FACTORS AFFECTING THE ORIGIN OF THE OVULATORY FOLLICLE IN HEIFERS WITH INDUCED LUTEOLYSIS

Seventeen nulliparous Holstein heifers were treated with a luteolytic dose of prostaglandin F2-alpha (PGF) on Day 8 (Day 0 = day of pretreatment ovulation) when the dominant follicle of the first follicular wave (Wave 1 ) was expected to be in the static phase. Wave 2 was first detected on Days 8 or 9 in 2 of the 14 and 3 of the 3 heifers that ovulated from Waves 1 and 2, respectively. That is, most heifers treated with PGF on Day 8 ovulated from the dominant follicle of Wave 1, except when Wave 2 emerged early. In heifers that ovulated from Wave 1, the dominant follicle increased (P < 0.0001) in diameter from Day 8 to the day prior to post-treatment ovulation (mean increase, 2.3 mm), even though it had apparently reached its maximum diameter before the induction of luteolysis. Heifers ovulating from the dominant follicle of Wave 1 differed from heifers ovulating from the dominant follicle of Wave 2 in the following ways: change in diameter of the ovulatory follicle from day of treatment to day of post-treatment ovulation (means, 2.3 versus 9.0 mm, P < 0.0001), diameter of Wave-1 dominant follicle on the day prior to post-treatment ovulation (means, 17.6 versus 13.7 mm, P < 0.0005); and length of the interval from day of treatment to day of post-treatment ovulation (means, 4.2 versus 6.3 days, P < 0.03). Following ovulation after the PGF-induced luteolysis, 11 heifers had two-wave estrous cycles and 5 had three-wave cycles. Three-wave cycles were associated with an earlier emergence of Wave 2 and a longer luteal phase (onset of luteal regression; Day 20.0 versus Day 17.5; P < 0.03).