الكاتب: Multiple References

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 F2a. Although the implants released 340 to 740 pg 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. l3e 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 postimplantation. 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.