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Synchronized Estrus and Fertility of Beef Cows after Weaning Calves for Short Intervals^1,2,3,

Michigan State University, East Lansing 48824

Abstract

In Exp. 1, the objective was to determine if interval of separating calves from cows (24 or 48 h) immediately before insemination affects detection and precision of estrus and pregnancy rates of lactating beef cows implanted with norgestomet. Separation of calves from cows for 24 h (n = 418) lengthened intervals to estrus, did not affect precision of estrus, reduced success of detecting estrus and lowered pregnancy rates relative to positive controls (48 h separation, n = 508). Cows with poor body condition, and not suckled for 24 h, conceived at lower rates than cows with similar condition that were not suckled for 48 h. Adverse effects of separation for only 24 h on fertility are apparently due to inadequate intervals between estrus and insemination at 48 h after removing implants. In Exp. 2, the objective was to determine effects of separating calves from cows for 48 h immediately before insemination on detection and precision of estrus and on pregnancy rate of ovulatory lactating beef cows injected twice with prostaglandin F₂ (PGF₂). Weaning increased detection of estrus but overall pregnancy rates did not differ between suckled (n = 256) and nonsuckled (n = 221) cows. But, weaning calves improved pregnancy rates of young (2 to 3 yr) cows and reduced fertility among middle (4 to 6 yr)-aged cows. Increased pregnancy rates after weaning calves for 48 h are due largely to greater detection of estrus and inseminating more cows. If facilities exist to separate calves from cows, weaning calves for 48 h immediately before artificially inseminating will increase pregnancy rate of beef cows after estrus is synchronized with progestin or PGF₂.

¹ Michigan Agr. Exp. Sta. Journal Article No. 11701.

² Gratitude is extended to G. D. Searle and Co. for Syncro-mate B^® and to The Upjohn Co. for Lutalyse^® used in these studies. Authors are grateful for technical assistance of B. Brubacher, B. Brusie, G. Koester and B. Pelkey and for cooperation by owners and managers of herds studied.

³ This research was supported in part by the Natl. Assoc. of Anim. Breed. and was conducted as part of a regional research project, NC-113, "Methods for Improvement of Fertility in Cows Postpartum."

⁴ Anim. Reprod. Lab.; Dept. of Anim. Sci.

⁵ Regional Livestock Ext. Specialist; Marquette, MI 49855.

⁶ MABC/Select Sires, Inc.; East Lansing, MI 48823.