Luteolytic efficiency of reduced doses of cloprostenol in the ewe. Effect of progesterone concentrations at the time of treatment

•Luteolysis after injection of small doses of cloprostenol was evaluated in the ewe.•Luteolytic efficiency was similar on day 6 and day 9 of the luteal phase.•A transient progesterone decline followed by recovery was a common finding.•High progesterone levels at the time of injection have a luteoprotective effect.•Possible mechanisms and implications are discussed.

Seventy six ewes were treated with 7.5, 12.5, 25 or 50 μg of cloprostenol on day 6 or 9 post-estrus to compare the luteolytic efficiency of the PGF2α analogue at each stage and to evaluate if progesterone concentrations at the time of treatment affect such efficiency. Blood samples were obtained before cloprostenol administration and 12, 24, 48, and 72 h thereafter. There was an effect of dose (p < 0.05) but not of day post-estrus on the proportion of animals completing luteolysis. As the dose increased, the proportion of ewes completing luteolysis also increased. Also, as the dose increased from 7.5 to 25 μg, more ewes showed a transient progesterone decline instead of an absence of response, indicating that in some ewes reduced doses initiated luteolysis but were not able to finish the process. Since the dose of 25 μg resulted in close to 50% luteolytic efficacy, this group was used to study the effects of progesterone concentrations at the time of treatment on the response to cloprostenol. Pre-treatment progesterone concentrations were higher (p < 0.01) in ewes experiencing luteolytic failure than in those that completed luteolysis. There was a negative correlation between initial progesterone concentrations and their reduction by 12 h post-treatment. It is concluded that high progesterone concentrations are associated with a reduction in sensitivity to small doses of cloprostenol. Possible mechanisms and implications of this luteoprotective effect are discussed.