A designed selection experiment with a true control population has been conducted for over 25 years. Holstein cows in the selection line have been mated to AI bulls with highest transmitting abilities for milk yield and no other traits have been considered. Genetic trend from 1969 to 1983 was estimated to be 126 kg milk per year, and environmental trend was 18 kg milk per year. Correlated responses were decreased percentages of fat and protein, greater health costs especially for mastitis, and improved conformation. Undesirable correlated responses to selection for milk yield were more than compensated by additional income over feed cost from milk sales. True control populations usually have not been included in designed selection experiments of dairy cattle.
Proceedings of the World Congress on Genetics Applied to Livestock Production, Volume XIV. Dairy cattle genetics and breeding, adaptation and conservation., , 74–77, 1990
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