Effectiveness of norgestomet implants in suppressing ovulation and estrus in heifers varies with stage of estrous cycle when implanted

Suppression of ovulation and estrus for 63 d by a 24-mg norgestomet ear implant with a delivery rate of 160 mug d(-1) was evaluated in beef heifers. Ovulation was suppressed in 80% of heifers implanted on day 7 of the estrous cycle, 40% on day 10, 33% on day 13, 53% on day 16, and 67% on day 19 (P < 0.05 for day 7 vs. days 10, 13 and 16). The norgestomet implant was more effective in suppressing ovulation and estrus when inserted on day 7 of the estrous cycle rather than later in the cycle.