An upcoming management decision is when and how to pregnancy test. If heat detection were perfect – every pregnant cow didn’t show heat and every empty cow did show heat – onfarm heat detection could do the job predicting when cows conceived and when they would calve.
However, about 5% of pregnant cows will display heat at some stage, and about half the cows that aren’t pregnant won’t show heat.
The job of detecting cycling cows becomes harder as fewer of the cows remain non-pregnant. Where there are fewer than three cows in heat at any time, detection rates decline.
The value of pregnancy testing depends on how much better active pregnancy testing is compared to observation alone, the cost of the testing, the timing of culling following pregnancy testing and hence the replacement cost minus cull value and potential savings in grazing costs.
If we have a 500-cow herd and 10% are truly not pregnant at the end of the mating period, we would expect about 98% of those defined as pregnant by ultrasound or manual pregnancy to calve. The reason this isn’t 100% is 1-2% of pregnant cows fail to carry the calf to calving. Conversely, some truly pregnant animals might be missed. The research figures suggest about 99% of the animals that are called pregnant will calve, and 85% of those that are called not pregnant will truly be empty.
To put that in absolute numbers, there might be 5-10 cows in a 500-cow herd that are incorrectly categorised after pregnancy testing. In comparison, if we relied on heat detection, 95% of animals not showing heat and assumed pregnant would calve, and 20 animals would be carried through that wouldn’t calve.
Of those cows assumed empty we would expect about 20 to show heat but be pregnant. If relying on heat detection we might end up with 40 animals incorrectly classified as pregnant or empty. The associated costs include carrying through empty cows and increased replacement costs for cows falsely culled.
The costs associated with carrying an empty cow through could include grazing off. So if, for example, we pay $25 a week for eight weeks for grazing, that’s $200 per animal falsely assumed pregnant. The costs associated with a false negative are more complex but where it results in a higher replacement rate, the direct cost of rearing more animals is incurred and-or a restricted ability to cull cows of lower genetic merit or high risk of other
diseases.
Assuming a 500-cow herd with a 10% empty rate, a $500 differential between a cull and a replacement animal, a cost of winter grazing for cows of $20 a week, and a cost of pregnancy testing of $3.50 each, pregnancy testing results in a net benefit of more than $10,100 for the herd for an expenditure of less than $1850, compared with heat detection. If we assume stock are wintered at home and fed 10kg drymatter/cow/day at 10c/kg DM, we are still making money by pregnancy testing and removing empty cows.
Pregnancy testing is most accurate between five and 12 weeks after a mating. If tested in this window fewer than 10% of cows will calve more than 10 days before or 10 days after the due date. Some cows will have a short or long gestation and so will calve some days away from due date. When pregnancy testing happens 15 weeks after a mating, nearly 30% of cows will calve more than 10 days early or late.
This is because of the difficulty of differentiating between a 12 and 15-week pregnancy. If the total length of mating is only 11-12 weeks, it might be tempting to pregnancy test the whole herd at 16 or 17 weeks after the planned start of mating.
However, the precision of expected calving dates will not be as good as splitting the pregnancy testing into two, at say 12 and 17 weeks after planned start of mating. This approach will result in about 25-30% of cows needing to be retested, depending on what proportion got pregnant in the first seven weeks of mating.
This equates to an additional cost of about $500 for our 500-cow example herd. Benefits of this approach are earlier culling decisions – important if we are heading into a dry summer – more precise calving spread estimates with ability to do better feed budgets and timing feed purchases, more precise drying-off times, ie being able to dry off late calvers later – depending on feed – and get more days in-milk, and earlier and more precise estimates of the reproductive performance of the herd.
Even in tight times, fundamental farm management tools remain cost-effective. Each cost needs to be evaluated and those that provide a return on investment should be continued.
