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Simulated Selection for Reproductive Rate in Beef Cattle¹

University of Wyoming², Laramie 82071

Abstract

Computer simulation was used to evaluate four selection strategies for reproductive success in beef cattle. The strategies were 1) no culling; 2) culling of nonpregnant heifers; 3) culling of nonpregnant heifers and cows nonpregnant twice; 4) culling all nonpregnant females. In addition, simulations were conducted utilizing method four for 20 yr, then changed to method three for an additional 20 yr. Cows were removed from the herd at age 10 if not previously culled. Because reproduction is a threshold character, an underlying normally distributed liability that influences whether or not a cow will produce a calf was assumed with a heritability estimate of .2, a mean of zero and a Standard deviation of one. Regressions of breeding value for reproduction and calving rate on year of selection were calculated assuming initial calving rates of 80 and 90%. Linear and quadratic effects for breeding value were present for all selection methods at both initial rates. All primary breeding value regressions were different than that for no culling. At the 90% initial calving rate, methods three and four were different from method two. There were no differences among any regressions for calving rate. Progress for breeding value was more regular than for calving rate; maximum progress for breeding value in 40 yr was approximately .32 Standard deviations for selection method four. Calving rate was more erratic, although calving rate increased approximately 7% as a result of 40 yr of selection using method four at the 80% initial calving rate. It appears that much of the improvement attained by any method of selection occurred in the first 20 yr. This result is logical because the initial herd was randomly generated and thus unselected. There was also little difference between selection method four over 40 yr compared with a method whereby the intensity of selection was reduced from method four to method three after 20 yr. These results suggest that occasional retention of a nonpregnant cow, although generally not economically feasible, will do little to influence the genetic composition of the herd, assuming that the cow has already produced at least one calf.

¹ Published with the approval of the Wyoming Agr. Exp. Sta. as Journal Article 1329.

² Dept. of Anim. Sci.