Milk records of up to the first three lactations of artificially sired Holstein daughters and their dams were used to estimate heritability from both daughter-dam regression and paternal half-sib correlation. The within-herd estimates h'om records expressed as deviations from herd-mate averages were :37, .30, and .24 from daughterdam regression for the first three lactations ; and the corresponding estimates h'om halfsib correlation were .24, .21, and .23. These results suggest a large maternal effect in the first lactation, a small amount in the second, and little in the third lactation. Analysis of the records not as deviations gave the same pattern for the daughterdam regressions, but confounding of sires with year-seasons apparently biased the half-sib correlations upward. The consequences of considering unequal heritability for different lactations if the apparent difference is due to maternal effects is illustrated for selection for first-lactation breeding value from up to three records on the cow herself and on her dam. Some recent reports (2, 7) have suggested that the first-lactation record of a dairy cow is more important than any later records for evaluating her breeding value. Little will be added to the accm~ey of prediction by using more than the first record. Evidence from other reports [see, for example, Rende] et al. (
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Vleck et al. (1966) studied this question.