Heterogeneous Within-Herd Variance. 2. Genetic Relationships Between Milk Yield and Calving Interval in Grade Holstein Cows

Genetic and phenotypic (co)variances for mature equivalent yield of 4% FCM and calving interval were simultaneously estimated by REML for grade Holstein cows in first and second parity. Data were 305d mature equivalent lactation records and calving dates for 299,441 daughters of 2489 AI sires first calving from 1970 to 1985 in California, New York, or Texas. Data were divided into three time periods, and herds were partitioned into three within-herd-year phenotypic SD classifications for milk yield. Average FCM yield and calving interval increased with SD in all time periods for both parities. Genetic variance of calving interval showed no trend with SD of yield, although residual variances increased as SD of yield increased Heritability of calving interval was less than .06 in all time periods and SD classes. Genetic correlations between first and second calving interval increased as SD of yield increased (.33 to .63), but phenotypic correlations were uniformly small. Genetic correlations between FCM and calving interval were largest in the highest SD class and for second lactations. Phenotypic correlations were small in every SD class. Even though milk yield and calving interval were positively (unfavorably) correlated genetically, the correlated response in calving interval was only about 1 d for each 100-kg increase in genetic merit for FCM yield