Effect of Female Culling and Age Distribution of the Dairy Herd on Profitability

Abstract

Deterministic models were used to simulate four female culling systems: (1) selection at birth on estimated breeding value, (2) selection on estmated breeding value based only on the cow's own records, (3) selection of the youngest cows, and (4) selection during first lactation on estimated breeding value. Milk yield was the only trait considered in selection. Each cow selection system was replicated with three sire selection systems. The physical inputs and outputs from each system were evaluated economically in factorial groups of rearing costs, milk and feed prices over twenty 13-mo periods.Within sire systems 1 and 2 (from populations making approximately 1% of the mean genetic improvement each 13 mo), cow system 3 had the slowest rate of genetic and phenotypic improvement. Although differences in rate of improvement among the other three systems were small, cow system 2 usually had the fastest rate of genetic and phenotypic improvement. Within sire system 3 (approximately 2% genetic improvement per period), the rank of cow system 1 declined and the rank of cow system 3 improved because of the shorter generation interval.In the medium economic classes, system 2 usually, but not always, excelled in net profit; rate of gain in profit was proportional to the rate of gain in milk yield for all systems. At the high rearing cost, however, system 1 was the most profitable even with less production than systems 2 and 4. This resulted from the lower cow depreciation cost associated with system 1. At the low rearing cost, the systems ranked the same as they did for milk production: 2, 4, 1, 3.