Hot, Hot Heat!

Hot, Hot Heat!

Heat Stress – Hot, Hot Heat!

No, it’s not an article about the Canadian indie band from ‘the noughties’! It’s the main stress our livestock are contending with currently, and it’s very obvious we need to be aware of heat stress. Rob Hall has seen the impact of extreme heat first hand when working in the USA and fortunately is able to offer some solutions to us in our hour of need!

Summer is well and truly with us, with the hot weather taking its toll on humans and cattle alike! Heat stress can be a huge problem for cows even on days which don’t seem excessively hot to us, with both milk production and fertility being affected. Yield may fall by as much as 20%, and dramatic dips in fertility are commonly seen. The effects of heat stress are long-lasting and the economic consequences can be disastrous if not carefully managed.

Historically high summer temperatures have not been a problem that many in the UK have had to worry about, but evidence is increasing to suggest that even the relatively low temperatures of British Summertime can lead to depressed dry matter intakes, lower milk yields, and reduced fertility through reduced estrous behaviour and poorer conception rates. There is also evidence to suggest that immune system function is depressed, leading to higher rates of mastitis and other infections. Lameness is increased as cows stand for longer to lose heat more effectively, leading to more sole bruising and sole ulcers for over two months after the heat stress incident.

The concepts of ‘stressors’ acting on dairy cattle was outlined by Bill in the previous newsletter. Cows can seemingly shrug off minor causes of stress and continue their productive lives, but multiple small stresses can amount to big problems. The cow which happily goes about her business getting in calf and producing milk regardless of the inconvenient ‘stressors’ may be the one who is pushed beyond the tipping point into illness by hot weather.

As with all stresses, the weather only becomes a problem when a cow can no longer adapt to deal with the issue. In the case of heat stress temperature isn’t the only factor – high humidity dramatically reduces the ability of cows to lose heat. Similarly, poor air flow increases the humidity of the air around the cow. The most accurate climatic parameter is the temperature humidity index (THI); a value which combines cow-side temperature
and humidity. A THI value between 72 and 79 indicates mild stress, 80 to 89 indicates moderate stress, and THI higher than 89 indicate severe stress. New evidence from the University of Arizona suggests mild heat stress can occur as low as THI 68 (22oC and 45% humidity), with milk yield losses from this point upwards. Even ‘mild stress’ can cause major problems and occurs at temperatures humans find comfortable. Waiting until we feel the heat is too late!

Temperature Humidity Indexz

THI scales show heat stress can be present at temperatures as low as 22oC if humidity inside buildings is high.

The key to tackling heat stress is identifying it before production losses and sick cows are seen on farm. To do this the most important indicators are respiratory rates above 60 breaths per minute and rectal temperatures above 39.2 °C. These will often be seen alongside changes in behaviour such as cows bunching toward the centre of buildings and increased standing

Focus on fresh cows

During the summer months, fresh cows are more susceptible to metritis, mastitis, ketosis, and other diseases as the heat reduces feed intake and decreased immune function during the critical transition period. Fresh cows are the group least able to cope with heat stress and act as an early warning sign for heat stress in the herd. Any decline in transition health should be investigated and aggressive interventions made to reduce the impact of the climate. Higher yielding cows are at greater risk of heat stress, as are larger and older animals. This is due to higher dry matter intake causing more heat to be generated by fermentation in the rumen, with additional heat generated by milk production.

Water access

Water intakes are variable and typically average 75 to 115 liters per THI scales show heat stress can be present at temperatures as low as 22oC if humidity inside buildings is high. Cows will often pant in an attempt to lose more heat in hot weather cow per day. However, in hot weather the same cow may require 190 to 230 liters per day – over twice as much! Cows that are producing more milk will need even more water. To ensure good intakes there must be at least 10cm water space for each cow, and at least two troughs for each group larger than 10 cows. As social creatures, cows like to drink in groups and will drink around 60% of their daily requirement straight after milking. It is recommended to use large volume troughs supplied with a minimum flow rate of 20 liters/ min to ensure troughs aren’t empty at peak times.


Studies have shown a 10% to 20% increase in milk production for cows offered shade in pasture compared to those without shade. For lactating cows this is essential, but this it equally important for dry cows and young stock. For fields without trees temporary shades can be moved around to avoid poaching areas of the field. Portable sun shades for cows at pasture can reduce poaching of shaded areas, and therefore reduce mastitis.

Focus on the collecting yard

One of the hottest places on the dairy farm is the collecting yard where the high density of cows prevents heat from escaping. Cows need a minimum of 3.5 to 4.5 m2 to allow adequate heat loss. This can be achieved by moving smaller groups of cows into the yard to reduce stocking density and promote heat loss. A large diameter fan above the cows can also help with cooling in this high-risk location. A similar problem exists in many parlours, where fans can help reduce the heat burden.

Building modifications

Simple building modifications can be made to improve airflow: removing alternate boards from walls, opening roof ridges and changing doors for gates without metal panelling. New skylights allow huge amounts of sunlight to heat the inside of the building – these can be painted to reduce the amount of light getting in, especially on south facing roofs.


While not the cheapest option, fans are fantastic at cooling cows by moving hot humid air away from them. Choose fans that are 36” to 48” wide and place them 8’ off the ground, 20’ apart. They should be angled downward at 15° to 25° to create continuous air flow through the entire length of a building. Aside from fans in the collecting yard and parlour, fans should only be considered after cheaper building modifications have been considered and implemented. However, costs can be reduced by using tubing to deliver streams of air further away from the fan. This reduces the number of fans required and can often be more effective at maintaining high enough air speeds to provide a cooling effect at cow level.

Sprinklers & Soakers

Those of you who have spent time in the USA will be familiar with sprinkler systems to either ‘mist’ or ‘soak’ the cows in the collecting yard or at the feed face. This helps cool cows by drawing heat from the cow as the water is evaporated. However, this evaporation is much less effective at higher humidity levels. It is not unusual for our UK weather to lead to relative humidity in excess of 80% for weeks on end. Obviously, spraying water around in a confined space rapidly increases this humidity further! For this reason sprinkler systems are far less effective in the UK than the USA and may even be counter-productive.

Dietary changes

Decreasing concentrates and supplementing fats can increase the energy density of the diet while decreasing the heat produced by rumen fermentation. However, it is wise to discuss this with your nutritionist to avoid creating new problems in trying to solve the original problem! As a general rule, do not increase the fats above 6.5% of dry matter. Decreasing the forage content or feeding higher quality
forages will also reduce the heat from fermentation.

Don’t add to the stress!

Avoid working cattle (moving, sorting, or transporting) or giving vaccinations on very hot days. Anything that adds to the stress of the cattle can make the difference between a cow that can cope with her heat stress and a cow that is pushed over the edge into illness.

For further information on how to get the best out of these principles speak to any of the vet team.

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2018-07-17T09:18:53+00:00June 27th, 2018|Dairy, Management|

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