Estimates of genetic parameters and correlation for yield traits and somatic cell score among lactations for records with and without bovine somatotropin

Records of registered Holstein cows for milk, fat, and protein yields and somatic cell score (SCS) were divided into three data sets. Data set 1 included records of cows in first lactation. Data set 2 included records of cows in second lactation. Data set 3 included records of cows for lactations three through five. In each data set, about 10% of records were from cows traded with recombinant bovine somatotropin (bST). Numbers of records were 65,720; 50,644; and 45,505 for Data sets 1, 2, and 3. With records of untreated cows considered to be one trait and of treated cows considered to be a second trait genetic parameters were estimated by REML with a two trait animal model. Records of cows with bST treatment had greater estimates of heritability for all data sets. Estimates ranged from 0.09 to 0.16 for treated records and from 0.09 to 0.27 for untreated records. Estimates of repeatability of records for third and later lactations for all traits were from 0.45 to 0.50. Estimates of genetic correlations between records of treated and untreated cows were high (0.991 to 0.999) for all traits. The impact of bST treatment on ranking of sires according to estimates of breeding value with three different models to adjust for bST effects was also investigated. With model 1, bST administration (given or not given) was ignored. With model 2, bST administration was used as a fixed effect. With model three, administration of bST was used to define the contemporary group (herd-year-month of calving-bST). Rankings on EBV for sires with bST treated daughters were similar for the three models. The fractions in common for the same proportion selected between any pair of models were high (0.930 to 0.999). Estimates of correlations among estimated breeding values for yield traits and SCS for sires with bST treated daughters with the three models for lactations all lactation were high (0.981 to 0.999). The three models also were used to estimate genetic correlations between yield traits and SCS. Estimates of genetic correlations between pairs of yield traits ranged from 0.35 to 0.85 for the three models for all data sets. The estimates of genetic correlations between yield traits and somatic cell score in first lactation were smaller: -0.03 to 0.12 than estimates for later lactations of -0.23 to -0.32. Estimates of environmental and phenotypic correlations among yield traits with the three models for all data sets ranged from 0.70 to 0.97. Estimates of environmental and phenotypic correlations among yield traits and SCS were from -0.10 to -0.23. For these data sets, treatment of some cows with bST had little effect on estimates of heritability and genetic correlations or on ranking sires based on estimated breeding value