Calving Intervals
Transcript of Select Sires' Reproductive Moment Program
on DairyLine Radio Which Aired Feb.9, 2006
With Ray Nebel,
Senior Reproduction and Herd Management Specialist, Select Sires Inc.

Ray, this week we are talking about calving intervals. What is the national average?

The US national average, Bill, is somewhere around 14 and half and 15 months. What that says is the herd turnover is every cow gets pregnant in the herd about every 14 and half to 15 months. About 20 years ago the goal was we should have a cow calve about every 12 months. With high production, larger herds and different challenges of dairy production today, that slipped from 12 months to 14 and half to 15 months. That’s not the desired goal, that’s where the national average is right now somewhere between 14 and half to 15 months.

So that’s not really where we want to be, where do we want to be?

Ideally, of course different producers have different goals, but recommendations from research say we need to be somewhere around 13 and a half. So we need to drop about 30 to 45 days on our average days open or about a month or month and half off our national average. There are a lot of producers that are there, but that really says we can get a fresher herd, have higher milk production, because cows spend less amount of time in late lactation where there is lower milk production.

So how many pregnant cows are needed to maintain that calving interval?

An easy equation is you want 100 percent of those cows pregnant; we just take what time interval, let’s just say 13 and half months so we take 100 and 13 and a half months and that says every month you need 7.4 percent of the cows pregnant and so a 13 and one half month calving interval says that every month we need 7.4 of the cows to get pregnant. We then take that times the herd size, say it is 100 cows, therefore we need seven to eight cows pregnant every month to maintain a 13 and a half-month calving interval. The next step would be, how many cows we need to breed every month to get those eight pregnancies. Again we go back and look at the records. If it has taken us three breedings per conception and we need eight cows pregnant, we need to breed 24 cows. What if we are not breeding those 24 cows, what if we are only breeding 15 cows every month? What we can do is go back and set up a synchronized breeding program and we can force, or bring in, more cows to breed, that way it says what kind of programs best fit what we need to accomplish on this farm.

A Select Sires Reproductive Solutions specialist could very well help with that, couldn’t they?

Very easily, and I guess the other point here is to look at where we are at now. To do that, we come into a herd today and say what are the days in milk? The days in milk would be from calving to where she is at this point of her lactation. That national average is 200 days, so if you take those 200 days and divided it by the average days in month, which is 30, and then multiply it by 7.4, it shows what percent of cows should be pregnant in that herd, and where they are today. At 200 days in milk it comes out to be 49 percent, so roughly half the herd should be pregnant at 200 days in milk. In contrast, if the herd is fresher or they are 150 days in milk, obviously we need fewer cows pregnant because we haven’t had the time to get the pregnancies. With 150 days of milk you only need 36 percent of the cows to be pregnant. To go in to get a baseline of where we are today, you figure out what the days in milk divided by the number of days in a month, times the desired calving interval, the number of pregnancies needed, and that tells us what percent of the herd should be pregnant today.