Tally Time – Measuring reproductive responses

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Operations that use artificial insemination (AI) and synchronization of estrus continually strive to increase pregnancy rates to AI. Working towards an improved response is a good thing, however it could be that you are already doing as good a job as your neighbor, but you are measuring the response differently.

Let’s start with the year I achieved a pregnancy rate to AI of 100 percent. At this extreme result, most would ask about how many head were involved, but they may not ask if the reported result was in the 60 to 80 percent range. In this case, only 3 females were inseminated and the results were either going to be 0, 33, 67 or 100 percent pregnant to AI. Reports of excellent responses mean more when you know how many animals were involved. If there are 20 animals total, one animal more or less pregnant changes the outcome 5 percentage points and if there are 30 head total, one animal changes the outcome by 3.3 percentage points. While our research efforts generally include an entire group of cows, others may selectively AI only the mature cows or only those that calved in a certain time period. These may be sound management choices for a particular producer, but may not be directly comparable to your results.

Often people use the terms pregnancy rate and conception rate interchangeably. The most accepted definitions for these terms are: pregnancy rate equals number pregnant divided by number synchronized or treated and conception rate is number pregnant divided by number inseminated. In the example of animals bred after detected estrus in Table 1, 90 animals are detected in estrus of the 100 animals treated and 54 are determined to be pregnant to AI; conception rate=54/90=60% and pregnancy rate is 54/100=54%. For AI after detection of estrus, conception rates will be higher than pregnancy rates. If animals are only inseminated after observed heat, the conception rate may be relatively high, but the pregnancy rate low because of failure to detect many animals in heat. In the case of fixed-timed AI, conception rate will be equal to pregnancy rate because all animals are inseminated. Pregnancy rate to AI is the key number from a practical standpoint.

Another factor that can influence results is the method of determination of AI pregnancy. Failure to return to estrus after an extended AI period is a reliable method especially if combined with actual calving dates. Other methods used might be based on a pregnancy determination, calving dates, calf color or DNA parentage test.

Table 1. Conception rate and pregnancy rate after estrus AI or fixed-timed AI.

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Currently, the most accepted method to report research results from synchronization of estrus and AI is to hold natural service sires out for 10 days after AI and pregnancy determination generally occurs between 30 and 40 days after AI with the use of ultrasonography. Variation in fetal growth rate increases the opportunity for error mis-identifying an AI pregnancy as pregnancy advances.

Figure 1 shows calving data from 1476 cows that were part of a multi-state synchronization research project where AI pregnancies were determined as described above and calving dates recorded for all cows. The shaded area indicates the overlap in calving dates between the cows confirmed pregnant to AI and natural service sired calves (overlap days 289 to 296 after the prostaglandin injection given just prior to insemination).

When comparing your results to others, keep in mind the potential differences in measurement technique that may influence your interpretation. If you want to compare your time in running the mile to mine, I’ll be using a sun dial to measure my speed.

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Source: Sandy Johnson, livestock specialist


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