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Target Weights For Breeding

published: November 16th 2008
by: Rick Rasby, Beef Specialist
source: www.beefblog.com

Substantial research has been conducted contributing to the traditional guidelines of developing heifers to 60 to 65% of mature body weight at time of breeding. In general, studies evaluating different postweaning rates of gain or target weights have used either different amounts of feed, or different types of feeds varying in energy and/or protein content to obtain differences in rates of growth.

A review of these studies conducted over the last several decades along with new research indicates the association among BW, puberty and heifer pregnancy rate appears to be changing over time. In general, research reports published through the late 1980s have shown much greater negative effects of limited postweaning growth on age of puberty and subsequent pregnancy, where as more recent studies indicate less of a negative impact of delayed puberty on pregnancy response. Several factors likely contribute to this change over time. Initial research in this area of interest corresponds to the industry shift from calving heifers at 3 years of age to calving at 2 years of age.

Thus, selection pressure for age of puberty was probably minimal in the animals used in the early studies. While selection intensity would have increased with the reduction in calving age of heifers, genetic progress would take time due to the long generation interval in cattle.

In the mid 1980s, researchers identified the association between scrotal circumference in bulls and age of puberty in their female offspring. Since then, scrotal circumference has been used as an indicator trait for puberty. The change occurring in scrotal circumference from 1985 to the present indicates substantial progress has been made, and a similar response in age of puberty would be expected (see breed association web sites for changes over time in EPD for scrotal circumference). Indeed, the inability of heifers to attain puberty prior to breeding may not be as problematic as heifers reaching puberty before weaning.

    There are data developed at the University of Nebraska were feeding replacement heifers to a traditional target weight increases development costs relative to more extensive heifer development. They reported similar pregnancy rates from the initial through fourth breeding season for heifers developed to reach either 53 or 58% of mature weight prior to breeding as yearlings. This demonstrated heifers developed to only 53% of mature weight could achieve similar initial pregnancy rates and retention compared to heifers developed to 58% of mature weight. In this data set, heifer development cost were reduced by $22 per head.