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Rain Impacts Forage Quality

August 26, 2019

Rain can have a variety of impacts on forage quality and yield, ultimately affecting overall feed and market value, North Dakota State University Extension livestock specialists say.

July in North Dakota was the 23rd wettest in the last 125 years, although areas in the north and north-central part of the state are experiencing drought conditions.

“Those who are lucky enough to receive moisture always look for the good in it; however, it has created challenges for producers trying to put up hay this summer,” says Karl Hoppe, livestock systems specialist at NDSU’s Carrington Research Extension Center.

How Growing-season Rain Impacts Forge Quality

“The stage of maturity at harvest is one of the most important factors influencing forage quality,” says Janna Block, livestock systems specialist at NDSU’s Hettinger Research Extension Center. “Generally, leaf-to-stem ratios decline as forage matures, which leads to increased concentrations of fiber components and reduced protein and energy content.”

Neutral detergent fiber (NDF) represents cell wall components of forages, including cellulose, hemicellulose and lignin. While hemicellulose and cellulose can be digested to some extent by bacteria in the cow’s rumen, lignin is indigestible. Lignin also will reduce digestibility of cellulose and hemicellulose.

Therefore, increased NDF content reduces overall fiber digestion by rumen microbes, which reduces the energy value of the forage.

The NDF content also is related to the “bulk” of forages, and high NDF can reduce the amount of forage that a cow can eat physically. Limited intake of low-quality forages may result in performance losses.

“In North Dakota, the majority of our perennial forage species harvested for hay are cool-season species,” says Miranda Meehan, Extension livestock environmental stewardship specialist. “These species initiate growth early in the spring and reach peak production in early July. To achieve the best combination of quality and quantity, this would be the optimal time to harvest.”

However, above-normal precipitation during the growing season leads to harvest delays, which means that many forages are overmature by the time they are harvested.

Temperature also is a major factor in how plants mature. Warm temperatures lower forage quality, compared with forages grown under cool temperatures, even when forages are harvested at the same maturity stage. The combination of relatively cooler temperatures and continued rain well into the growing season has kept many forages in a vegetative state, which could mean that forage quality may not be reduced as much as expected.

How Rain During Harvest Impacts Forage Quality

Rainfall that occurs between cutting and baling can have a variety of impacts on forage quality.

“Yield and digestibility losses between 6% and 40% have been reported, depending on factors such as species, timing and amount of rain, stage of maturity at harvest and drying conditions,” Meehan notes. “A study of hay quality conducted by NDSU Extension in 2015 reported lower total digestible nutrient content of samples that had been rained on.”

Leaching of plant sugars (carbohydrates), vitamins and minerals out of plant tissues is a major concern. Crude protein content may be increased in rained-on hay because it composes a larger portion of forage dry matter as sugars are lost.

If hay is baled while it is too wet, much of the crude protein may be lost to heat damage, Block cautions. High moisture content of forages can cause continued respiration in harvested forage, which further contributes to carbohydrate losses. Leaf shatter and loss can occur, particularly with alfalfa.

In addition, the potential for increased microbial activity can reduce forage quality and increase the risk of mold and mycotoxins.

Recommended dry matter percentages for baled hay are approximately 18% or less for large round bales, 16% or less for large square bales and 20% or less for small square bales, Hoppe says.

Spontaneous combustion may occur when internal temperatures exceed 150 F. Most of the time, this will occur within a few weeks of baling, but it also can occur with heavy rainfall after hay is in the stack.

Quality losses due to heating may occur as well. Some proteins may bind with fiber, which makes them unavailable for digestion. The presence of mold and mycotoxins also can be detrimental to livestock.

“Given the large list of variables that can impact forage quality in a wet year, forage analysis is extremely critical,” Hoppe says. “If you’ve never tested hay before, this is the year to do it. But remember, forage analysis is only as good as the samples that are submitted.”

Detailed instructions for forage sampling can be found at

County offices of NDSU Extension may have a hay probe available for producers to borrow, and can provide information on where and how to submit samples. The National Forage Testing Association maintains a list of certified laboratory facilities on its website at

At a minimum, analysis should include an estimate of total digestible nutrients and crude protein (CP), which are used as the basis to determine forage quality and develop rations for livestock at various stages of production. However, this year, evaluating other components of the forage might be worthwhile as well.

The content of NDF will be an important factor to consider. Some labs also may offer analysis for acid detergent fiber nitrogen or acid detergent fiber protein, which are measures of heat-damaged protein. If this result is greater than 10%, the lab will adjust CP levels to determine the amount of available protein.

Laboratory analysis of forages typically will cost between $20 and $40.

“These tests are inexpensive when considering the importance of hay quality to your bottom line,” Block says. “A forage test early in the season will help producers determine the quality of the forage base and whether supplementation will be necessary to meet livestock requirements. The bottom line is it is best to test rather than try to guess how the unusual growing conditions this year may have affected hay crops.”

Source: North Dakota State University