NORGESTOMET IMPLANTS SYNCHRONIZE ESTRUS AND ENHANCE FERTILITY IN BEEFHEIFERS SUBSEQUENT TO A TIMED ARTIFICIAL-INSEMINATION

Three trials involving 128 heifers were conducted to determine whether norgestomet implants administered during the mid- and late luteal pha ses after breeding could be used to synchronize a second estrus in non pregnant, inseminated heifers without adversely affecting pregnancy in pregnant heifers. All heifers were initially synchronized with Syncro -Mate B and artificially inseminated 47 h after implant removal. On d 9 (Trial 1) or d 12 (Trial 2) after the timed AI, the heifers were ran domly assigned to treated or control groups. Treated heifers received two silicone implants containing 10.0 mg of norgestomet each (Trial 1) or one silicone implant containing 3.6 mg of norgestomet (Trial 2). S ilicone implants were removed on d 21 after the initial AI. In Trial 1 , the calving rate to the initial AI of the control heifers was 35 vs 55% for the norgestomet-implanted heifers ( P > .05). In Trial 2 the c alving rate to the initial AI of the control heifers was 9 vs 45% in t he treated heifers (P < .01). At the return estrus 52% of the control heifers returned to estrus within a 3-d period, whereas 93% of the nor gestomet-treated heifers returned to estrus within a 3-d period (P < . 01). Norgestomet treatment had no effect on serum progesterone concent rations of the pregnant heifers on d 21 after the initial AI. In Trial 3, both control and treated heifers were administered silicone implan ts containing 3.6 mg of norgestomet on d 12; additionally, the treated heifers received an injection containing 3.0 mg of norgestomet and 5. 0 mg of estradiol valerate. Norgestomet implants were removed on d 21. The calving rate of the control heifers was 10 of 19 (53%), compared with a calving rate of 1 of 21 (5%) of the heifers that were implanted with norgestomet and injected with norgestomet and estradiol valerate on d 12. In summary, treatment with norgestomet implants on d 9 or 12 , through d 21, increased calving rates of treated beef heifers and sy nchronized the estrus of nonpregnant, treated heifers.