Efficacy of Norethindrone Acetate and Norgestomet Implants in Suppressing Estrus in Female Beef Cattle

Abstract

Beef females with corpora lutea on day 12 of the estrus cycle were implanted with 11.5 mg norethindrone acetate implants (7 implanted and 7 nonimplanted controls) at the same time they were administered a luteolytic dose of prostaglandin F_2aL. Although the implants released 340 to 740 μmlg of norethindrone acetate daily, the implants did not suppress estrus. Norgestomet implants were inserted into beef females 5 days after estrus and implants were left in situ for 16 days. The numbers of cows in the norgestomet study were 27 nonimplanted, 19 with 6-mg implants, and 21 with 8-mg implants. Control cows were not detected in estrus until day 17 of the estrous cycle (12 days after the time of implantation) and 10 of the 27 cows (37%) were detected in estrus over a 5-day period beginning on day 17 (12 to 16 days after the time of implantation). None of the cows implanted with 8-mg implants were detected in estrus while the implants were in situ. However, 3 of the cows with 6-mg norgestomet implants were detected in estrus 13 to 16 days post-implantation. Based on implant secretion data, cows with 6-mg implants began displaying estrus when the implants released less than 138 pg of norgestomet daily. Eight-milligram implants released 137 pg of norgestomet or greater per day during the entire 16-day implantation period.