Despites estrous behavior, in a study examining ovulation in dairy cows, the risk of ovulation failure was 3.9 times higher for AI performed in the warm period (12.4% of 663 AIs) versus the cool period (3.4% of 1254 AIs) [46]. All cows showed estrous signs, and estrus was confirmed (as the absence of a CL and presence of a preovulatory follicle) by palpation per rectum at AI . The ovulation rate (detection of at least one CL in the ovaries) was determined
by ultrasound 11 days after AI. In cows suffering ovulation failure, luteal structures were absent and the preovulatory
follicle became persistent follicle (8–15mmdiameter) in 23 cows and resembled ovarian cyst (18–32 mm diameter) in
102 further cows