Part Lactations. III. Progeny Testing with Part Lactation Records
Analyses of monthly records of artificially-bred cows indicate that under New Zealand conditions records accumulated up to a specified test day early in lactation are suitable for progeny testing. Heritability slightly exceeds that for lactation yield and the genetic correlation between the part record and lactation yield exceeds 0.90. As a result, the rate of progress in improving additive genetic merit for lactation yield when selecting on part records differs very little from when selecting on lactation yield. Approximately 40, 60, and 80 daughters in sire proofs using part records give the same information about lactation yield as a proof using lactation yield on 30, 40, and 50 daughters, respectively. Proofs evaluated on both kinds of records for the same group of sires were highly correlated and the part lactation proofs were just as repeatable as those based on lactation yields.
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