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Last week was a major heat wave; many areas reached over 30’C every day of the week: some of the hottest September weather ever recorded. The weekly average THI scores showed heat stress on all of our monitor farms throughout Britain and, for all but the northerly farms, the stress averaged a Moderate rating with predicted milk losses generally over 2.0 l/cow/day. Such losses are predicted from the THI values but agree closely with the actual losses reported by our monitor farmers.

Heat stress of this magnitude accumulates – cows do not clear their daily heat load overnight and it builds through the week. It then takes several days to clear the back-log so cows will be suffering for the first part of the week and any mitigation should be kept running at full capacity.