CONVENTIONAL BREEDING PROGRAMS AND GENETIC-RESISTANCE TO ANIMAL DISEASES

The power of selection is illustrated by the increase in the average a nnual yield of dairy cows in Sweden in kg fat-corrected milk from 2,50 0 kg in the year 1900 to 8,200 kg in 1997. More than a half of this im provement has a genetic origin. The breeding goal for dairy cattle has successively been broadened to cover traits other than production. Th e prerequisites for a successful breeding programme and the consequenc es of the process are presented in this article. The dramatic improvem ent of the reproduction techniques and statistical tools for genetic e valuation of breeding stock and selection, which have occurred during the last 50 years, is also discussed. The factors which affect the gen etic gain are described in detail to demonstrate the possibility of ma king genetic gains in traits reflecting disease resistance. The Swedis h procedure of disease recording and the procedures for genetic evalua tion of fitness traits, such as disease resistance and female fertilit y, are described. The existence and consequences of negative genetic c orrelations between production and fitness traits are dealt with, as i s the procedure for multi-trait selection. An illustration of methods for performing multi-trait selection in real life, and the consequence s of such selection in an envisaged Nordic selection scheme, are given . Finally, the authors review the present genetic trends for fitness t raits of dairy cows in Sweden.