Simulation of Genetic Differences in Fertility of Beef Cattle Herds in Two Different Environments

Résumé du livre

The individual animal version of the TAMU Cattle Production Systems Model was used to simulate genetic differences in potential fertility. Simulations were conducted to evaluate the effects of different potential reproduction levels on conception rate and herd structure and productivity. The model was validated using data from Mississippi State University and the Welder Ranch in Calhoun County, Texas. Genetic differences in potential fertility were evaluated by simulating (1) reduced maintenance requirements; (2) decreased sensitivity to poor body condition; (3) decreased sensitivity to weight loss; (4) increased tendency for lactational anestrus; and (5) different weights at which heifers could begin cycling (40, 43 and 50% of mature weight). The herds in the reduced maintenance simulations were more fertile, lived longer, calved earlier, and produced more efficiently than the base herds. Larger increases in fertility were found for the poorer environment of the Welder Ranch than in the more adequate Mississippi environment. Cows and heifers grew faster when they were more able to utilize existing feed for production rather than maintenance. Reducing sensitivity to condition or weight loss allowed the simulated cows to overcome biological controls that would normally limit estrous activity. Simulated cows were generally in poorer condition, but continued to cycle and conceive at a rate greater than the base level. Reduced sensitivity to weight loss did not cause as much difference from base in any category as did the body condition simulations. Increased tendency for lactational anestrus only slightly decreased overall conception rate, but large differences within age groups were observed. Simulations in which altered genetic potentials were combined showed few differences that were not found for the single factor simulations for conception rate, herd structure or herd efficiency. Decreasing weight at puberty tended to increase conception rates and herd efficiency without changing weaning weights, herd structure or sales composition. Selection for fertility is a difficult task made more difficult by the time required to assess fertility. Cooperative research between reproductive physiologists and animal breeders to determine methods to genetically improve fertility and to identify fertile animals early in their life is suggested.