Pregnancy Rates (Part One)
Transcript of Select Sires' Reproductive Moment Program
on DairyLine Radio Which Aired Feb. 10, 2005
With Ray Nebel,
Extension Specialist and Professor of Reproductive Management,
Virginia Tech
This week’s Reproductive Moment is with Ray Nebel. Ray, this week you are talking about pregnancy rates.
Pregnancy rates, Bill, are really the combination of heat detection rate, times conception rate. So, another way to look at it is how fast cows get pregnant. It is the number of cows that actually do get pregnant, divided by the number of cows that are eligible to get pregnant.[*] That is figured using a 21-day period, which is the average estrus interval. In any 21-day period how many cows actually got pregnant, and how many were eligible to get pregnant. The three most popular records-processing centers in the country -- Dairy Comp, PC Dart and Dairy One -- all have some derivation of pregnancy rate, but they look at it in a 21-day period. They calculate the number of open cows that are eligible to breed, and during the 21-days how many of them actually did become pregnant? So what's your rate, or how fast did your cows get pregnant?
What is the average nationwide, and what should be a goal?
The national average for pregnancy rate is 14 percent. So, 14 percent of the eligible cows are actually getting pregnant every 21 days. Now that seems really low, and most dairymen are not used to thinking of it in those terms – they are used to a conception rate. But, the normal conception rate we talk about is 35 to 40 percent. So, you can see where it is kind of a culture shock when they first hear of a pregnancy rate of 14 percent. But, it is different. A conception rate is how many times your cow would come up with a pregnancy divided by how many times she has been bred. If you breed her two times and she gets pregnant, that is a 50 percent conception rate. But that doesn’t take into account that we miss a heat. Therefore, pregnancy rate takes in both heat detection and conception. So, the 14 percent average seems low, but when you look at what it is really doing, that is where we are.
How do you go about measuring or monitoring them?
Measuring or monitoring is really monitoring the pregnancy rate. On DHI records – Dairy Herd Improvement programs, they record when cows calve, and when you want to start breeding them. We call that period in between the voluntary waiting period. So, in cows past the voluntary waiting period, we would expect cycling, that they would be identified in estrus every 21 days. If we take an example of a voluntary waiting period being 60 days, it says cows past 60 days should be in heat every 21 days. So, those cows should be identified. Then, after an interval of 45 days, which is the normal interval that we check pregnancies, how many of those cows were bred and conceived? So, it calculates a pregnancy rate for every 21 days. There are three different ways that pregnancy rates are evaluated.
First, by date. What that means is you would run a date list and it says every 21-day interval usually goes back nine months because the average gestation length is nine months. So for every 21 days it asks, "Of the cows that were eligible to become pregnant in that 21-day period, What percentage actually got pregnant?" In the summer you would expect it to go down into single digits, especially where there is heat stress down in the south or out in the southwest, or western part of the U.S. Then, at this time of year, there could be some of our highest pregnancy rates, where we should be getting at least low twenties to mid-twenties. Pregnancy rates of 20 to 25 or 27 are not uncommon for this time of year. The goal for the whole year should be somewhere around 20 percent. The twenty percent pregnancy rate is a very acceptable pregnancy rate.
The second list is by lactation number. That is the pregnancy rate for first lactation, second lactation and older cows. This is a way to identify what group of cows you are having the most problems with.
The last list is by days-in-milk. Starting at your voluntary waiting period, how fast are you getting cows pregnant based on when you want to start?
Those three lists, by date, by lactation number and by days-in-milk, would be a critical way to look at pregnancy rates.
Ray Nebel, Extension Specialist and Professor of Reproductive Management at Virginia Tech.
* Example: In a certain 21-day period, 200 cows were eligible to get pregnant, and 30 cows actually got pregnant. To determine the pregnancy rate, divide the number that actually got pregnant (30) by the number that were eligible to get pregnant (200). 30/200 = 0.15, then multiply that by 100 to get percentage, in this case 15 percent.
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