Index selection of beef cattle for growth and milk production using computer simulation modelling

The Texas A&M University (TAMU) Beef Cattle Production model was expanded to include basic concepts of quantitative genetics. The traits simulated were birth weight, yearling weight, mature weight and milk production. The progeny inherited attributes from both the sire and the dam. The incorporation of genetic concepts into the model allowed for the introduction of variation between individuals and generations. This was achieved by interfacing the original model with stochastic genetic subroutines including a restricted selection index for desired genetic change. The index included birth weight and yearling weight. In addition, a function for estimating dystocia was also added. The model was used to simulate selection strategies for a small-to-moderate size breed of cattle and a large-size breed using a restricted selection index. There was an increase in both birth and yearling weight after 20 years of selection in the small-to-moderate size breed, and there was also an increase in dystocia. Antagonistic selection to decrease birth weight and increase yearling weight was simulated for the large breed. Birth weight and dystocia problems declined while yearling weight increased for all classes of animals. In both experiments correlated responses were recorded for mature size and milk production. It was concluded that the modified TAMU Beef Cattle Production model offers breeders an opportunity to compare different selection strategies and evaluate different breeding plans.