Survivorship approaches to measuring and comparing cull rates for dairies

Records of 19 482 cows from 9 herds in the San Joaquin Valley of California, USA, were used to estimate culling rate in dairy herds. The cull rate derived from data on non-culled cows was estimated as the weighted slope of the cumulative proportion of cows remaining in a herd after first parturition, where cumulative proportion was computed using a cohort life-table with intervals of 1 month. Cull rates ranged from 9.0 to 13.8% per 12 months of age, compared with culling density rates of 22.2 to 39.7 culled per 100 cow-years. It is suggested that variation in culling among herds during the first lactation, and particularly during the first part of the first lactation, may be an important consideration in future studies of optimal culling practices. A survivorship approach to measuring culling provides an estimate of the rate of removal that is not biased by age, in contrast with currently used methods.