Production Losses in Dairy Cattle Due to Days Open

Losses in milk yield due to delayed breeding were determined frmn 4,910 complete lactations of Holsteins in the North Carolina Institutional Breeding Association. Production of 756 cows to the end of the lactation terminating nearest 48 months after first calving was utilized in the first phase of the study. Regression analyses within herd and year of first calving with age at calving, total days dry, and total days in milk as covariates, revealed a quadratic relationship between days open and production. There was an average decrease of 2.40 1.09 kg of milk and 0.112 0.040 kg of fat for each additional day open. A parallel study utilizing shorter production intervals included 1,580 cows with first and second lactations, 1,050 cows with second and third lactations, and 700 cows with third and fourth lactations. Regression analyses were within herd-yearseason of calving, with age at calving, days open, and days dry as eovariates. Yields per day for the interval from 210 days after calving through 305 days o2 next lactation, were used as dependent variables. Days open were not uniformly expensive for all lactations of an animal. An increase of 1.16 kg of milk for each additional day open was obtained for first lactations. For second and third lactations there was a decline of 3.58 and 3.68 kg of milk, respectively, for each additional day open. The differential response for the first lactations was attributed to their higher persistency. A calving interval of 13 months for first ealvers and ]2 months for second and later ealvers was suggested as an optimum length for attaining nmximum production. Considering the reduced calving rate, as well as the loss in milk yield, net decrease in income for each additional day