Mike Rankin, Senior Editor, Hay & Forage Grower
(Previously published online with Hay & Forage Grower: December 24, 2024)
Hopefully, 
no haymaker puts up a crop that they know will mold; however, many haymakers have pushed the moisture limit and hoped the mold fairy wouldn’t pay a visit. Sometimes, she just comes uninvited.
“Most hay can become moldy when it is baled too wet, left in the field for too long, or stored outside in shaded areas where rain or humidity can slow down the drying process and increase the risk of mold,” notes Rocky Lemus, an extension forage specialist with Mississippi State University. “Hay that is cut and baled with a high amount of dirt, or hay where the plant shows signs of fungal diseases, could also contain spores that will germinate if exposed to moisture during storage.”
Lemus also mentions that hay stored in high stacks or ensiled at high moisture can become moldy. Under ideal moisture conditions, spores from different mold species can germinate within 24 to 72 hours.
It’s typically not recommended to feed moldy hay. Some molds produce mycotoxins, which can induce health issues in livestock. In total, moldy hay can cause a drop in feed intake, low fertility, greater water consumption, poor hair coats, estrogenic imbalances, lameness, poor kidney and liver function, respiratory disease, diarrhea, and colic in horses.
“Horses, which are cecal digesters, are at the highest risk of mold susceptibility among common livestock,” Lemus writes in his monthly forage newsletter. “They can develop a respiratory disease called recurrent airway obstruction, also known as heaves.”
Animals aren’t the only ones that moldy hay can negatively impact, Lemus points out. The spores in moldy hay can cause a condition called “farmer’s lung” in humans, where the fungus can grow in lung tissue after breathing in too many spores.
Best not to guess
Sometimes feeding moldy hay results in no serious consequences, but other times it does. This makes the decision of whether to feed moldy hay difficult. Ultimately, it depends on the type of mold present, which is difficult to discern without having the forage tested for a mold spore count and mycotoxins. Concerning the latter, there are nearly 400 known types.
“If the presence of mycotoxins is unable to be verified, it is important to carefully monitor herd health regarding reproductive efficiency, feed utilization and gain, and overall health status,” Lemus asserts. “Even if mycotoxins are not present in the hay, mold can lower the hay’s nutritional value. Although some producers might consider mixing moldy hay with other types of supplemental feed to dilute the problem, it could increase the risk of eating hay that would normally be refused,” he adds.
Nearly all hay will contain some mold; it’s just that sometimes it’s not visible. Many forage testing laboratories can test hay for different types and amounts of spores and specific mycotoxins. A sample is collected and submitted like a hay analysis for nutrient value.
Typically, the cost for a mold and mycotoxin analysis ranges from $60 to $90 per sample, and it might take a week or more to complete. Mold spore counts are reported as colony forming units per gram (cfu/g), and hay feeding risks are classified based on the amount of cfu/g as follows:
- Low risk (under 500,000)
- Relatively safe (500,000 to 1 million)
- Feed with caution (1 to 2 million)
- Closely observe animals and performance (2 to 3 million)
- Dilute with other feeds (3 to 4 million)
- Discontinue feeding (over 5 million)
For more informed decisions about risk and cautions, consult with your nutritionist about safe levels depending on the livestock type and growth stage.
Prevention is the best cure
“Many of the mold issues and the mycotoxin issues start in the field before or during baling,” Lemus notes. “Severe mold issues can be prevented by baling at 12% to 18% moisture, stacking hay in well-ventilated areas, alternating bales so air can circulate, avoiding placing tarps tightly and completely enclosing the hay that prevents air movement, not stacking hay too high, using a hay preservative if hay is marginally wet, and inspecting the hay for mold before feeding it to animals.”
The forage specialist emphasizes that moldy hay is less palatable, resulting in lower feed intake or higher animal refusal. Ultimately, this leads to poor weight gain, reduced milk production, and diminished animal performance. “Use common sense and good observation as your best decision aids concerning feeding or testing moldy hay,” Lemus concludes.