J. Anim Sci.
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J. Anim Sci. 1989. 67:1175-1182.
© 1989 American Society of Animal Science
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A Comparison of the Breeding Performance of Yearling and 2-Year-Old Hereford Bulls1

R. C. Greer2, J. J. Urick3, M. D. Frank2 and L. G. Carroll2

Montana State University, Bozeman 59717 and Livestock and Range Research Laboratory, Miles City 59301

Abstract

Fort Keogh Livestock and Range Research Laboratory, Miles City, single-sire breeding herd records provided 1,728 observations on the breeding performance of yearling and 2-yr-old novice Line 1 Hereford bulls. Breeding results from the joint performance of the male and female, with the female as the elementary unit of observation, formed a Bernoulli experiment, in which the female either was pregnant or was not pregnant. Thus, the measure of performance is the probability of pregnancy, the probability being conditional on certain factors (e.g., female traits, bull traits and herd characteristics). The Logit model, not subject to the shortcomings of a linear probability model, was utilized to compare the breeding performance of the yearling with the 2-yr-old novice bulls. The observed difference in herd pregnancy rate between yearling and 2-yr-old bulls was explained largely by a lower probability of pregnancy when the female was a yearling heifer in a predominately heifer herd exposed to a good gain ratio yearling bull, or the female was of any age in a mixed-age herd exposed to a medium gain ratio yearling bull. The breeding performance of 2-yr-old novice and good or excellent gain ratio yearling bulls assigned to mixed-age female breeding herds was not statistically different. Decreased breeding performance was found for 2-yr-old cows nursing a calf. However, this result was attributed to the cow, because no discernible difference in performance due to bull age or other traits was found.

Footnotes

1 Published with the approval of the Director of the Montana Agric. Exp. Sta. Journal Series No. J-2273.

2 Dept. of Agric. Econ. and Econ.

3 Fort Keogh Livest. and Range Res. Lab., USDA, SEAAR.






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Copyright © 1989 by the American Society of Animal Science.