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Rate of Conception in Beef Cows Pasture-Bred during a Controlled Breeding Season

United States Department of Agriculture¹

Abstract

An attempt has been made to determine the rate of conception with pasture-bred beef cows during a controlled breeding season that extended over a period equivalent to 6 normal estrual cycles. An indirect method of obtaining the basic data had to be employed as the matings were not actually witnessed by an observer. The 282nd day from the date the bulls were placed in the cow herds is taken as the basic date from which the rate of conception is determined by counting back 282 days from the date of birth of all normal calves. As some calves were born previous to the basic date there is an overlap the result of some calves being carried in-utero less than 282 days. A similar overlap is assumed for calves carried in-utero more than 282 days. By correcting for this overlapping the number of cows that conceived in each 20-day interval during the breeding season is computed.

Using the method described in this study it was determined that, when pasture-bred, about 52 percent of the cows that gave birth to normal live calves became pregnant within the first 20 days of the breeding season. At the end of 40 days 80 percent and at the end of 60 days about 90 percent had become pregnant. All cows that eventually dropped normal calves had become pregnant at the end of 120 days.

In this study slightly more than one-half of the cows calved within 70 days of the start of the breeding season. Only 8 or 1.4 percent calved after the season started.

There is no material difference in the rate of conception for heifers and cows. In the 100 per cent fertile herd an average of 1.81 heat periods elapsed for conception, 1.80 for cows and 1.83 for heifers.

¹ Bureau of Animal Industry, Jeanerette, La.