ECONOMIC-ASPECTS OF PERSISTENCY OF LACTATION IN DAIRY-CATTLE

Persistency of lactation is a trait of economic importance in dairy ca ttle because of its impact on fertility, health, and feed costs. When level of production is based on a 305-day lactation, persistency also affects returns from milk production for lactations with lengths diffe rent from 305 days. The objective here was to quantify the impact of p ersistency of lactation on feed costs and milk returns, relative to co nstant 305-day yield. A bio-economic model that allowed for optimisati on of insemination and culling decisions was used. For individual lact ations of 305 days, persistency affected feed costs only but for other lactation lengths, persistency had a greater impact on milk returns p er lactation than on feed costs. Under an optimised insemination and c ulling strategy, which resulted in an average calving interval of 12.4 months, the economic value of persistency was $13.6 (Canadian) per ph enotypic standard deviation but increased with level of persistency. O n a genetic standard deviation basis and with a heritability of 0.15, the economic value of persistency was 3.4% relative to the economic va lue of 305-day production. The economic value of persistency was almos t tripled when average calving interval was 13 months. Consideration o f health and reproductive costs will further increase the economic val ue of persistency. Persistency had a substantial impact on optimum ins emination decisions. With high persistency, insemination was profitabl e longer into the lactation and the optimum time of first insemination was delayed for high producing cows. (C) 1998 Elsevier Science B.V.