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Genetic and environmental (co)variance components for 4% FCM yield were simultaneously estimated by REML for grade cows from herds stratified by within-herd SD for 305d mature equivalent milk and time period. Data were lactation records from 299,441 daughters of 2489 AI b d s that calved for the first time from 1970 to 1985 in California, New York, or Texas.
Sire and residual variance components for FCM increased with SD in all time periods and were slightly larger for second than for first lactations. Residual components were of uniform size across time periods within each SD class, but sire components were as much as 21% less for first lactations and 27% less for second lactations in the latter periods than in the earliest one. Heritabilities of first lactation FCM were largest (.154, .263, and -226 for low, medium, and high SD classes) in the earliest time period. Variance components far log-transformed records in each time period differed less than on the original scale, but both sire and residual components remained largest in the most variable herds. Genetic correlation coefficients between yields in first and second lactation were greater (.75 to -99) in all SD classes and time periods than phenotypic correlations (.40 to .61). Heterogeneous phenotypic variation for milk yield was attributed to genetic and environmental components. However, genetic variation only increased with environmental opportunity (variation).