Not every cow diagnosed as pregnant will calve, but this pregnancy loss can be caused by a huge variety of factors and it can be difficult to know when you have a problem that needs further investigation.
Pregnancy loss typically shows up in two different forms:
The causes of pregnancy loss vary depending on when it occurs. Infectious causes of losses in early or mid-pregnancy include viruses (BVD & IBR), Leptospirosis and the parasite Neospora. In late pregnancy there are a much wider range of reasons for abortion, with common causes including fungal abortion (due to feeding of mouldy silage), Neospora, and consumption of Macrocarpa pine. However, there is a certain amount of pregnancy loss that is unavoidable, and you will always have some cows that show up empty after a positive pregnancy diagnosis. This is due to a wide variety of non-infectious factors that can also contribute to pregnancy loss.
It can often be difficult to work out whether the pregnancy loss your herd is experiencing is “normal”, and invisible pregnancy loss costs the farm money through feed (and possibly grazing fees) for unidentified empty cows. A 300-cow herd doing their pregnancy testing in February can expect around 11 cows to lose their pregnancy after scanning, but up to 20 cows could lose their pregnancy before this is considered too high. Not every cow that loses her pregnancy between scanning and dry off will show visible heats, so an additional pregnancy scan in late lactation can be useful to identify extra empty cows. In the example 300-cow herd described above, we would expect a pregnancy scan in early May to detect seven to 10 extra empty cows. These can then generate extra income by being sent to the works instead of consuming feed through the dry period, being treated with dry cow therapy, and possibly increased grazing costs where dry cows are grazed off farm.
If you are concerned about the pregnancy loss levels in your herd then your first step is to talk to your vet. The number of normal unavoidable pregnancy losses varies for each herd depending on factors like herd size, timing of your pregnancy testing, and type of pregnancy testing used. Milk pregnancy testing can result in apparently higher levels of pregnancy losses because it takes several weeks for the pregnancy hormone to decline in milk after a pregnancy loss, so cows can be diagnosed pregnant for a short period after they have in fact lost the pregnancy. If your herd’s pregnancy loss rate is too high then a variety of diagnostic testing can be used to identify the cause, including blood testing the empty cows, and examination of the aborted calf and placenta (if available). It is not possible to diagnose the cause of pregnancy loss for every cow, but we have the highest chance of achieving a diagnosis when we can collect both the aborted calf and at least part of the placenta.
Please talk to your vet if you are concerned about the pregnancy losses in your herd, or if you are interested in the benefits of pregnancy scanning prior to dry off.