White cows are more prone to feeling the bad effects of prolonged sunshine at very high temperatures. In some cows, after prolonged sunshine you start to see red skin blotches on the pink skin under the white hair that can cause irritation and annoyance to the cow.

Photosensitivity, sometimes referred to as a sun allergy, is an immune system reaction that is triggered by sunlight. Photosensitivity reactions are usually characterised by an itchy eruption on patches of sun-exposed skin. The best thing to do for affected animals is to keep them indoors while the sunshine continues. Some vets will prescribe an anti-inflammatory injection depending on the extent of the irritation.

Water

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The main thing to do is to make enough clean water available to cows on these very hot days. Spring-calved milking cows producing over 20 litres per day will consume up to 70 to 80 litres (15 gals) per head per day on very hot days. Try to let cows access to extra water troughs on the way to and from the parlour so the field troughs are under less pressure and young cows get a better chance to compete for available water.

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Photosensitisation: what is it and how to treat it