The sub-zero blasts Old Man Winter threw at us are but a memory. We smile as we turn our faces to the warm spring sun.

Over the next few months, however, that same sun could cause major disruptions in your dairy herd. As a result of heat stress, you could see:

  • milk production drop 10 to 25%
  • increased days open
  • depressed immune systems
  • decreased fertility

Two sources of heat stress can have an impact: environmental temperatures and the heat produced internally from cows digesting feed, known as basal nutrient metabolism. The latter source is a lesser factor than high environmental temperatures.

However, as cows increase milk production and feed intake, they produce more heat from nutrient metabolism. This aggravates any heat stress caused by environmental sources. As a result, higher producing cows will experience heat stress before lower producing or dry cows, according to published research by J.G. Linn, University of Minnesota.

Dairy cattle have a normal body core temperature of 101.3 to 102.8 degrees F (38.5 to 39.3 degrees C). The thermoneutral, or comfort, zone for cows is an environmental temperature range of 41 to 77 degrees F (5 to 25 degrees C). Within this zone, the heat produced by their normal bodily functions approximately equals the heat lost by their bodies.

Environmental heat comes from solar radiation and hot weather. High humidity and lack of air movement in barns or holding areas aggravate the problem. Three temperature-humidity ranges play a significant part in heat stress:

  • Temperature of 100 degrees F (38 degrees C) and 20% humidity. You need to begin serious measures to alleviate heat stress.
  • Temperature of 100 degrees F and 50% humidity. This is a danger zone for cattle and you should implement immediate steps to address the problem.
  • Temperature of 100 degrees F and 80% humidity. This is lethal for cattle.

Fresh air makes milk

It's been said that "there is milk in fresh air." Cattle can't dissipate body heat as we do. They can perspire at only 10% of the human rate. Consequently, you have to use mechanical means to help your animals maintain their core body temperatures within the thermoneutral or comfort zone.

One of the most efficient and least expensive ways to provide a comfort zone, and maximize milk production, is to ensure optimal ventilation in the holding area. This may require opening the sides of the barn by removing metal sheeting and installing screening. If your barn has concrete walls, install fans to ensure adequate air movement in all sections of the barn. If netting is installed in the outside walls, you can raise it to increase air flow. Increasing the roof venting will also enhance air flow.

Beat the heat outdoors

Outdoors, cattle should have the benefit of shade in feeding and resting areas. If these areas lack trees on your farm, you can erect netting over them.

When installed over a clean surface impervious to water, such as concrete, misters can also help to reduce heat stress. Install misters so that they wet only the front one-third of the concrete pad. This prevents development of wet or muddy areas which could lead to mastitis problems.

Set misters to deliver no more than a fog. If you see water dripping from your cows' udders, adjust them to deliver less moisture. Also, avoid installations that can make feed wet and develop mould.

Adjust feeding too.

Heat stress reduces feed intake eight to 12%, and sometimes more, causing reduced volatile fatty acid production in the rumen and decreased milk production. To address this problem, pack more nutrients into smaller volumes of feed. Requirements for lactation don't change, but the amount of energy your cows need to remain cool increases.

During the hot months, your cows deplete minerals more readily through excessive water loss. Properly using buffers in the diet can help compensate.

Be sure to confer with your feed consultant before making any drastic changes to ration formulation.

Altering feeding patterns can also help to ensure your cows consume adequate feed. Multiple feeding—three times per day, for example—ensures feed will be fresher, thus increasing dry matter intake. This also lets you or your herdsperson observe the cattle more closely. During hot weather, cattle prefer to eat at night and after milkings. Feeding 60 to 70% of the ration between 6 p.m. and 8 a.m. increases milk production successfully during hot weather, according to Linn's research.

Water, water everywhere

Your cows' water requirements increase dramatically as environmental temperatures increase. Provide additional sources of water for them. At environmental temperatures above their body temperature, however, their water intake decreases because of reduced dry matter intake and inactivity, note the University of Missouri's J.F. Keown and R. Grant.

You should increase the recommended ratio of one waterer for every cow during hot, humid weather. Cattle should have access to an unlimited quantity of fresh, clean water in an accessible area.

Cooling the water to the preferred temperature of 70 to 80 degrees F (21 to 27 degrees C) has also proven effective in increasing intake. Research published by R.G. Dado and M.S. Allen in the Journal of Dairy Science concludes that the value of added milk production should offset the cost of cooling the water.

Hot weather checklist

Reduced dry matter intake is the main reason for decreased production during times of heat stress. As intake decreases, the cow uses more energy in an attempt to maintain a comfortable body temperature. That reduces the availability of nutrients and energy for milk production. So, when hot, humid weather sets in, be prepared to:

  • modify the environment to maximize ventilation
  • provide shade or misters to cool the cattle
  • modify rations as a supportive measure and an enhancement to environmental cooling
  • increase the amount of water available to the herd