [P < 0.01]). After normal ovulation and CL development, luteolysis took pla ce on days 5 or 6 after ovulation, and animals ovulated on days 9 and 10. I t is suggested that early induction of ovulation with GnRH can cause shorte ned luteal function in cattle and, ultimately, reduced fertility. (C) 1999 Published by Elsevier Science B.V. All rights reserved.