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Management tips to lessen dangers of feeding moldy hay

Contributed by Steven Paisley Published on 14 November 2016
moldy hay

Many producers, blessed with an abundance of spring and summer moisture, are quickly discovering this year’s hay crop has a little more mold growth than they are used to.

While above-average precipitation is a good problem to have, there are additional management concerns to address after opening that bale pile and discovering some spoilage.

Source of the mold

Although excess moisture is typically the cause of mold, it can be created by a number of situations. Hay that is baled too wet can have mold throughout the bale, while bales that have been exposed to moisture after being stacked may have mold on the outside layers.

Handling moldy hay

Often the dark, almost black dust associated with the mold is the biggest health issue. The dust is decomposed forage particles, mold organisms and, most importantly, mold spores. Inhaling the dust, mold and spores should be minimized, especially when feeding to horses and freshly weaned calves. Some potential effects of feeding moldy, dusty hay include reduced intake and respiratory health responses associated with inhaling the spores. In addition, an immune response to the inhaled and consumed mycotoxins can produce hormonal and endocrine effects in the animal.

Mold versus heat-damaged hay

Hay baled when slightly wet can also go through a heating process. This damage has different implications for hay quality than mold. When the internal temperature of a bale stays in the 125ºF to 150ºF range, mold spoilage typically results. Heat damage, also known as the Maillard reaction, occurs if the bale temperature exceeds 150ºF. When this occurs, protein and carbohydrates in the hay begin to react to the high temperatures, going through a browning process – the Maillard reaction.

Although we actually want the Maillard browning reaction to occur when we are cooking hash browns in a skillet, heat-damaged forage, although often tasty to cattle, has a dramatically reduced feed value. The browning reaction essentially makes the nutrients unavailable to the animal, dramatically reducing the feed value of the forage.

Testing for mycotoxins and mold

Though there are good testing procedures for the limited number of molds found in grain, testing for molds in forages is quite difficult. There are dozens of mold types that may affect forages and very few tests that evaluate forage-specific molds. Most feed labs offer an overall mold count, or estimate of the mold contamination, which may be useful when determining the severity of the problem. In general, look for visual cues to determine severity. White, grey and black molds are typical. Large mold problems that have unusual mold colors are more cause for concern as they may potentially produce mycotoxins.

Feeding moldy hay

Steps can be taken to reduce the harmful effects of feeding moldy hay. If possible, feed the hay in a well-ventilated area. Control dust by mixing hay with wet feeds such as silages, beet pulp, wet distillers' grains or even liquid feed supplements. Diluting the moldy hay with other forages will reduce exposure. This will improve the overall safety of feeding moldy forages. Because of the reduced quality and unavailable nutrients in heat-damaged hay, make sure animals are supplemented with higher quality forages or grains.

Cattle have ruminal microbes that destroy a high number of mycotoxins associated with molds, while horses and swine are more susceptible to mold problems. Pregnant and lactating animals are at a higher risk than feeder cattle and non-pregnant animals. Some indicators that mold levels are too high include dramatically reduced intake and pneumonia-like respiratory issues such as hacking or coughing.


While there are some risks associated with feeding moldy forages, planning ahead, blending, controlling dust by including wet feeds or liquid supplements to condition or dampen the hay, avoiding feeding to pregnant or lactating animals, and feeding in a ventilated area can all help reduce the impacts.  end mark

Steven Paisley
  • Steven Paisley

  • Extension Beef Cattle Specialist
  • University of Wyoming
  • Email Steven Paisley

PHOTO: If feeding moldy hay, steps should be taken to reduce the harmful effects. Staff photo.