Heat stress and lameness

It has been a dry hot summer for most of the country so far this year, and it doesn’t look like letting up any time soon. There has been less lameness because of it but that doesn’t mean that it will stay that way. If you don’t manage this hot period properly you may experience a lot of lameness in the autumn. The reason for this is that underfeeding and heat stress are both contributors to lameness at a later stage. In NZ there is already a tendency for underfeeding even at the best of times, although there will be many people disagreeing with me on that one, but how many farmers have experienced an increase in milk production when they milk fewer cows? So, when we are in a drought, underfeeding is even more prevalent – especially if there is a shortage of silage or other feeds available.

The other issue is heat stress. Not much research has been done on the effects of heat stress on lameness, but there is evidence of a link. When a cow is heat stressed her first response is to decrease DMI. This makes sense because as the rumen processes the food it creates a lot of heat. This reduction in food intake has a direct impact on milk production. A cow will also spend more time standing. Often on hot days you can see herds of cows just congregating around the water trough. This is a classic sign of heat stress. The temperature in which heat stress occurs depends on humidity and wind speed.

The higher the humidity and the lower the wind speed the lower the heat stress threshold will be. It is believed that the optimum ambient temperature for a cow is between 5-15 ֯C so heat stress could kick in as low as 20 ֯C.  That puts it into perspective a little especially when you look at how hot it can get in the holding yard and in the cow shed. If you are finding it hot, the cow is probably boiling. Excactly what it is about heat stress that causes an increase of lameness is a question that needs more research. It could be the prolonged period of standing, it could be the reduced DMI, and it could be the stress hormones released at the time of heat stress. I am inclined to think they all have a part to play. I guess the important question is how we manage it. Fortunately, in NZ the nights are not as hot compared to Australia, North America and other places in the world where the temperatures are well in the 20’s day and night for weeks on end. Some practical things you can do to combat heat stress are: put misters in the cow shed and maybe sprinklers in the yard. With the sprinkler make sure the water is not too cold as it is not nice to get hosed down with ice-cold water when you are very hot – that would induce a different stress! Another thing that would help is to reduce or take all the minerals out of the water all together on hot days.

You may be nervous about that but water is a much more important mineral than copper or any other mineral you put in the water. Adding the minerals to the water makes it horrible to drink and therefore decreases the water intake. It’s also important to ensure your water troughs are cleaned regularly, you don’t clean your dishes once a month (hopefully), and neither should you clean the water troughs infrequently. It may also be necessary to consider improving accessibility to water for all cows through installing additional water troughs to counter the effects of herd hierarchy.

As always I welcome your comments and observations info@dhi.ac.nz

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