Effect of positive genetic trend for mature size on age-of-dam adjustment factors for weaning weight in Angus field records.

Abstract

Weaning weight and postweaning gain field data from the American Angus Association were used to estimate age-of-dam adjustment factors for 205-d weaning weight. A two-trait model was used to take into account all genetic information available in the breed. Correction factors were developed for age-of-dam categories by sex of calf and management (creep or no creep) within maternal-grandsire birth-year groups. The maternal-grandsire birth-year groups were included to determine the need for updating age-of-dam adjustment factors for weaning weight. The mathematical model for 205-d weight included an overall mean, weaning contemporary group, age of dam by sex of calf and management code within maternal-grandsire birth-year group, animal direct effect, maternal genetic effect, permanent environmental effect, and residual error. The model for postweaning gain included an overall mean, postweaning contemporary group, animal direct effect, and residual error. Age-of-dam additive adjustment factors differed from the current factors used for the Angus breed. Young dams did not receive as large an adjustment as the current Angus adjustments recommended. Grouping the age-of-dam categories by birth year of maternal grandsire yielded different additive adjustment factors for 205-d weight. Noncreep calves tended to have larger age-of-dam adjustments than creep-fed calves did. Results indicate the need to consider including age of dam as a fixed effect in the animal model evaluation, so that adjustment factors may be specific to the genetic changes for growth in the breed.