TECHNICAL AND ECONOMIC-EFFECTS OF CULLING AND REPRODUCTION STRATEGIESIN DAIRY-CATTLE HERDS ESTIMATED BY STOCHASTIC SIMULATION

The objective of this study was to create operational replacement guid elines under various conditions concerning reproductive performance, s upply of replacement heifers and individual milk yield. Nine culling s trategies were defined by three average insemination periods and by th ree discrimination policies between high- and low-yield cows. The effe ct of the nine culling strategies was analysed with combinations of he at detection rate and time of initiation of breeding after calving und er two replacement heifer purchase policies: purchase (open herd) and no purchase (closed herd). The strategies were evaluated using a stoch astic simulation model that simulated production and reproductive stat us in herds composed of dual-purpose cattle with additional young stoc k. The evaluation of the strategies was made in a situation without a milk quota under typical Danish conditions in 1993. Results showed tha t discriminating between high- and low-yield cows improved net revenue significantly in open herds but not in closed herds. Irrespective of the purchase policy, using longer insemination periods increased net r evenue significantly in herds with poor reproductive performance; wher eas net revenue in herds with good reproductive performance tended to increase by using shorter insemination periods. The culling rate is a poor figure when evaluating culling strategies and culling strategies should be assessed at herd level rather than per cow.