Synchronization of estrus in beef cattle with norgestomet and estradiol valerate

Fifty-six cows received a norgestomet implant and an injection of norgestom et and estra diol valerate; half (n = 28) received 500 IU equine chorionic gonadotrophin (eCG) at implant removal, 9 d later. A third group (n = 25) r eceived 2 doses of cloprostenol (500 mu g) 11 d apart. Estrous rate was hig her (P < 0.05) for cows given norgestomet and estradiol plus 500 IU eCG (75 .0%) than for those receiving cloprostenol (44.0%); for those receiving nor gestomet and estradiol alone, it was intermediate (67.8%). Pregnancy rates to artificial insemination (after estrus or timed) were higher (P < 0.05) f or cows given norgestomet and estradiol than for those given cloprostenol ( 23 of 28, 82.1% vs 13 of 25, 52.0%), and intermediate (67.8%) for those giv en norgestomet and estradiol plus eCG. In a second experiment, for heifers treated with norgestomet and estradiol plus eCG (n = 15) or with 2 doses of cloprostenol (n = 16), estrous rates were 66.7% vs 56.2% (P > 0.5), ovulat ion rates were 100.0% vs 81.2% (P = 0.08), intervals from implant removal o r cloprostenol treatment to estrus were 48.0 +/- 4.4 hours vs 61.3 +/- 7.0 hours (P = 0.12) and to ovulation were 70.4 +/- 4.4 hours vs 93.2 +/- 7.5 h ours (P < 0.01), respectively; pregnancy rates were 41.7 and 35.7%, respect ively (P > 0.5). Norgestomet and estradiol were as good as (heifers) or sup erior to (cows) a 2-dose cloprostenol regimen. In cows given norgestomet an d estradiol, injecting eCG at implant removal did not significantly improve estrous or pregnancy rates.