Conventional breeding programmes and genetic resistance to animal diseases.

The power of selection is illustrated by the increase in the average annual yield of dairy cows in Sweden in kg fat-corrected milk from 2,500 kg in the year 1900 to 8,200 kg in 1997. More than a half of this improvement has a genetic origin. The breeding goal for dairy cattle has successively been broadened to cover traits other than production. The prerequisites for a successful breeding programme and the consequences of the process are presented in this article. The dramatic improvement of the reproduction techniques and statistical tools for genetic evaluation of breeding stock and selection, which have occurred during the last 50 years, is also discussed. The factors which affect the genetic gain are described in detail to demonstrate the possibility of making genetic gains in traits reflecting disease resistance. The Swedish procedure of disease recording and the procedures for genetic evaluation of fitness traits, such as disease resistance and female fertility, are described. The existence and consequences of negative genetic correlations between production and fitness traits are dealt with, as is the procedure for multi-trait selection. An illustration of methods for performing multi-trait selection in real life, and the consequences of such selection in an envisaged Nordic selection scheme, are given. Finally, the authors review the present genetic trends for fitness traits of dairy cows in Sweden.