Management and Feeding during Heat Stress

27 June 2018

This week has seen record temperatures throughout Ireland and therefore the risk for heat stress is at its highest. Heat stress occurs when the heat that a cow produces is greater than her capacity to lose heat.

Heat stress can have significant detrimental impacts on milk production, reproductive performance and immune function. Research has shown that the negative effects on reproduction due to heat stress occurs before milk production declines.

Cows are particularly prone to heat stress due to the relatively large amount of heat generated from the rumen digestion of feed. During heat stress, cows will spend more time standing in an attempt to lose more heat, which may put more pressure on feet resulting in lameness. High yielding cows generate more heat than low yielding cows and are at more risk of heat stress. In addition, older cows are less able to deal with heat stress compared to younger cows.

The cow’s comfort zone (also known as the thermoneutral zone), has an upper limit of 25 oC and at temperatures above this, the cow is at risk of heat stress. Both temperature and humidity affect the amount of heat stress that cows undergo. Using the heat stress information from American studies and inputting our recent conditions (temperatures up to 30oC at a typical humidity of 55%), cows are at a significant risk of moderate to severe heat stress.

Compared to other animals, cattle cannot dissipate their heat very effectively. To maintain their normal body temperature cows try to increase heat loss by increasing blood flow to the skin, increase breathing rate/panting, increase in sweating and drooling. This means that energy is redirected to maintain body temperature rather than for milk production. In addition, as the majority of heat production in cows is due to rumen fermentation, cows will reduce their intakes to limit heat production.

In the present conditions, it is key that we try to alleviate the heat stress that cows experience to support milk production, reproductive performance and immune function.

Environmental management

A cow’s body temperature needs to remain a constant 38.8 oC (± 0.5 oC) for efficient metabolism. To do this, the heat generated from normal rumen digestion and metabolism must be lost to the environment. However, when the environmental temperature is high, this heat is lost more slowly, causing the cow’s body temperature to rise resulting in heat stress.

In America, where heat stress is a common occurrence, they have a number of techniques to cool the environment and get air moving, which helps the cow lose heat to prevent heat stress:

  • Allowing access to shade such as housing, gives cows an option to escape the direct sun
  • Minimize moving cows during the heat of the day  
  • Minimize the time when cows are gathering together in the collecting yard
  • Fans get air moving to help cool the surrounding area and dissipate heat – especially in areas where cows are gathered such as collecting yards
  • Sprinklers or hoses can be used in collecting yards to cool cows waiting to be milked. One downside is that wetting cows may increase the moisture in lying areas and increase the risk for environmental mastitis.

Dietary management

Water intake increases dramatically with heat stress as a means to lose heat to the environment. Providing plenty of cool, clean water is critical, especially before and after milking. Providing enough water is a combination of two factors : -

1. Adequate flow rate.

If cows are queueing around water drinkers, this is a sign that the flow rate is insufficient for the herd of cows. If this is the case, typically cows lower in the social order, such as lame cows and heifers, are most likely to ‘lose-out’ at water drinkers and be at risk for heat stress. Milking cows typically require more than 100 litres of water per day and will drink between 2 and 6 times a day. In hot weather, water intakes can increase 10 – 20%, particularly in high yielding cows, resulting in 110 – 120 litres of water consumption per day.

The water supply should be able to deliver 1.1% of bodyweight for the cattle per hour. Therefore, a 600kg cow needs 6.6 litres of water per hour. Flow rate can be practically measured by the amount of time taken to fill a 10 litre bucket. If it takes a minute to fill, flow rate is 10 litres/minute, or 600litres/hour, enough to support a 90 cow herd.

2. Adequate number of drinkers.

Although initially this may seem like something that cannot be easily or quickly changed, some farmers have already redirected water pipes across the hedge from drinkers in other fields, and plumbed them into spare drinkers.

Dietary manipulation can reduce heat stress by increasing the energy density of the ration. Low quality ‘stemy’ forages generate more heat by fermentation in the rumen. High quality forages are digested faster and result in less heat being produced.

  • High quality grass is more suitable as less heat is produced through rumen fermentation than higher fibre grass.
  • Offering higher levels of concentrates will support dry matter intake and help cows maintain energy requirements, even though overall dry matter intakes are less. This will also limit heat production from fermentation as forages generate more heat by fermentation than concentrates.  
  • As rumen protected fats are energy dense, they may also have a place in helping the cow to meet her energy requirements with a lower intake. • However, care must be taken to properly balance diets to avoid digestive upsets, acidosis and displaced abomasums.
  • Increasing the level of buffers (such as Acidguard, magnesium oxide, sodium bicarbonate or calcium carbonate) in the diet will protect against rumen acidosis, which helps to offset the reduction in salivary bicarbonate to the gut due to reduced intakes, reduced cud chewing and losses through drooling.
  • Yeasts have been shown to improve fibre digestion and stabilize the rumen environment.
  • The mineral content of rations should be reviewed so that the minerals lost in sweat, such as sodium, potassium and magnesium, are adequately replaced in the diet. Speak to your local Trouw Nutrition commercial nutritionist for recommendations on the mineral package needed to deal with heat stress.

In conclusion, the present weather conditions are placing cows in moderate to severe risk of heat stress. However, there are some management and dietary changes that can be made to help limit the detrimental impacts of heat stress and maintain productivity.