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May has been an interesting month for nutrition on dairy farms. Discussions have dealt with last year's forages, prospects for corn and alfalfa this year and how the wet and cold affected nutrition and health of animals. Inventories of last year's forage crop are diminishing, but the problems encountered with last year's corn and corn silage crops linger on. The wet and cold weather have delayed corn planting, which will likely reduce yield, but what effect the delay will have on quality will be determined over the growing season. There were anxious days early in the month wondering about winter kill of alfalfa again this spring. Thankfully, this did not happen, so discussions have shifted to the quality and quantity of first cutting alfalfa needed for feeding this summer and refilling inventories. The central point of all these discussions is the weather and as all farmers know, we have no control over it.
While wet and cold weather dominate discussions now, the hot humid days of summer are just around the corner. The Southwest and other sections of the country are already experiencing days over 100° F and the hot weather will soon be here in the Midwest. Farm discussions should now be on hot weather feeding of cows and the transition away from cold weather diets.
The high potassium (K) content of alfalfa makes it one of the best forages to feed in the summer when K requirements of cows increase during heat stress. The 2013 winterkill of alfalfa left dairy farms with no alfalfa for feeding in 2014. An early harvest of alfalfa is needed for feeding this summer, as this will be the only source of alfalfa other than purchased hay. The typical targeted RFV for alfalfa in dairy cow diets is about 160, but this year with low fiber digestible corn silage in many rations, targeting a higher quality first cutting alfalfa for summer feeding should be the goal.
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. Never the less, nutritional alterations in the diet can help reduce heat stress. The areas to focus on with diet changes are energy and minerals,
During hot weather, high feed intakes contribute greatly to heat stress of cows. The natural reaction of the cow is to decrease feed intake. Increasing fermentable carbohydrates and decreasing fiber in the diet during the summer may seem like a good way of compensating for lower energy intakes, however, this approach can result in acidosis. Cows ruminate less when heat stressed producing less saliva to buffer the rumen. The bicarbonate buffering agent in saliva also is lower during heat stress as cows lose more bicarbonate through urination and panting.
To reduce the potential for acidosis, diets should contain 30 percent NDF or more with 70 percent of the NDF coming from high quality forages. Here is where harvesting some high quality first cutting alfalfa this year can be a big boost to summer diets. Including both forages and high fiber by-product feeds with high NDF digestibility will help keep energy intake up while reducing the potential for acidosis.
Heat stressed cows lose body weight (BW) which has been thought to be fat reserves in response to a decreased energy intake and increased energy requirement for heat dissipation. However, recent research by Lance Baumgard and colleagues at Iowa State indicates the BW loss is not fat reserves, but water, gut fill (reduced feed intake) and muscle mass. Heat stressed cows are mobilizing body protein tissue and have a lower blood NEFA level than the early lactation cow that mobilizes adipose tissue to support milk production. Thus, adding fat to the diet can help minimize the negative energy balance experienced by heat stressed cows. The addition of 2 to 3 percent fat to diets, particularly rumen inert fats, can help maintain energy intake as feed intakes decrease. Increasing the total metabolizable protein in the diet of heat stressed cows, particularly from rumen bypass protein sources, seems like a good idea, also, to help compensate for muscle protein loss and stimulate water intake.
Cows lose K and sodium (Na) in response to heat stress. Potassium is lost through sweat while Na is excreted via urine to balance the loss of K. Increasing K to 1.5 percent or greater and Na to 0.5 percent of the diet dry matter (DM) is recommended during heat stress periods. Feeding four ounces of salt plus eight ounces or more of sodium bicarbonate can easily meet Na requirements. Alfalfa is one of the best non-mineral feed sources of K, and here again, a high quality first cutting of alfalfa can be very beneficial. Balancing for dietary cation anion differences (DCAD) is another way of accounting for these two elements. Lactating cow diets should have a minimum DCAD of +30 milliequivalents (mEq) per 100 grams of DM, and preferably closer to +40 mEq, during summer months. Magnesium should be 0.35 percent of the diet DM or greater as high dietary K levels can interfere with magnesium absorption from the rumen.
Digestion aids like yeast cultures, fungal products and buffers are beneficial during heat stress periods. In addition, do not forget about hoof health. Research from Wisconsin indicates heat stressed cows can be on their feet up to 16 hours a day. The foot environment in free stall barns also will be wetter from more urination and sprinklers. Consider feed additives such as zinc methionine and biotin along with regular footbath 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 percent 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 cool feeling to cows.
Implementing a nutrition program that helps lactating cows cope with heat stress before being stressed is more effective than changing diets during heat stress. As if there was not enough to do already the last days of May, add changing to summer time diets to the list.
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