Genetic parameters and trends of milk, fat, days open, and body weight after calving in North Carolina experimental herds.

The main objective of this study was to estimate genetic trends for 3.7% FCM, fat yield, days open, and predicted body weight after calving in six experimental dairy herds owned by the State Farm Division of the North Carolina Department of Agriculture. Body weights were predicted from heart girths measured at or before the first test day after calving. Data analyzed were 23,052 records from 8575 cows, daughters of 681 bulls. Heritabilities and breeding values were estimated using the multiple-trait, derivative-free REML programs and a single-trait repeatability model. Breeding values of cows were averaged by and regressed on birth year to estimate genetic trends. Genetic correlations between traits were estimated by correlating breeding values. Estimates of heritability were 0.25 for 3.7% FCM, 0.28 for fat yield, 0.03 for days open, and 0.17 for predicted body weight. Unfavorable genetic relationships were found between yields and days open and between yields and body weight. Genetically, cows that were heavier after calving produced less milk and fat but conceived earlier than smaller cows. Genetic changes in yields and days open were greater for cows born after 1970, but the greatest genetic changes were after 1980 (FCM, 94.7 kg/yr; fat yield, 3.46 kg/yr; days open, 1.1 d/yr). Breeding values for body weight increased for cows born from 1950 to 1970, decreased until 1980, and increased for later parities. The results of our study suggest that AI organizations may need to include fertility traits in progeny testing and relax the emphasis on increased body weight.