Direct and correlated responses to selection for milk yield: Results and conclusions of regional project NC-2, "Improvement of dairy cattle through breeding, with emphasis on selection"

Measurement of direct and correlated responses to single-trait selection fo r milk yield was the major objective of regional project NC-2. The NC-2 Tec hnical Committee included representatives from Alaska, Illinois, Indiana, I owa, Kansas, Michigan, Minnesota, Nebraska, South Dakota, Wisconsin, and th e USDA. All representatives, except Illinois, Kansas and Nebraska, maintain ed a selection line formed by using AI sires selected for high estimated tr ansmitting abilities for milk and a second Line that served as some type of a control. Stations varied in criteria for selection of bulls for control lines. Farms were managed similarly, including feeding and management of se lection and control lines as one herd, random mating within line, and restr icted culling policies. Selection for milk yield effectively increased milk production. All selecti on lines increased milk and net income per lactation more than control line s. Realized gains matched or exceeded gains expected from estimates of bree ding values. Yields of milk components increased, but component percentages decreased appreciably for selection lines. Reproduction of nulliparous ani mals was not affected, but days open for lactating selection cows increased in some of the individual projects. Selected cows tended to have larger he alth costs, specifically for mammary treatment. Udder and conformation trai ts did not deteriorate for selection lines, although control lines with sel ection of sires on genetic evaluations for type received higher type scores . There should be few reservations about undesirable responses correlated w ith selection for milk yield.