Relationship of fertility to milk yield in Swedish cattle.

Data consisted of 48,830 and 21,136 records of Swedish Red and White and Swedish Black and White cattle. Fertility in virgin heifers and in first lactation cows was measured by first service pregnancy rate and number of services per pregnancy. Production trait considered was FCM produced in first 100 d of lactation. The heritabilities for fertility traits in virgin heifers and first lactation cows were about .05 in both breeds. The genetic correlations between fertility measures within parity were close to one in both breeds, suggesting that these were measures of the same trait. The genetic correlations between fertility in virgin heifer and in first lactation were about .88 for Swedish Red and White breed and between .6 and .7 for Swedish Black and White breed. The association between fertility and production in first lactation cows was antagonistic, with genetic correlations between first service pregnancy rate and milk yield of -.13 and -.32 for Swedish Red and White and Swedish Black and White breed, respectively. The association between first service pregnancy rate in virgin heifers and milk yield in first lactation was also antagonistic, with genetic correlations of -.14 and -.41 for Swedish Red and White and Swedish Black and White breed, respectively.