Short communication: genetic and environmental relationships between milk yield and kidding interval in dairy goats

First-parity 305-d milk yield and intervals between first and second kiddings from 1975 through 2005 were analyzed to estimate genetic and environmental parameters for United States Alpine, LaMancha, Nubian, Saanen, and Toggenburg dairy goats. The data set included 43,612 does with first-parity milk yield and 25,863 does with first kidding interval. Parameters were estimated by REML using bivariate mixed models within and across breeds. Fixed effects were month and herd-year of kidding within breed and month and breed-herd-year of kidding across breeds. Random effects were animal and residual. The heritability estimate for first-parity milk yield was 0.36+/-0.01 across breeds and ranged from 0.35 to 0.38 within breed; heritability for first kidding interval was 0.05+/-0.01 across breeds and ranged from 0.00 to 0.15 within breed. The estimate of the genetic correlation between first-parity milk yield and first kidding interval was positive (unfavorable) across breeds (0.35+/-0.09). Estimates of environmental correlations within and across breeds were positive (0.16-0.25). The presence of unfavorable genetic relationships between milk yield and kidding interval indicates a need to include reproductive performance as a selection criterion.