Determining The Value of Rained On Hay

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by Dan Undersander, University of Wisconsin and Krishona Martinson, University of Minnesota

Rain occurring while cut hay is laying in the field causes both yield and quality losses that reduce the value of the crop as an animal feed and a marketable commodity.

Weather-induced losses are caused by:

  • Prolonged plant respiration reducing soluble carbohydrates and overall energy content
  • Leaching of soluble carbohydrates, protein, and certain minerals from the hay
  • Leaf shattering and loss, removing the highly digestible and high protein portion of the forage
  • Microbial activity metabolizing soluble carbohydrates and reducing energy content
  • Color bleaching

How much does rainfall reduce dry matter yield?

Several researchers have studied the effects of rainfall on cut alfalfa. Wisconsin researchers measured dry matter losses of 22% when alfalfa was exposed to 1 inch of rain after 1 day of drying (curing). Similar hay dried without rain damage lost only 6.3% of the initial yield. Losses appear to be greatest after partial drying of the forage has occurred. In this same study, alfalfa exposed to 1.6 inches of rain over several days suffered a 44% loss in dry matter. Michigan researchers conducted several different studies to examine the effects of rainfall on field cured alfalfa. The first study reported maximum dry matter losses of 34%. In a second study, rainfall intensity was kept constant at about 0.7 inches but spread over periods of 1 to 7 hours. Dry matter losses ranged from 4 to 13%, with highest losses occurring when the rain was spread over a longer duration. Overall, dry matter losses were much lower in these experiments even though rainfall amounts were about 2 inches.

Other species have been studied as well. Yield losses of birdsfoot trefoil appear to be less than alfalfa, while red clover shows even less dry matter loss due to rain, and grasses suffer the least amount of dry matter losses. Dry matter losses are most crucial to the person responsible for baling the hay. Dry matter losses usually represent a significant decrease in income since less hay is available for baling, feeding, and selling.

How does rainfall reduce dry matter yield?

Three primary factors are involved in dry matter losses; leaching, respiration, and leaf loss. Leaching is the movement of cell solubles out of the plant. Components of the plant that are very water soluble are leached out of the forage and lost when rain occurs. Unfortunately, most of these compounds are those highly digested by the animal. They include such components as readily available carbohydrates and soluble nitrogen, minerals, and lipids. About one-half of the dry matter leached by rain is soluble carbohydrates.

Respiration (breakdown of soluble carbohydrates by plant enzymes) occurs at nearly 2% dry matter per hour in fresh forage, and declines almost in proportion to the decrease in moisture content until the plant reaches approximately 60% moisture. Every time the forage is wetted by rain, respiration is either prolonged or begins again in cases where the cured forage was below 60% moisture. In either case, additional dry matter is lost.

There is some disagreement in the research literature regarding the amount of leaf loss that occurs in cut alfalfa as a direct result of rainfall. In Wisconsin studies, leaf loss ranged from 8 to greater than 20% as a percent of the initial forage dry matter when rainfall amounts were from 1 to 2.5 inches. In Michigan studies, direct leaf loss was much lower (0.5 to 4.2%). Perhaps the issue of leaf loss from rainfall is a moot point. Experience and common sense tell us that rain damaged alfalfa is more predisposed to leaf shatter after it dries, and rainfall often means additional raking and more lost leaves.

How does rainfall intensity and forage moisture affect losses?

Research is conclusive on these two points. Given the same amount of total rainfall, a low intensity rain will result in more leaching of soluble compounds than a high intensity rain. Also, as forage moisture content declines, it is more prone to dry matter loss from rain. In Wisconsin rainfall studies, the maximum loss in dry matter (54%) was a treatment where 2.5 inches of rain fell on hay that was nearly dried.

How does rainfall affect forage quality?

Perhaps nothing is more frustrating than to see excellent quality alfalfa turn into unsuitable feed with each passing rain and subsequent raking. Most rainfall studies are in agreement that wetting of field dried alfalfa has little impact on protein concentration. For rained-on hay, it is common to see relatively high protein values in comparison to fiber concentrations, unless significant leaf loss occurs. With the leaching of soluble carbohydrates, structural fibers (acid and neutral detergent fibers) comprise a greater percent of the forage dry matter. Depending on numerous factors, the digestibility of rained-on hay may decline from 6 to 40%. Changes in fiber components are thought to occur by indirect mechanisms, where the respiratory activity of microorganisms has a concentrating effect on fiber components by oxidizing carbohydrate components; additional fiber is not made during the wetting process.


Rained on hay can be a suitable forage, but quality depends on several factors. Forage quality tends to be retained if rain occurs soon after cutting when the forage has had minimal time to dry; the rainfall was a single event compared to a multiple day or drawn-out event; rainfall intensity was higher versus a longer, lower intensity event; and the forage has not been re-wetted numerous times. Analyzing forage for nutrient content is recommended, but can be especially useful when determining the quality of rained on hay.

A note from Lindsey Grimes, BioZyme Nutrition Field Support

How can Amaferm® help?

When forage quality is impacted by added precipitation, the feed additive Amaferm®, found in all VitaFerm products, can be used to improve forage digestibility. Amaferm is a precision prebiotic used to stimulate the enzyme activity and growth of beneficial microorganisms that already exist within the digestive tract. This natural feed additive maximizes the energy value of feed by improving digestibility through increased fungal branching, more microbial enzyme activity, and faster bacterial growth. From this increase in digestibility of feedstuffs, a producer could expect to see increased milk production as well as increased gain and feed efficiency.

Amaferm improves digestibility regardless of forage quality, but works best when energy is limited. In the case of hay that has been rained on, the leaching, respiration and leaf loss that occurs decreases the dry matter content of the forage and consequently decreases the amount of cell solubles that can be utilized as an energy source. Compromised forage quality from excess rain on field dried forages also limits the amount of protein available for digestion.

As previously mentioned, the digestibility of rained-on hay may decline anywhere from 6 to 40%. However, with the addition of Amaferm into the diet, total digestibility can be increased up to 9%. VFA production can be increased up to 16%, allowing more energy to be utilized by the animal. This boost in energy is equivalent to supplementing 1 lb of corn. Microbial protein can be improved upwards of 34% with the inclusion of Amaferm®. This provides enough additional protein to equal 1 lb of soybean meal or dried distillers grains.

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