Replacement policy in dairy herds on farms where heifers compete with cows for grassland—Part 3: A revised hypothesis

Abstract

A preliminary investigation of replacement policy on dairy farms where heifers compete with cows for grassland suggested that profitability falls as replacement rate increases, because the number of cows able to be milked and the maturity of the herd override the gains in milk yield per cow due to genetic improvement by culling for insufficient yield and by the greater use of a sire of high genetic merit. This hypothesis was inadequate to cover all the situations investigated in two factorial experiments involving changes in herd health status, replacement rate, age at first calving, calving index and the merit of AI sires used in the herd from an initial state for these variables of 0·22, 36 months, 13 months and Standard, respectively. Overall, these was a clear indication that the major influence on profitability was the number of cows in the herd. An increase in replacement rate was not always associated with a fall in profitability, nor was an improvement in yield per cow always associated with a rise in profitability relative to the performance of a control herd after 15 simulated years. The manipulation of replacement rate and age at first calving to achieve an increase in the number of cows in the herd was not necessarily incompatible with a desire for herd improvement either phenotypically (yield per cow) or genetically (herd and heifer genotype), particularly if premium bulls were used.