Relations between Parts of Lactations and Producing Ability of Holstein Cows

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Abstract
Relations between various parts of completed lactations of at least 243 days duration were considered in 599 lactations of 253 Holstein cows in the Iowa State College herd. Factors for estimating 305-day production from cumulative part records need to be different in first lactations and in later lactations. Average repeatabilities for single month milk and fat production and for cumulative month milk and fat production were 0.41, 0.32, 0.57, and 0.51, respectively. Average intrasire daughter-dam regressions for single month milk and fat production and for cumulative month milk and fat production were 0.15, 0.09, 0.25, and 0.19, respectively. Genetic correlations between cumulative part and whole milk and fat production were over 0.9. Selecting on the cumulative part record would improve production nearly as much as selecting on the complete record itself, the efficiency values ranging from 0.74 to above unity.