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OSU Extension

College of Food, Agricultural, and Environmental Sciences

June 18, 2019 - 8:42am --

It is turning into another challenging and frustrating year to make hay.  The extended forecast through the end of June calls for above normal rainfall.  I recently read an article in Hay and Forage Grower on-line entitled “Cursing the raindrops”, in which author Mike Rankin addressing this year’s weather patterns said, “Those putting up high-moisture forage have an uphill battle.  If you’re in the dry hay business, it’s a Mount Everest situation.”  The age-old question for anyone trying to make hay with rain in the forecast is mow sooner rather than later and risk rain on the cut forage, or wait for a weather break and lose quality as the forage continues to mature? 

            Rain on mowed forage causes a reduction in quality and can result in dry matter (quantity) losses as well.  According to a July 2017 University of Delaware blog post quoting Brian Pugh, Oklahoma State Extension Area Agronomy Specialist, there are four ways rain reduces forage quality and causes dry matter loss.  Rain results in leaching of soluble carbohydrates, vitamins and minerals, rain can increase and prolong plant respiration, can cause leaf shattering and can increase the microbial breakdown of plant tissue.  The extent of the rainfall damage depends upon the moisture level of the forage when rain occurs, the amount of rainfall received, the intensity of the rain, and the length of time over which the rainfall occurs.

            The best-case scenario for rain and mowed forage is a quick rain shower within a few hours or less after mowing, followed by sunny skies and drying weather.  In this situation leaching of soluble nutrients is minimal, plant respiration is very minimally affected, no leaf shattering occurs, and microbial breakdown is not an issue.  The goal in dry hay production is to get mowed forage moisture level below 30% as quick as possible because plants continue to respire and burn up soluble carbohydrates above that moisture percentage.  Research has demonstrated that the highest respiration losses occur with forage above 40% moisture.  The rate of respiration loss slows greatly below 40% moisture. 

According to a University of Wisconsin fact sheet entitled “Rain Damage to Forage During Hay and Silage Making”, normal dry matter losses due to respiration of mowed forage is about three to four percent of the dry matter harvest.  The publication says, “Each time cut forages are wetted by rain, respiration is prolonged or begins again in cases where cured forage is already below 30 percent moisture.”  Some Wisconsin research looked at the impact of a one-inch rain on alfalfa following one day of drying.  The dry matter loss was 22%. 

Researchers in Michigan examined the effect of a 0.7-inch rain on field-cured alfalfa spread over a period ranging from one to seven hours.  In that study, dry matter loss ranged from four to thirteen percent with the highest losses reported for rainfall spread out over a prolonged time.  Back to that University of Delaware blog post, it cited research from the University of Kentucky that reported a loss of five percent dry matter per inch of rain on mowed forage with forage digestibility reduced by ten percent or more due to leaching of nutrients and leaf shatter. 

Some other key points about rain damage to mowed forage, summarized from the University of Wisconsin publication are:

  • For a given rainfall amount, a low intensity rain results in more leaching of soluble compounds as compared to a high intensity rainfall event.
  • Leaching of soluble carbohydrates results in a percentage increase of structural fibers (acid and neutral detergent fiber) in rained on forage.  Research indicates that forage digestibility of rained on hay is reduced anywhere from six to forty percent.

With rain on the horizon, producers are tempted to bale hay too soon, beyond safe moisture limits.  Wet hay can result spontaneous combustion and barn fires.  I’ll close with some words about monitoring wet hay excerpted from a recent article in the OSU Extension CORN newsletter by Mark Sulc, OSU Extension Forage Specialist:

“Use a hay temperature probe and monitor the internal temperature of the hay during the first three weeks after baling.  It is normal for hay to go through a “sweat” in the few days after baling. Internal temperatures of 110 F in the first five days after baling are quite common in our region and are not a big concern.

Hay bale temperatures of 120 to 130 F will likely result in mold growth and will make the protein in the hay less available to animals. While those temperatures are not high enough to cause hay fires, the concern is if the mold growth continues and pushes temperatures upward into the danger zone.  If the temperature in the hay continues to rise, reaching 160 to 170 F, then there is cause for alarm. At those elevated temperatures, other chemical reactions begin to occur that elevate the temperature much higher, resulting in spontaneous combustion of the hay in a relatively short time period.”


Rory Lewandowski is an OSU Extension Agriculture & Natural Resources Educator and may be reached at 330-264-8722.

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