Summary. Post-partum acyclic beef cows received continuous long-term treatment with GnRH (200 or 400 ng/kg body wt/h) or the GnRH agonist buserelin (5·5 or 11 ng/kg body wt/h) using s.c. osmotic minipumps which were designed to remain active for 28 days. All treatments increased circulating LH concentrations whereas FSH remained unchanged. Ovulation and corpus luteum (CL) formation as judged by progesterone concentrations ≥ 1 ng/ml occurred in 0/5 control, 4/5 200 ng GnRH, 4/4 400 ng GnRH, 4/5 5·5 ng buserelin and 3/5 11 ng buserelin cows. The outstanding features of the progesterone profiles were the synchrony, both within and across groups, in values ≥ 1 ng/ml around Day 6, and the fact that most CL were short-lived (4–6 days). Only 3 cows, one each from the 400 ng GnRH, 5·5 ng buserelin and 11 ng buserelin groups, showed evidence of extended CL function. Cows failed to show a second ovulation which was anticipated around Day 10 and this could have been due to insufficient FSH to stimulate early follicular development, or the absence of an endogenously driven LH surge. The highest LH concentrations for the respective groups were observed on Days 2 and 6 and by Day 10 LH was declining, although concentrations did remain higher than in controls up to Day 20. The fact that LH remained elevated in treated cows, but CL were nevertheless short-lived, suggested that endogenous LH secretion in post-partum cows following a GnRH-induced ovulation is not limiting to CL function, and that other factors cause early demise of the first CL. The doses of GnRH used were not directly limiting to CL function since in a second trial extended CL were observed when GnRH was preceded by progesterone.
Keywords: cow; post partum; continuous GnRH; ovary