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AN EVALUATION OF TWO SOURCES OF BIAS IN ESTIMATED BREEDING VALUES FOR WEANING WEIGHT IN BEEF CATTLE
Abstract
Field records from 15 herds in the American Angus Association production testing program were studied to evaluate the bias in estimated breeding values (EBV) associated with genetic trend and positive assortive mating. The data were analyzed within herd and collected over 13 to 22 years with records numbering from 91 to 567 annually per herd. All weights were expressed as ratios with the contemporary group means calculated for animals from the same herd, year, sex and management regime. Regression techniques were used to estimate separate contributions to genetic trend associated with sires and with dams. Sire contributions to trend were calculated as the negative pooled within sire regression of offspring weaning weight ratio on year of calf birth and ranged from .007 (+OR-) .229 to 1.299 (+OR-) .241 ratio units. Dam contributions were estimated as the pooled within dam regression of weight ratio, expressed as a deviation from the offspring's paternal half-sib contemporary average, on year of calf birth. The dam contributions ranged from .055 (+OR-) .062 to .679 (+OR-) .112 units per year. Total genetic trend was estimated as the sum of the sire and dam effects and was 1.471, 1.131, .333, .606, .583, 1.313, 1.367, .612, .211, 1.498, .885, 1.108, .406, .405 and .779 units per year for the 15 herds. Annual estimates of breeding value were calculated using the actual weaning weight ratios and ratios adjusted for estimated genetic trend. Trends were assumed to be consistently linear. Bias was then estimated as the difference between the two estimates and ranged from .375 to 3.857 units in dam estimated breeding value. The bias was a function of the magnitude of the trend and the generation interval. The pooled within dam regression of estimated bias on progeny number was quadratic and measured the change in bias over time. The impact of this bias is minimal early in life but increased rapidly after the fourth or fifth calf. When annual genetic trend was less than .75 ratio units, the loss in selection response did not exceed 2.5 percent. In the herds with trends between .75 and 1.1 units per year, the loss was six to eight percent. In the remaining five herds where annual genetic trend estimates exceeded 1.1 units, the loss ranged from 10 to 25 percent. All herds practiced positive assortive mating. The resulting bias was measured as the pooled within dam regression of dam EBV on sire EBV expressed as a deviation from the year-sire average value. An EBV was calculated for each dam for each progeny with all other progeny data deleted. Any individual and paternal and maternal half-sib data available in any year was included in each calculation. The average herd bias associated with positive assortive mating ranged from .051 to .081 units per mating and was consistent from herd to herd.
Subject Area
Livestock
Recommended Citation
ZOLLINGER, WILLIAM ARTHUR, "AN EVALUATION OF TWO SOURCES OF BIAS IN ESTIMATED BREEDING VALUES FOR WEANING WEIGHT IN BEEF CATTLE" (1981). ETD collection for University of Nebraska-Lincoln. AAI8118190.
https://digitalcommons.unl.edu/dissertations/AAI8118190