Part Lactations. III. Progeny Testing with Part Lactation Records

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Abstract
Heritability estimates for dairy records accumulated over the 1st 3-5 months of lactation are reported in this paper as being not significantly lower than that for the complete lactation yield. The genetic correlation between part records and lactation yield exceeds 0.90 and 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. Mathematical expressions are developed for finding the number of daughters needed to prove a sire on part lactation records such that the proof will contain the same amount of information about the sire''s additive genetic merit for lactation yield as a proof based on lactation yields. Approximately 40, 60 and 80 daughters with part records are equivalent to lactation yields of 30, 40 and 50 daughters. 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.