By Country News on January 20, 2016

It would be a brave man that would bet against this summer being hot and dry, so we must be aware of the problem of cows overheating. All mammals including dairy cows are capable of maintaining a relatively constant body temperature in a wide range of environmental temperatures.


This ability to stabilise body temperature is essential to various biochemical reactions and processes that occur in the lactating cow. When cows are heat stressed, this constant body temperature is not maintained. The increased body temperature changes the whole metabolism of the cow leading to reduced performance and eventually (if temperature exceeds 42° C) death.


The most comfortable temperature range for lactating dairy cows

is between 5° C and 25° C. Above 25° C cows will eat less in order to reduce the amount of heat generated by digestion in the rumen. They will also redirect blood from the internal organs to the skin to try and cool down which also has the effect of reducing the rate of digestion. It has been calculated that dry matter intake is 90 per cent of normal at

30° C, 75 per cent of normal at 32° C, and only 67 per cent at 40° C.



Cows under heat stress will show the following signs and behaviours:

·      Eat less;

·      Produce less milk;

·      Become less active;

·      Seek out shade and breezes;

·      Stand in dams – can lead to outbreaks of environmental mastitis;

·      Increase their respiratory rate (open-mouth breathing);

·      Sweat more; and

·      In severe cases become recumbent and die.


As well as affecting milk production, heat stress dramatically lowers conception rates, shortens oestrus periods and delays or interrupts early development of the embryo. In hot and humid areas, conception rates as low as 10 per cent have been recorded.

It has also been shown that the fertility of maiden heifers is less affected by heat than that of lactating cows. This is likely due to the extra heat produced by milk production.


Heat stress does not prevent the occurrence of normal oestrus (heat) cycles but it will reduce the length of the heat period from 18 hours to about 10 hours with a greatly reduced intensity. This makes the job of heat detection even harder and inadequate heat

detection is one of the commonest causes of reproductive inefficiency in dairy herds.


Heat stress can also increase the incidence of early embryonic death so even though conception may have occurred, the developing embryo may die in the following two or three weeks.

It has been shown that there is a critical 48-hour period around joining when heat stress will result in embryonic death later on, giving rise to variable intervals between heats. Some herds that are milking three times daily and doing their AI after the night milking, have reported a perceived increase in pregnancy rate to AI.




Where cows are severely affected by heat stress the main priority is to reduce the body temperature. This is most readily achieved by cold hosing while applying a fan to the animal to give maximum evaporative cooling.

Avoid adding to the stress by handling the animal quietly and provide cool drinking water. Additional fluid therapy and supportive drugs may be required.




In our climate most heat stress is due to the effect of direct sunlight. The following points are some practical ways in which heat stress can be avoided or reduced:

·      Graze cows in paddocks with plenty of shade;

·      Graze cows in paddocks with the best pasture at night as they will be more inclined to eat when it is cooler;

·      Provide shade and sprinklers in dairy yards – putting sprinklers on timers (10 mins on 10 mins off) gives maximum cooling and reduces the risk of contaminating udders with run off from the cows backs;

·      Graze cows in nearby paddocks so as to reduce the distance walked;

·      Plant shade trees, especially along northern and western boundaries;

·      Hold cows waiting for AI in a cool place, not on concrete;

·      Provide fans in the dairy;

·      Get as many cows as possible in-calf early to avoid having to mate in December/January;

·      Provide plenty of cool water close to grazing areas; and

·      Erect shade shelters or have sheds over feed pads.

By -       Rochester Veterinary Practice

By Country News on January 20, 2016

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