The use of station and field tests for the improvement of milking performance in dairy cattle. IV. Culling effects in herd and station records

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Abstract
SUMMARY The effects of culling, natural wastage and different correction and exclusion procedures on estimates of lactation parameters have been examined in test station records of 1042 Danish Red cows. A total culling rate of 20% was applied to the initial material, of which 11% represented obligatory culling for disease, injury and infertility and 9% ‘paper’ culling for performance at four different stages of lactation. Exclusion and correction of short records generally resulted in overestimates of the sample mean and underestimates of the selection differential produced by culling. In all instances, the variance of the sample prior to culling, and the loss of variance attributable to culling, was underestimated in samples containing short or corrected records. It was concluded that field data cannot provide material suitable for objective studies of selection and that a reappraisal of the effects of age, lactation, season, etc., in the controlled conditions of test stations may well be justified. An absence of culling in test stations contributes to the large estimates of variance and heritability characteristic of station material.