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Impact of Culling on Sire Evaluation by Mixed Model Procedures¹

Department of Animal Science, North Carolina State University, Raleigh 27650

ABSTRACT

Records from the North Carolina State University dairy where each of 130 females produced second records regardless of first lactation yield provided data to compare sire evaluations from three mixed model procedures: single trait evaluations of first and second lactations (Model 1); evaluations of both lactations together including a random component for cow effects (Model 2); and a multitrait procedure where first and second lactation evaluations were calculated simultaneously (Model 3). Relationships among the 45 sires were included in all models. Culling was simulated at intensities of 10, 20, and 30% on deviation of first lactation from population means. Variation in Model 2 evaluations was least affected by increases of culling intensity. Evaluations by second lactation from Model 1 were most affected by culling. The effect of culling on variability of sire evaluations by Model 3 was not large but was dependent on the genetic correlation assumed between first and second lactations. Expected values for correlations between sire evaluations on first and second lactations were derived and tested.

¹ Paper No. 8314 of the Journal Series of the North Carolina Agricultural Research Service, Raleigh.

² Department of Dairy Science, Virginia Polytechnic Institute and State University, Blacksburg 24061.