Yields of milk, fat, and protein have heritabilities of about .25; herita- bi1ities of percentages fat and protein are about .5 to .6. Reproductive traits have heritabilities of <.10. Cows must continue to reproduce while sustaining high levels of production. There is competition for nutrients between the mammary gland and uterus, particularly when cows conceive for the next lactation. Most evidence, to date, indicates a genetic antagonism between production and reproduction once cows calve and are under the stress of production; however, conception in virgin heifers may be complementary to production in first lactations, but the evidence is not strong on the latter. The question is whether selection for daughter reproduction can be justified economically. Mixed-model multiple- trait analyses should be applied to more complete models to attempt to better understand the relationship between production and reproduction.

A. E Freeman

Proceedings of the World Congress on Genetics Applied to Livestock Production, Volume XI. Genetics of reproduction, lactation, growth, adaptation, disease, and parasite resistance., , 3–13, 1986
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