Efficient manipulation of estrous cycle via exogenous treatments is critical for fertility in dairy cows following timed artificial insemination (TAI) programs. Luteolysis is a key event that warrants reproductive success in manipulated estrous cycles. The main objective of this thesis was to characterize luteolysis following different doses of cloprostenol sodium (CLO) and its effects on luteal blood flow (LBF) and fertility of lactating dairy cows. The first objective was to determine the effect of luteolytic and sub-luteolytic doses of CLO in dairy heifers with an early and mid-cycle corpus luteum (CL). Measurement of LBF was indicative of complete, and partial luteolysis following various doses of CLO. Heifers receiving multiple doses of CLO had complete disappearance of LBF 4 d post-treatment. The second objective was to assess the effects of different CLO dose strategies during TAI programs on pregnancy rates per AI (PR/AI) in lactating dairy cows. We also aimed to verify the association between LBF around TAI with PR/AI in cows that had a d 7 and 14 CL. Cows were treated with a single full dose, two full doses 24 h apart or a double dose of CLO. There was no evidence of differences in PR/AI between CLO doses. But, 3rd+ parity cows treated with a single full dose of CLO had greater pregnancy loss form d 24 to 34 post-AI. Treatment with double dose of CLO resulted in similar PR/AI independent on synchronization status. Amount of LBF at 2 and 4 d post-treatment was a predictor of PR/AI 34 d post-AI. Cows with decreased LBF of both d 7 and 14 CL below a median cutoff had greater PR/AI 24 d post-AI. Multiparous cows treated with double doses of CLO avoided lower fertility in non-synchronized cows and greater pregnancy losses compared to cows treated with a single full dose.
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