Genetic parameters for six measures of length of productive life and three measures of lifetime production by 6 yr after first calving for Hereford cows.

Genetic parameters for length of pro- ductive life given the opportunity (LPL|O), measured as days between first calving and disposal conditioned on one of six opportunity groups, L1 through L6 (e.g., L2 is length of productive life in days given the opportu- nity to live 2 yr after first calving), and lifetime produc- tion (LP), measured as the number of calves born (NB), number of calves weaned (NW), and cumulative wean- ing weight (CW) by 6 yr after first calving, were esti- mated using records of 1,886 Hereford cows from a se- lection experiment with three selected lines and a con- trol line. Weaning weights were adjusted to 200 d of age and for sex and age of dam. Estimates of heritability and genetic and environmental correlations were ob- tained by restricted maximum likelihood with bivariate animal models, with year of birth of cow as a fixed effect and direct genetic and residual as random effects. Genetic trends were estimated by regressing means of estimated breeding values by year of birth and line on birth year. Estimates of heritability (SE) for LPL|O ranged from 0.05 (0.01) to 0.15 (0.03). Estimates of ge- netic correlations (SE) among LPL|O ranged from 0.74 (0.14) to 1.00 (0.00), and estimates of environmental