Prepare Now for Summer Feeding Program

Jim Linn and Mary Raeth-Knight

May 2007

The hot days in May are a reminder that the heat of summer is coming. As we move into June, July, August and even September, the hot days of summer will challenge our dairy cows. It is not too early to begin making changes in your diets to help cows manage the stresses of heat and humidity.

When cows actually feel heat stress is a function of both temperature and humidity. In very dry conditions, 10 to 20% humidity, cows may not show signs of heat stress until temperatures are in the mid to upper 80s. In very humid conditions, above 75%, heat stress can start at 70 degrees. Research from the University of Georgia a few years ago found a lag of about two days between heat stress conditions and when cattle showed the signs of heat stress. Indicators that cows are experiencing heat stress are:

Feed intake is reduced

Milk production declines

Cows are less active and stand more

Cows often crowd together, especially around water tanks

Panting and open mouth breathing

Rectal or milk temperature increases

Dietary modifications will help cows cope with heat stress, but diet has a far less impact on mitigating heat stress than does altering the environment. Sprinklers, fans and shade are much more important for heat abatement than diet change. Nevertheless, nutritional alterations in the diet can help reduce heat stress. The three areas to focus on with diet changes are feed intake, energy intake and keeping cows healthy.

• Total feed intake has a big impact on the amount of heat produced by the cow during the digestion of feedstuffs. During hot weather, high feed intakes contribute greatly to the heat stress of cows and, therefore, the natural reaction of the cow is to decrease feed intake. Diet changes to higher fermentable carbohydrates and decreased fiber during summer months has been one approach to keeping both feed and energy intakes up. However, this approach can result in acidosis as cows will ruminate less reducing the amount of saliva produced to buffer the rumen. In addition, the buffering capacity of the saliva is lowered during heat stress as cows lose bicarbonate, the buffering component in saliva, through urination and increased panting. Therefore, any potential benefit to maintaining feed and energy intakes by increasing fermentable carbohydrates in diets could be negated by acidosis.


• To reduce the potential for acidosis in the summer, diets should contain a minimum of 30% NDF with 70% or more of the NDF coming from high quality forages. Including both forages and high fiber byproduct feeds with high NDF digestibility will help keep energy intake up while reducing the potential for acidosis. Having 8 to 12% on the top screen of the shaker box that is good consumable forage particles is important for rumen stimulation. Keep diet starch levels below 25% to help prevent acidosis.


• Fats are high in energy and digestibility, which results in less heat produced during digestion than with other feeds. The addition of 2 to 3% fat to diets, particularly rumen inert fats and whole cottonseed, can help maintain energy intake as feed intakes decrease.


• Cows lose potassium and sodium in response to heat stress. Potassium is lost through sweat while sodium is excreted via urine to balance the loss of potassium. Increasing potassium to 1.5% or greater and sodium to 0.5% of the diet dry matter (DM) is recommended during heat stress periods. Sodium levels can be achieved by feeding 4 ounces of salt plus 8 ounces or more of sodium bicarbonate. Balancing for dietary cation-anion differences (DCAD) is another way of accounting for these two elements. A DCAD for lactating cows above +30 milliequivalents (mEq) per 100 grams of DM should be the target during summer months. Magnesium should be 0.35% of the diet DM or greater as high dietary potassium levels inhibit magnesium absorption from the rumen.


• The best time to feed cows is evening. The effect of body heat produced during feed digestion during the evening and nighttime will have the lowest additive effect on environmental induced heat stress. Also, feeds will remain fresher in the bunk longer during nighttimes than during the day.


• Protein, particularly rumen degradable protein, should not be overfed during summer months. Feeding protein above requirements increases the workload of the liver and kidney to excrete the extra nitrogen increasing energy requirements and body heat production. Keeping total crude protein in the diet to around 17% with about 60% of the protein being rumen degradable and evaluating amino acid balance is a good heat stress protein feeding strategy.


• Other additives. Rumen stabilizing and digestion aids like yeast cultures, fungal products and buffers have been shown to be beneficial during heat stress periods. And don’t forget about hoof health. Recent research from Wisconsin indicates cows will be on their feet up to 16 hours a day under heat stress conditions. The foot environment also will be wetter from more urination and possibly sprinklers. Consider feed additives such as zinc methionine and biotin along with regular foot bath usage to help maintain good hoof health.


• Water is the most important nutrient of all to minimize heat stress. Cows need access to plenty of clean fresh water. Water intake can increase 50% above normal levels during heat stress. Drinking water helps cool off cows as it is a heat sink drawing body heat into the water to warm it after ingestion giving a cooling feeling to cows. Cows prefer to drink water that has a temperature of about 70 degrees.