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Timing of Preovulatory Endocrine Events, Estrus and Ovulation in Brahman x Hereford Females Synchronized with Norgestomet and Estradiol Valerate^1,2,3

Texas Agricultural Experiment Station^,5, Beeville, 78102

Abstract

The purpose of these studies was to investigate the pattern and timing of preovulatory endocrine events, estrus and ovulation in Brahman x Hereford (F₁) heifers synchronized with norgestomet and estradiol valerate. In Exp. 1, 66 nulliparous and 191 primiparous Brahman x Hereford (F₁) heifers were used to estimate the interval from norgestomet implant removal to onset of estrus. The mean interval from implant removal to onset of estrus was 29.8 ± .5 h, with 80.9% exhibiting estrus within 48 h. Endocrine and reproductive characteristics were examined in detail during Exp. 2 with 37 primiparous heifers. Continuous observation for estrus, 6-h or 2-h blood sampling and ovarian palpation per rectum were employed. All animals were artificially inseminated 48 h after implant removal. Mean interval from implant removal to onset of estrus and to onset of the luteinizing hormone (LH) surge were closely related (r = .91;P < .0001). Mean intervals from implant removal to ovulation, onset of estrus to ovulation and onset of LH surge to ovulation were 59.1 ± 2.5 h, 23.3 ± 1.4 h and 23.1 ± 1.6 h, respectively. Approximately 73% of heifers exhibited estrus within 54 h after implant removal (optimal timing); conception rate was 59.3% in this subgroup. Conception rate of heifers that did not exhibit estrus within 54 h after implant removal or exhibited an LH surge later than 12 h after estrus (delayed timing) was 10%. Assessment of plasma estradiol-17ß concentrations suggested that retarded selection and (or) maturation of the preovulatory follicle following implant removal delayed estrus and lowered conception in up to 28% of females timed-inseminated at 48 h.

¹ Texas Agric. Exp. Sta. Tech. Article No. 22607; contribution to regional project W-112, Reproductive Performance of Domestic Ruminants.

² Supported in part by Ceva Laboratories, Overland Park, KS and by a Behmann Brothers Foundation Fellowship to J. C. Mikeska.

³ Mention of trademark or a proprietary product does not constitute a guarantee or a warranty of the product by the Texas Agric. Exp. Sta. and does not imply its approval to the exclusion of other products that also may be suitable.

⁴ Reprint requests and correspondence.

⁵ Beef Cattle Reproduction Lab.

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