Impact of Culling on Sire Evaluation by Mixed Model Procedures

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Abstract
Records from the North Carolina State University [USA] dairy where each of 130 females produced 2nd records regardless of 1st lactation yield provided data to compare sire evaluations from 3 mixed model procedures: single trait evaluations of 1st and 2nd lactations (model 1); evaluations of both lactations together including a random component for cow effects (model 2), and a multitrait procedure where 1st and 2nd 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 1st lactation from population means. Variation in model 2 evaluations was least affected by increases of culling intensity. Evaluations by 2nd 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 1st and 2nd lactations. Expected values for correlations between sire evaluations on 1st and 2nd lactations were derived and tested.