Heritabilities and Genetic Correlations for the First Three Lactations from Records Subject to Culling

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Abstract
Lactation records on 13,544 cows representing daughters of 90 Quebec Holstein artificial insemination sires were used to estimate genetic parameters of the first 3 lactations for milk, fat and protein yield and percent of fat and protein. A restricted maximum likelihood procedure derived from a multiple trait mixed model was used to estimate simultaneously sire and error variances and sire covariances free of bias from cow culling for all 3 lactations. For yield traits sire variances across lactations were relatively constant whereas error variances increased with advancing lactation. Heritabilities decreased for all yield traits but increased for percentage traits with succeeding lactation. Genetic correlations between different lactations ranged 0.83-0.92 for the five production measures. Production traits of different lactations are essentially the same trait genetically; combining all lactation records as a single trait in sire and cow evaluations is appropriate.