Incidence rates of cystic follicles in Holstein cows according to 15-day and 30-day intervals.

Cystic ovarian follicles occurred in 198 out of 1599 (12.4%) Ontario Holstein lactations. These occurrences were grouped by date of diagnosis into 15-day and 30-day intervals after calving, and only open cows were considered. The incidence rates for the 15-day intervals form zero to 150 days of lactation varied form 0.2% to 3.5%; the highest rate was 31 to 45 days. The incidence rates for the 30-day intervals from zero to 210 days of lactation varied form 1.4% to 6.8%, depending on the method of calculating the denominator of the rate. The consistently highest 30-day rate was at 31 to 60 days (4.7% or 4.8%).