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Economic trade-offs between replacement rates and improved genetics
Genetic progress in sires used for artificial insemination (AI) is rapidly increasing. This means that replacement heifers are increasingly much better genetically than cows. Economically, this should lead to increased voluntary culling and thereby decrease cow longevity. On the other hand, lower culling rates are often viewed favorably because the costs and environmental impact to maintain herd size are generally lower. Thus, there is an economic trade-off between genetic progress and longevity in dairy cattle. Objective was to investigate these trade-offs.
USDA results show that the annual increase in average predicted transmitting ability (PTA) of Net Merit dollars of Holstein sires is accelerating from $20/yr when the sire entered AI in 2000 to 2004, to $52/yr in 2005 to 2009, to $86/yr in 2010 to 2014. We expect that heifers born in 2015 are about $50 more profitable per lactation than heifers born in 2014.
An elegant but older study is from Allaire (1981). He found that the economically optimal cull rates were in the range of 25% to 27%, compared to the lowest possible cull rate of 20%. There was only a small effect of using the best surviving dams to generate the replacement heifer calves. Genetic progress from sires also had little effect.
Using a spreadsheet model to determine genetic lag in Net Merit PTA between service sires and dams shows that increased cow cull rates reduce the genetic lag only marginally. The ratio of annual genetic trend in sires’ PTA for Net Merit and genetic lag was 6.6%, 7.7%, 8.7% and 9.4% for the annual cull rates of 20%, 30%, 40%, and 50%, independent of the magnitude in sire genetic trend. These results confirm the findings of Allaire (1981) that cow depreciation costs overwhelm the value of the genetic superiority of the replacement heifers.
Van Arendonk (1985) showed that the effect of changes in genetic improvement in milk revenue minus feed cost on herd longevity was relatively small. Reduced involuntary cull rates improved profitability, but also increased optimal voluntary culling. Finally, an economic optimal culling model with prices from 2015 confirmed that optimal annual cull rates were insensitive to heifer prices and therefore insensitive to superior genetics in heifers.
In conclusion, economic cow longevity depends more on the difference between heifer raising costs and cow cull prices than on genetic progress. This is confirmed by old and new studies.
Keywords: genetics, longevity, economics