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There has been considerable traffic through my social media about the risks of heat stress to dry cows. Dahl in Florida has shown quite dramatic effects on calves, their subsequent performance and even their daughter’s performance. When you read his papers it is clear that Florida always has heat stress and just turning off the fans puts a high heat load on the animals. This is very different to the usual British weather – so do these findings apply to British cows?

Konig’s group in Germany have looked at calf birthweight data from 2005-2016 in 56 large herds in northern Germany (JDS, 2023) Their daily average THI readings (from weather stations) ranged from 10 to 74 so these data are probably more comparable to the British situation. Their analyses showed that for every ten units increase in THI the calf birth weight fell 230g. These results are in line with those reported by Dahl so they are starting to indicate that the impacts seen in Florida may happen in northern Europe and Britain (as they analysed data from 2005-2016 I would hope they will soon publish heifer performance data as well).

The biggest effect of THI on calf birth weight was seen when analysing the THI for the period 50-56 days before birth. On many farms with two dry cow groups this is the less intensively managed ‘far-off’ group and this group will very often be out grazing poor pastures. The most important practical controls for this group will be water supply and shade provision. Make sure there is enough trough space (ideally 10cm linear length per cow in the group) and the troughs fill up fast enough. These cows need ample shade provision (4-6 sq m, preferably under trees) – so a new reason to avoid having the dry cows following the milkers around the grazing platform.