Analysis of Herd Life in Guernsey Dairy Cattle

Heritabilities and genetic and phenotypic correlations for 48-and 72-mo herd life were estimated with multiple-trait REML, from sire models incorporating sire relationships. Two traits were defined for 48-and 72-mo herd life, true herd life and functional herd life, which were adjusted for milk production prior to culling. Heritabilities for 48-and 72-mo herd-life traits were low, ranging from .02 to .07; genetic correlations among herd-life traits ranged from .82 to .95, and phenotypic correlations ranged from .80 to .97. Genetic correlations be- tween the herd-life traits and first lactation milk, fat, and protein production ranged from .37 to .81. Genetic correlations were lower between functional herd life and milk, fat, and protein production than among true herd life and these same variables. Multipletrait REML from sire models, which included sire relationships, was used to estimate genetic and phenotypic correlations between 48-mo true or 48-mo functional herd life and linear type traits for registered Guernsey cattle. The genetic correlations were used to compute weights for indirect prediction of true and functional herd-life transmitting abilities from linear type traits transmitting abilities. The predic-