Consequences of selection for milk yield from a geneticist's viewpoint.

The annual genetic trend for milk yield of Holsteins in the United States has accelerated with time and had means of 37 kg during the 1960s, 79 kg during the 1970s, 102 kg during the 1980s, and 116 kg from 1990 to 1996. Selection programs of the dairy cattle breeding firms in the United States have become more selective and effective with time, and selection goals continue to place major emphasis on yield traits, which clearly impact profitability of dairying. Traits other than yield are also included in selection goals of the industry. Type traits, especially those related to udder conformation, body size, and angularity have been included in selection programs and have altered the appearance and physiological functions of Holstein cows. Selection programs have continued to increase the body size of Holsteins despite mounting evidence that smaller cows have advantages for survival and efficiency. Favorable emphasis on cows that appear sharper might result in cows that are more prone to metabolic problems. The high intensity of current selection in the United States has brought about a rapid increase in genetic relationships among animals. Increased relationships will inevitably result in undesirable levels of inbreeding in the commercial cow population unless dairy producers turn to crossbreeding.