On my regular routine fertility visits lately I have noticed fewer pregnancies than I would normally expect. Additionally, I have scanned at least six cows that are either losing embryos or look like they will lose embryos. It has made me reflect on what has been going on in recent weeks.
An obvious event was the heat wave we had at the beginning of September. What was notable about that heat wave was just how hot it was at night, combined with very high humidity levels. In reality, milking cows are under heat stress when the temperature is above 16 degrees – even more-so when it is humid. Anyone who works with livestock will be aware of the behavioral impact on animals in the heat – increased respiration rate, standing instead of lying, bunching under trees or in parts of the shed that we didn’t realise were cooler.
Heat stress causes all sorts of unwanted processes physiologically. It is thought that heat stressed cows get ‘leaky guts’ so that bacteria and toxins, usually contained within the gut, are absorbed into the blood stream, causing widespread inflammation. As a result, the stress caused by the heat is compounded. Stress factors such as these, even if short lived, might lead to embryonic loss. The most vulnerable time for a new pregnancy is from 4-12 days after service, when the embryo is undergoing considerable growth. The embryo itself is tiny and the chemical environment it is exposed to can have a massive (and potentially detrimental) impact on the young embryo. So thinking back, and considering the cows scanned this last week, they would all have been served 30-40 days ago, and therefore very vulnerable to embryo loss.
Heat stress is a difficult problem to deal with. Our temperate climate potentially makes the problem worse because heat is short-lived, which means cows are not adapted for it. It tends to be accompanied by high humidity, which reduces the cows ability to shift heat. Many farms don’t have the infrastructure – such as big fans – to help modulate the stress. Heat stress is here to stay and I expect that more and more farmers will find themselves investing in fans in the future. Let’s hope the government recognises the impact on health, welfare and production and begins to offer grants on forced ventilation systems.
Graeme McPherson, Veterinary Surgeon