Distribution of ovarian follicular populations in the dairy cow within 35 days after parturition

Abstract

Ovaries were obtained at slaughter from 12 Holstein dairy cows at 15, 25 or 35 days after their 4th calving. Non-atretic and atretic antral follicles were separated into 6 size classes according to size and the numbers in each class were expressed as a percentage of the total for each ovary. Non-atretic follicles of diameter 0.16-0.28 mm decreased from 27.5% at Day 15 to 1.5% at Day 35 whereas those of 0.29-0.67 mm and 0.68-1.57 mm diameter increased from 37.4 to 47.2% and from 11.5 to 17.3% respectively (all P less than 0.05). The proportions of follicles measuring 1.58-3.68, 3.69-8.56 and greater than 8.56 mm remained almost constant. The atretic follicles of 0.29-3.68 mm varied significantly in number according to the post-partum interval and to whether they were in the ovary containing the CL of pregnancy. It is concluded that the CL of pregnancy and/or the conceptus have a carry-over effect on the rate of growth of the antral follicles even after parturition.