Effect of Genetic Differences Among Herds on Accuracy of Selection and Expected Genetic Change

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Abstract
Improperly accounting for genetic differences among herds affects the accuracy of estimates of breeding values. The loss in expected genetic change resulting from this lower accuracy was quantified. Expected genetic change was calculated by sequential selection theory and was expressed in terms of accuracy of estimates of sire and cow breeding values. Simple linear models were used to explain milk production. Selection index theory was used to calculate accuracy as the expected correlation among indices and breeding values. The expected genetic change was calculated and plotted for variations in accuracy of estimates of breeding values. Expected genetic change increased from .4 to .6 kg and 1.1 to 1.2 kg of milk/cow per year for each .01 increase in accuracy of estimates of breeding values of cows and sires, respectively. Evaluation of the potential for genetic differences among herds indicated that the maximum improvement in the expected genetic progress would be around 10 kg of milk/cow per year.