|Van Vleck, Lloyd|
Submitted to: Genetics and Molecular Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/11/2006
Publication Date: 3/9/2007
Citation: Al-Seaf, A., Keown, J.F., Van Vleck, L.D. 2007. Estimates of correlations among yield traits and somatic cell score with different models to adjust for bovine somatotropin effects on Holstein dairy cows. Genetics and Molecular Research. 6(1):67-78. Interpretive Summary: Estimates of heritability for yield traits (305 days of lactation, mature equivalent, and milked twice a day) and somatic cell score for Holstein dairy cows were similar for each of three models for records in first, second, and third and later lactations. Model 1 ignored whether cows were treated with bovine somatotropin. Model 2 included a fixed factor for whether the cow was treated with bovine somatotropin. Both Models 1 and 2 included as contemporary group the herd, year and month of calving. Model 3 included treatment or not with bovine somatotropin as part of the definition of contemporary group. Estimates of heritability for yield traits obtained in this study ranged from 0.12 to 0.20. Estimates of heritability for somatic cell score were identical for each of the three models and ranged only from 0.08 to 0.09 for different lactations. Estimates of genetic correlations between pairs of yield traits ranged from 0.35 to 0.85 and were similar for the three models over all lactations. Estimates of genetic correlations between milk and fat yields increased from lactation one (0.35) to lactation two (about 0.43) to later lactations (about 0.54). The increase in estimates of genetic correlations between fat and protein yields followed a similar pattern (0.52 to about 0.63 to about 0.71). Estimates of genetic correlations between yield traits and somatic cell score in first lactation were smaller in magnitude (-0.03 to 0.12) than small but favorable estimates for later lactations of -0.23 to -0.32. For the three sets of lactations used in this study, no important differences in estimates of genetic parameters were found among the three models: the model unadjusted for bST treatment and for two models that adjusted for bST. Slight to no differences between estimates with different models within lactations indicate that adjustment for bST has negligible effects on estimates of heritability and genetic, permanent environmental, and temporary (residual) environmental correlations. Whether these results would be true if a larger proportion of records were from cows treated with bovine somatotropin (about 7% in this study) is not known. What the effects of the three models on rankings of sires and especially of cows on estimated breeding values was not the goal of this study. Further study is needed to determine whether Models 2 and 3 would result in similar rankings for estimated breeding values even though estimates of genetic parameters were similar for both models for these data.
Technical Abstract: Records of Holstein cows from the Dairy Records Processing Center at Raleigh, NC were edited to obtain three data sets: 65,720 first, 50,694 second, and 65,445 later lactations. Correlations among yield traits (preadjusted to 305 days of lactation, mature equivalent, and twice a day milking) and somatic cell score were estimated with three different models:1) bovine somatotropin (bST) administration ignored, 2) bST administration as a fixed effect and 3) administration of bST as part of contemporary group (herd-year-month-bST). Heritability estimates ranged from 0.13 and 0.17 for milk, 0.12 to 0.20 for fat, 0.14 to 0.16 for protein yields, and 0.08 to 0.09 for somatic cell score. Estimates were smaller for later than for first lactations. Estimates of genetic correlations among yields ranged from 0.35 to 0.85 with no important differences between estimates with the 3 models. Estimates for lactation 2 were in agreement with estimates for lactation 1, although estimates of genetic correlations for later lactations were generally greater than for lactations 1 and 2 except between milk and protein yields. Estimates of genetic correlations between yields and SCS were mostly negative or small (-0.45 to 0.11). Estimates of environmental correlations among yields were similar with the three models (0.77 to 0.97). Estimates of environmental correlations between yields and somatic cell score were negative (-0.22 to -0.14). Estimates of phenotypic correlations among yield traits ranged from 0.70 to 0.95. Estimates of phenotypic correlations between yields and somatic cell score were small and negative. For all three data sets and traits, no important differences in estimates of genetic parameters were found for the two models that adjusted for bST and the model that did not.