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Iowa Beef Center
313 Kildee Hall
Iowa State University
Ames, IA 50011-3150
Phone: 515-294-BEEF (2333)
Fax: 515-294-3795

last updated: July 15, 2015


Rain Damaged Hay can be Costly for Farmers


photoAMES, Iowa – Rain is a needed ingredient for growing crops. However, the frequent rains in Iowa this summer have become a challenge for hay producers planning to mow, bale and store hay. Denise Schwab, beef program specialist with Iowa State University Extension and Outreach, said it’s important to recognize the impact that rain can have on hay.

When moisture content of hay increases, so does the microbial activity and decay, which generates heat. When the heat levels increase, there is the potential of spontaneous combustion.

“This isn’t common, but it can happen when hay is stored in a wet condition,” Schwab said. “In most hay fires, farmers lose not only their hay, but also the barn it is stored in. In fact, because of a fire a couple years ago my friend lost most of the surrounding land which was covered by a Conservation Reserve Program contract.”

Hay should be baled with a moisture content of less than 20 percent; however, bale size also plays a part in determining moisture levels. Small square bales have more surface area per unit that allows the bale to breathe and dissipate moisture easier. The small bales can be baled at the 18-20 percent moisture range, but large round or square bales should be baled when cuttings are in the 16-18 percent moisture range.

All hay will heat up due to the natural respiration of the plant, and temperatures below 125 degrees Fahrenheit are considered normal. Temperatures above 125 degrees Fahrenheit need to be monitored. When hay is between 125 and 150 degrees Fahrenheit, a process called the Maillard reaction causes “carmelization” of the hay and reduces forage quality.

Schwab advised that when hay reaches 150 degrees Fahrenheit, farmers should check it daily and consider spreading out the hay to provide air movement and possible cooling.

“When temperatures exceed 175 degrees, check the temperature every two hours and alert your local fire department,” Schwab said. “Hay this hot will be nearly black in color, have reduced feed value and can be dangerous to move. Avoid adding oxygen to hay in this temperature range, which may cause combustion. If the hay being moved exceeds 190 degrees Fahrenheit, have the fire department on hand in case of spontaneous ignition.”

While most hay never reaches the point of combustion, it can be damaged in terms of feed value.

The most dramatic effect of high moisture on feed value is the reduction in digestibility. Heat damaged proteins have been the primary focus of nutritionists, but researchers from the U.S. Dairy Forage Research Center in Marshfield, Wisconsin, suggest that reduced energy values of heat damage are an important impact.

“Data shows a decrease of 11 percent in total digestible nutrients (TDN) from heat damage hay,” Schwab said. “This comes from oxidation and the loss of digestible sugars and cell solubles during the heating process. The concentration of undigestible fiber is increased, and the TDN levels are reduced. Forage testing of heat-damaged hay is critical to evaluate the true value of that feed.”

Another component of feed value is the growth of molds and mycotoxins. Wet, tightly baled hay provides a moist, dark environment for mold growth. Usually, molds affect palatability and subsequent intake of hay by livestock. Mycotoxins are the secondary products that some molds produce and can be toxic to animals if fed at high enough levels.

Schwab said producers should consider feeding moldy hay only to less sensitive animals and be sure it’s mixed with non-moldy hay.

“Monitor those animals closely for any signs of toxicity and allow them to sort through the poor hay for the better hay,” Schwab said. “There is one exception: horses are more susceptible to mycotoxins and should not be fed moldy hay.”

In the past, some farmers have applied salt to the surface of hay that was baled wet, but research has shown little benefit from this practice. While the theory is good, the amount of salt needed would be very large and expensive, and would likely reduce the palatability of the hay.

What should a forage producer do if wet weather persists? Here are four tips to manage potential moisture damage:

  1. Mow hay into wide windrows or use a rake or tedder to spread out hay and speed drying.

  2. Consider using a preservative to reduce spoilage if hay is too damp for storage.

  3. Consider making wet baleage if the window of time for haying is too short.

  4. Test forages to determine feed value prior to the winter feeding season.

For more information on managing damaged hay with high moisture content, contact your local ISU Extension and Outreach beef specialist or the Iowa Beef Center at


The Iowa Beef Center at Iowa State University was established in 1996 with the goal of supporting the growth and vitality of the state’s beef cattle industry. It comprises faculty and staff from Iowa State University Extension and Outreach, College of Agriculture and Life Sciences and College of Veterinary Medicine, and works to develop and deliver the latest research-based information regarding the beef cattle industry. For more information about IBC, visit


Contact: Denise Schwab, ISU Extension and Outreach beef program specialist, 319-472-4739,

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