CULL RATE CONSIDERATIONS

Part 1 of this series, Cull Rate Calculations, explained how culling metrics are calculated, with the theme, if you don’t measure it, you can’t manage it. The counterpoint to “If you don’t measure it, you can’t manage it,” is called McNamara’s Fallacy – named for former defense secretary Robert McNamara who reputedly made errors because he disregarded things that could not be measured or counted. The point is, we need to apply some judgement to the numbers we calculated in part 1. I recall doing cull rate calculations when I nominated a herd for a DCRC award several years ago. I was shocked to discover that this high performing herd had a cull rate of about 50%. I dug deeper and cull rate considerations showed that at least half of the cows that were coded as culls were sold for replacements.

Cull rate is an essential tool for managing future herd size. You need to know how many cows are leaving the herd in order to determine how many replacements will be needed to take their place. Cull rate is not a good tool for evaluating herd management efficiency. One farm might have 20% cull rate because the supply of replacements is managed and cows stay healthy and stay in the herd for a long time. Another farm might have a 20% cull rate because fertility is low, replacements are not being produced fast enough, and unprofitable cows stay in the herd to fill space in the barn or on the balance sheet. A 40% cull rate might mean that cows have to go because they are unhealthy or unproductive. That same 40% cull rate might mean there is an opportunity to sell replacements like the earlier example. Managed culling, with a planned cull rate ensures a supply of replacements so you don’t have to milk unprofitable cows that you would rather not milk.

Cull rate is usually calculated on an annual basis but robot performance is very sensitive to fluctuating cow numbers. The number of animals culled each month may be at least as important as the number culled each year. Overcrowding robots can increase fetching and decrease production per cow. Understocking robots means underutilized capacity for production. The examples below show the effect of seasonal fluctuation in cow numbers and culling patterns.

The blue bars represent total daily milk production and the orange line represents the number of cows. In graph (1) cow numbers fluctuate through the year and production fluctuates even more. On the right side of the graph, cow numbers are increased in an effort to maintain production, but unproductive cows are retained, production per cow drops, and total production drops too. Inefficient use of the robots is compounded with lower producing cows, with less drive to the robot, and lower production per milking.

Graph 2 shows how managed culling, matched with a planned supply of replacements results in consistent production throughout the year. That means consistent use of the robots. It also means consistent use of maternity pens, and calf and heifer raising facilities

Culling decisions for individual cows are based on health, production, and longevity. On a herd basis, culling decisions are actually based on breeding decisions. Breeding decisions determine when replacements are available. When replacements are available, cows leave the herd. The next articles in this series will focus on those breeding decisions and the replacements coming into the herd.