INDUCED AND SYNCHRONIZED ESTRUS IN CATTLE - DOSE TITRATION OF ESTRADIOL BENZOATE IN PERIPUBERTAL AND POSTPARTUM COWS AFTER TREATMENT WITH AN INTRAVAGINAL PROGESTERONE-RELEASING INSERT AND PROSTAGLANDIN-F2-ALPHA

Peripubertal beef heifers (n = 57) and postpartum multiparous cows (n = 52) were used to determine the optimal dose of estradiol benzoate (E B) to induce and synchronize estrus after treatment with intravaginal progesterone inserts (IVP4, EAZI-BREED(TM) CIDR(R)). All females recei ved an IVP4 for 7 d (d 0 = insertion day) with a 25-mg injection of PG F(2 alpha) (Lutalyse(R)) on d 6. At 24 to 30 h after IVP4 removal, fem ales were randomly assigned to be injected subcutaneously with EB at t he following doses: heifers 0,.2,.38, or .75 mg and cows 0,.25,.5, or 1 mg. Furthermore, seven heifers and seven cows from each dose group w ere bled every 4 h for 76 h starting at EB injection. Serum was collec ted and assayed for LH and estradiol-17 beta (E-2) Observations for si gns of estrus were made twice daily for 21 d after removal of IVP4, an d females were artificially inseminated 8 to 20 h after detection of e strus. The percentage of females showing estrous behavior was increase d by EB (P < .04); the greatest response was at .38 mg in heifers (86% ) and 1 mg in cows (100%). Dose x time interaction affected (P < .01) E-2 concentrations in heifers and cows; the animals that received the higher doses of EB had greater E-2 concentrations in a shorter time th an those that received the smaller doses. The percentage of cows and h eifers with an acute preovulatory LH release (peak LH) was affected by dose, with a linear (P < .01) and a quadratic (P < .01) response. Hig hest concentrations of LH during peak LH were affected by dose with a linear(P < .01) response in heifers and linear (P < .01) and quadratic (P < .08) responses in cows. Heifers receiving .38 mg and cows receiv ing .5 and 1 mg of EB had the highest peak LH. Time to LH peak had a l inear (P < .03) response in heifers and had linear(P < .04) and quadra tic (P < .05) responses in cows. Pregnancy rate was affected (P < .02) in heifers by whether or not they were anestrous before IVP4 treatmen t (those with estrous cycles = 52% vs those that were anestrous = 22%) and in cows by dose of EB (P < .01; 8, 23, 21, and 67% for 0,.25,.5, and 1 mg, respectively). In conclusion, in females treated with IVP4 a nd PGF(2 alpha) to induce and synchronize estrus, an injection of EB i ncreased concentrations of E-2 and LH and increased number of animals showing estrus. Also, EB increased pregnancy rates in cows. Optimal re sponses were at .38 mg EB for heifers and at 1 mg EB for cows.