On Friday our monitor farms in Yorkshire and Monmouthshire saw a gap in the clouds and had a hot, sunny afternoon. Was this enough to cause heat stress?
A couple of sunny hours is probably nice, a chance to warm up and dry out (I am sure I have spotted cows ‘soaking up the rays’). However if this extends to a warm day or a hot afternoon this will be unpleasant for the cows and they will strive to change their behaviour. They will start shade seeking and may stand up more to pick up any draughts. There may be enough heat load to trigger physiological signals to reduce feed intake as that will reduce metabolic heat production. But will there be enough pathological changes induced to impact on fertility and in utero calf health?
All our heat stress monitoring is just based on monitoring the environment (just the temperature and humidity actually) and then deducing how much heat stress a cow might be feeling. No account is taken of differences in the cow’s own metabolic heat production nor her genetic capacity to tolerate heat stress nor to acclimatise to prolonged hot spells.
We need to start measuring what is happening in the cow – when does her heat load build up to an intolerable level – when does she start over-heating. We can then start to assess when we have done enough to control her problem – when can we turn the fans down?