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Bennet Cassell
Extension Dairy Scientist, Genetics & Management
(540) 231-4762; bcassell@vt.edu
Dairy farmers have been selecting for higher production long enough that several “correlated responses” have appeared in dairy cows. For instance, experience (and data) have taught us that somatic cell scores go up and fertility rates go down if selection is only for higher milk yield for several generations.
The industry responded by developing genetic evaluations for somatic cell score and daughter pregnancy rate. Selection for lower cell scores and higher fertility, in conjunction with higher yields, is turning the tide of genetic change in a favorable direction for these two traits. Higher producing cows have increased appetites and tend to be more aggressive eaters. Does this characteristic affect their growth rates as heifers?
A long term study at the Scottish Agricultural College in Edinburgh, Scotland compared Holsteins selected for maximum production of fat and protein to controls selected for average production. Complete details are in the Journal of Dairy Sci. 89:322-329.
Heifers in the maximum production line grew faster and were heavier at first calving than heifers in the control line. The size differences disappeared by the end of third lactation, however. So – there has been a correlated response to selection for higher yields, as growth rates and size at first calving have incr eased.
Most producers would consider these to be favorable responses. However, the changes call attention to the need for better heifer nutritional programs to meet growth requirements than were necessary a few generations ago.
The dairy cow of today is a highly specialized beast and requires increasingly specialized management systems to reach full genetic potential – even as a heifer!