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Fertility Rates of Cows Bred Following Estrous Cycles of Varying Lengths^{1, 2,}

Department of Dairy Science, State College of Washington, Pullman

ABSTRACT

This study shows no practical reason for delaying service because of estrous cycle irregularity, if the cow is otherwise normal and the cycle is at least 17 days. The conception rates for cycles of less than 17 days were 34% (P < 0.01) among normal cows not bred at the previous estrus, as compared with 50% for all services in this grouping. Similarly, those bred following intervals of 26–27 days were generally below average, but were not significant. Conception rates among otherwise normal cows which failed to conceive at the previous estrus were significantly lower for cycles of 20–22, 32–34, and 35–37 days, and were significantly higher for cycles over 50 days in length. The average was 40% for all services in this cow-group. Among otherwise abnormal cows not bred at the previous estrus, conception rates were significantly lower for estrous intervals of 2–16 (19%), 26–28 (21%), and 29–31 (12%) days in length. This cow-group averaged 29%. Only cycles occurring 50–100 days after a previous breeding had significantly higher conception rates in the group of abnormal cows (26 vs. 21%). Abnormal cows repeating for service anytime within 37 days profitably might have been withheld from service (19–22 days, a possible exception), because the chances for conception were one-third greater for cycles exceeding 43 days in length. The study involved 19,369 services following estrous cycles of known lengths. The data were from one herd of Holsteins for a 25-yr. period.

¹ Scientific Paper No. 1673, Washington Agricultural Experiment Stations, Pullman. Project 888.

² This work was suuported in part by funds from Regional Project W-2, "Causes and Prevention of Breeding Failure in Dairy Cattle." W-2 is a cooperative study by agricultural experiment stations of the Western Region and the Dairy Research Branch, USDA.