Effect of Concentrations of Progesterone on Follicular Development and Fertility in Beef Cows Jason B. Candler The present study was performed to investigate the effects of low ascending concentrations of progesterone on fertility in lactating beef cattle. Cows on the low ascending progesterone treatment received PGF 2 α from days 3 to 4 after estrus. All cows received PGF 2 α on day 9, and estradiol benzoate on day 11. Cows were inseminated artificially 12 hours after estrus. Concentrations of progesterone were lower in treated versus control cows. Pregnancy rate did not differ between treated and control cows. Pregnancy rate was greater in cows that ovulated a follicle from the right (78%) versus left (52%) ovary (p < 0.01). Low ascending progesterone early in the estrous cycle did not decrease fertility to the same extent seen with extended periods of low progesterone.