Usefulness of Part Records to Estimate the Breeding Values of Dairy Cattle

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Abstract
Complete lactations for 24,602 Hoisteins were used to measure heritabilities and repeatabilities of monthly and cumulative production, to obtain genetic and phenotypic correlations between part and whole records, and to measure the relative genetic gain in whole records from selection on part records. Repeatabilities were largest in the center months and increased with added months of cumulative production. Heritabilities of milk for parts increased gradually through the 1st lactation but for all lactations varied around .10 increasing to .16. Genetic correlations between monthly and total production were generally . 9 or larger and were largest for the center of the lactation. Phenotypic correlations of monthly with total production in the same or subsequent lactations were largest in months 4 to 6. Adding a 2nd month to the 1st contributed the largest increase in accuracy of predicting that complete lactation, and the correlation exceeded .9 by the time the 5th month was added. A succeeding lactation could be predicted nearly as accurately by a part of the 1st lactation as by the whole 1st. Selection on production for a single month will provide less genetic progress than selecting on the complete record, but cumulating parts should increase relative efficiency with each added month.