Mycotoxins in corn silage cause problems in dairy rations

FREEPORT, Ill. — Corn diseases can produce mycotoxins that impact the corn silage in dairy cattle rations.

“Tar spot or southern rust can contribute to the mycotoxin accumulation,” said Damon Smith, professor and extension specialist at the University of Wisconsin.

“This disease is pretty new, we’ve had tar spot since 2015,” said Smith during a presentation at the 2026 Illinois Dairy Summit, hosted by the Illinois Milk Producers Association and University of Illinois Extension.

“It is considered an established pathogen,” he said. “It can overwinter here, so we have locally derived epidemics.”

Southern rust is a little different than tar spot.

“The spores have to be blown in each year,” Smith said.

Usually, southern rust is not a problem in the Upper Midwest.

“Some years we get prevailing wind currents that push the spores up from the Deep South where they overwinter and infect our plants, so it is all about timing,” Smith said.

For a lot of corn diseases, he said, the goal is to delay the inevitable.

“We are trying to push what is going to happen down the road far enough so the corn plant can finish its job,” he said.

Weather is one factor that impacts corn diseases each year. Smith showed a graph of temperatures from 1950 to 2024.

“The daytime highs in the graph have actually been pretty flat, but what is changing is the night where we are getting warmer,” the professor said.

“That is why tar spot is doing so well because it is doing all of its work at night when it is warmer,” he said. “Fungi love warm and wet so this is directly influencing what we see with these pathogens.”

Another factor is more fluctuating temperatures during the growing season, which drives precipitation.

“These wet, dry cycles impact when we see diseases and how bad they will be,” Smith said.

“Tar spot started in this part of the U.S. very early last year, but it got shut down quickly because we got very warm in July,” he said. “But those conditions were conducive for southern rust that took off, and when it got cooler in September, southern rust shut down and we saw tar spot take off.”

Ear rot and stalk rot can also be a problem when making corn silage.

“Some years we have worse stalk rots and in some years we have worse ear rots because the spores can move into the canopy at different times of the season,” Smith said.

There are many different fungi producing all sorts of mycotoxins.

“Where I see testing go off the rails is when they are targeted on a specific toxin,” Smith said. “But I think we need to be getting mycotoxin screenings on anything we can test for, especially the whole fusarium group, because you don’t know what’s there unless you are testing for it.”

The extension specialist talked about a research project where they hand chopped corn plants for silage, but they removed the ear portion of the plant before chopping.

“So, we had paired samples ear versus stalk,” Smith said.

“We tested for the presence of fungus and mycotoxin to try to understand where the fungus is growing and where the mycotoxin is accumulating,” he said.

The researchers looked at the deoxynivalenol or DON levels.

“The correlation is pretty high — when the presence of the fungus is in the ear, we found DON in that ear,” Smith said. “The same is true for the stalks. Where we find the fungus in the stalk, we also found pretty high levels of DON.”

In addition, the researchers wanted to know if the presence of DON in the ear, influences the stalks.

“Those correlations were very low, almost nonexistent, so that tells us the infections in the stalk are distinct from the infections in the ear,” Smith said.

“We have to manage those things differently,” he said. “Some years we will have high stalk infections and some years we will have high ear infections.”

Wisconsin researchers followed the level of DON through the ensiling process.

“What is interesting is we get a spike during the first 30 days of DON still accumulating and then it levels off,” Smith said.

“The speed at which we make the silage anaerobic generally is influenced by the quality of the silage we put up. The foliar diseases not only affect the nutrient quality of the material, but they are also generally drier.”

Foliar diseases suck the life out of the plant and they also make it more difficult to make good silage.

“You don’t get the bunker to pack properly and the fungi continues to grow,” Smith said.

“We’ve had plots some years that when we went into Labor Day weekend, the corn was at 70% moisture, and when we came back on Tuesday after the holiday, the moisture was down to the low 50s,” the professor said.

“With both Southern rust and tar spot, I encourage you to chop those fields first to try to keep the moisture in the optimal range,” he said.

Fungicides can help to manage corn diseases and the timing of the applications is a really important part of the management plan.

“A lot of the ear rot problems come from an infection during the silking time,” Smith said.

Canadian researchers treated cornfields with fungicides at VT to 11 days after silking and evaluated the DON levels.

“The sweet spot was at silking to five or seven days after silking in terms of reducing DON levels with a fungicide and then it wears off,” Smith said. “So, by the time they got two weeks out, it’s like they didn’t do anything at all.”

And, the specialist said, researchers in Wisconsin have seen similar trends.

“This is a multi-prong approach — it’s not just the fungicide, hybrid, how we harvest or when we harvest,” he said. “Each one is part of the program to figure out what fits in your operation.”

Smith encourages dairymen to test their feed routinely.

“I know it seems expensive, but it’s probably cheaper than dumping a lot of feed,” he said.