The Estimation of Environmental and Genetic Trends from Records Subject to Culling

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Abstract
In the past attempts have been made to estimate genetic and environmental changes in dairy herds by fitting constants to periods when the records were made, to genetic groups, and to cows. If the cows which are culled from the herd have made lower records than those remaining, the estimated genetic trend is biased upward and the estimated environmental trend is biased downward. The causes of these biases are described, and 2 methods, based on maximum likelihood, are presented for obtaining unbiased estimates of these trends. The 1st method assumes that repeatability is known. The 2d obtains joint estimates of repeatability and trends.

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