Why is heat detection rate so important?

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Measuring reproductive performance can be a challenge. Many of the measures give historic rather than current data. In other words, they tell us about past reproductive performance – not what's going on today. Another drawback is that they tell us we have a problem, but don't do a very good job of pointing out solutions.

How fast we get cows pregnant after they freshen depends on three things:
1. How long you wait after calving before you are willing to breed her (voluntary waiting period).
2. How effective you are at finding heats after the voluntary waiting period begins (heat detection rate).
3. How effective you are at impregnating cows once they are bred (conception rate).

The rate at which cows get pregnant in your herd will determine how many cows are likely to have extended lactations. You certainly don't want cows spending extended periods of time in less profitable portions of their lactations.

Pregnancy rate is a function of both the heat detection rate and the conception rate. In fact, to find your pregnancy rate, simply multiply your heat detection rate by your conception rate. The heat detection rate tends to be monitored less closely than the conception rate; however, it is the key to getting cows pregnant.

The table at right shows what your pregnancy rates will be at different heat detection rates if you have a 50 percent conception rate.

Another way to look at this is shown in the graphic at the bottom of this page. It shows how many cows will remain open at different days in milk for different pregnancy rates. How many cows go past 150 days in milk without conceiving is a benchmark that is used to monitor pregnancy rate.

For example, the chart shows that with a pregnancy rate of 15 percent, you would have 50 percent of your cows open at 150 days. Conversely, with a pregnancy rate of 50 percent, you would have less than 10 percent open.

I'm not so worried about the exact day a cow gets pregnant, but I don't want a lot of cows to get way out in lactation before conceiving. My goal is to have less than 15 percent of the herd still open at 150 days in milk.

I generally watch two things to estimate the heat detection rate. First, how quickly cows get bred after their voluntary waiting period (days to first breeding), and second, the number of cows which show up open at pregnancy exam. If you watch these two parameters, you will have a pretty good idea of whether your current heat detection rate measures up.

Mark Wustenberg is a veterinarian in Bay City, Ore., and
operates Kilchis Dairy Herd Services with his wife, Judy.


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30% heat detection rate x 50% conception rate = 15% pregnancy rate
50% heat detection rate x 50% conception rate = 25% pregnancy rate
70% heat detection rate x 50% conception rate = 35% pregnancy rate
100% heat detection rate x 50% conception rate = 50% pregnancy rate  



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