Abstract
Individual cow test day records collected between December 1979 and June 1986 were used to generate part-lactation feed intake and 4% FCM measures for 9361 Ayrshire and 93,336 Holstein cows in first lactation. Grain energy and total energy intake and FCM measures were for the first 70 and 90 d of lactation and from d 70 to 305 and d 90 to 305. Separate analyses by breed were carried out according to a multiple-trait mixed model. Sixty-six Ayrshire and 416 Holstein sires were treated as random in the analyses. Heritability estimates for both grain and total energy intake in early lactation were from .04 to .06 for Ayrshires and .10 to .13 in Holsteins. Heritabilities were between .12 and .18 for the corresponding measures in both breeds for the late lactation period. Genetic correlations between energy intake and FCM within a time period were .79 and higher for the early period and .93 and higher for the later lactation period. Measures of FCM in early lactation had higher genetic correlations with later lactation feed intake and production than did the early measures of feed intake.
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