SELECTION LIMITS: HAVE THEY BEEN REACHED WITH THE DAIRY COW?

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Abstract
Genetic theory suggests that at some point genetic progress for milk production might plateau either through exhaustion of genetic variability or through development of antagonistic genetic relationships between milk yield and components of fitness. Although there have been no long-term selection experiments with dairy cattle, empirical evidence from field data indicates that selection limits for increased milk production have not been reached nor will they be in the foreseeable future. The rate of genetic improvement in milk yield is accelerating. Rather than witnessing a decline in genetic variability, as genetic theory would indicate, we seem to be experiencing an increase in genetic variability as production levels increase with time which is likely due to improved management allowing for greater expression of genetic variability. There is some evidence of genetic antagonisms between milk yield and fitness traits, fertility and health measures in particular, and this could impose a limit to selection for increased milk production. The solution to this problem is probably through improved management of high producing cows, to reduce stress associated with high production. Key words: Dairy cattle, selection, genetic variation

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