Friday, March 29, 2024

Is that cow pregnant?

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Pregnancy testing is a routine procedure, but the value of the information will depend on how accurate it is, when testing occurs and how the data is subsequently used.
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Pregnancy testing determines whether the cow is pregnant. This is useful information to calculate the overall empty rate and to make individual cow culling decisions. But dated pregnancy testing provides additional information useful to make individual cow decisions such as dry-off dates and to calculate the six-week in-calf rate and expected calving pattern for the herd.

Real-time ultrasound (scanning) or manual palpation can provide information about pregnancy status (yes or no), stage of pregnancy and pathology.

When testing occurs in the best window, between about 12 and 14 weeks after planned start of mating, it can be very accurate with 95% of the dates of conception estimated by pregnancy testing within a week of actual conception date (see Figure 1).

However, not all cows calve exactly 282 days (the length of pregnancy) after a confirmed conception date as there is variation in pregnancy length, with 95% of cows calving within 10 days either side of the expected calving date. Turning this around, 5% of cows (1 in 20) will calve more than 10 days before or more than 10 days after the due date.

Further variation occurs where service dates are unknown, as commonly occurs once the bulls are out, and estimated conception dates can’t be corrected to known service dates. In this case, variation in actual calving date from the date estimated from pregnancy testing is due to the variation in the estimate of how many days pregnant and the variation in gestation length.

In this situation even very experienced scanners will end up with a small number of cows calving more than 20 days from the expected calving date. About five to 10% of cows that are confirmed pregnant about five weeks after a specific service will undergo embryo loss. Where early pregnancy testing is undertaken and cows are not reconfirmed as pregnant, a proportion of the cows confirmed pregnant at the first test might abort and turn up either empty or calve several cycles later if they re-conceive.

There have been many attempts to develop indirect ways of determining pregnancy.

Cows which don’t conceive to a given service are likely to have low progesterone concentrations in blood and milk at about three weeks after that service. If samples are collected at days 18, 21 and 24 days after mating, and if progesterone is low in one or more of these samples, it is very unlikely the cow is pregnant.

However, false positive progesterone pregnancy tests occur where a cow is incorrectly bred in the middle of the cycle (and hence 21 days later will be in the middle of the next cycle) or where pregnancy loss occurs and the corpus luteum (the source of progesterone) has not undergone luteolysis by the time of testing.

Progesterone pregnancy testing also requires collection of milk samples at precise times after breeding, which in a large herd is logistically challenging.

Proteins produced by the foetus, pregnancy specific proteins (PSPs) appear in blood and milk from about 30 days after conception and can be measured.

PSPs are reasonably reliable in determining whether a cow is or isn’t pregnant, but can’t provide information on stage of pregnancy. These tests also require collection of samples from individually identified cows, submission of those samples to the lab and a time lapse to reporting the results.

Scanning or manual pregnancy testing remain the quickest and most cost-effective pregnancy tests. Like any test they are not perfect, but precision can be improved by pregnancy testing at the right time (between about five to 14 weeks after a mating), comparing against known AI dates, ensuring that cows are clearly identified (to minimise recording errors) and providing a good physical environment for the person doing the testing to minimise fatigue.

Scott McDougall is a vet with Cognosco, Anexa Animal Health, Morrinsville. He will be writing this regular column on a two-monthly basis alternating with another vet.

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