June 9, 2014
Although mycotoxins are unavoidable contaminants of cereal grains, being aware of the contamination possibilities and taking proactive steps will go a long way in managing their impact.
Several strategies starting in the field and continuing to storage can help manage the impact mycotoxins have on grain, said Erin Bowers, post-doctoral research associate, Iowa State University, Ames, Iowa, U.S., during the 2014 GEAPS Exchange in February. Bowers was one of several speakers who participated in the sessions presented by NC213, a multi-state research consortium of scientists, industry representatives and government agencies.
These include implementing a testing strategy, efficient control measures, maintaining cleanliness, being aware of growing conditions, looking for hot spots of contamination and an effective marketing strategy.
In North America, the five primary mycotoxins are: aflatoxins, deoxynivalenol (vomitoxin), fumonisins, zearalenone and ochratoxins. Each is caused by different fungi that thrive under varying conditions. For example, aflatoxin affects corn and the producing fungi prefer hot, dry conditions while vomitoxins can affect corn, wheat and barley and the main fungi prefer cool, wet conditions.
Mycotoxins are a major concern, because they can cause serious health conditions in livestock and humans, Bowers said. Aflatoxins cause an increase in mortality rates, weight loss in animals, a decrease in egg production in poultry and a drop in milk production in dairy cattle.
Fumonisins cause feed refusal and pulmonary edema in swine and ELEM in horses. Vomitoxins have been associated with nausea, vomiting, diarrhea, feed refusal and a drop in milk production in dairy cattle. Zearalenone can have reproductive effects in ruminants and ochratoxins have been associated with increased mortality, a decrease in weight gain and a decrease in egg production in poultry.
In the field
Reducing mycotoxin production starts in the field with management decisions that support plant health, Bowers said. Irrigation can reduce plant stress and deter aflatoxin-producing species. However, try avoiding irrigation directly during anthesis for wheat and silking for corn since splashing is a significant way in which fungal spores are transferred onto a plant. These growth periods are when the plant appears to be highly susceptible to infection, Bowers said.
Although no till is popular because it takes some of the burden off working the land and prevents erosion, it can also encourage the growth of fungi, she said. A lot of fungi like to colonize on crop debris. It’s easy for those spores to get stirred up with wind and water, and then infect a plant. Tillage helps disperse that fungal inoculum load and keeps it away from the plant, Bowers said.
Crop rotation among a host (corn) and non-host plant (such as soybeans) is also a good option. If the fungus has a host crop that it likes to colonize every year, it will build up high populations. But rotating a less ideal host plant with a good host will limit its growth.
Insect management is important because they can carry spores on their bodies and into the plants, spreading infection.
Bt corn has been shown to reduce fumonisins, and there is also evidence that it helps with aflatoxins. Hybrid selection can help in areas with aflatoxin contamination. The fungi likes to grow down the silk of corn; tight-husked hybrids have been effective in reducing fungal colonization, Bowers said.
When grain is delivered to a storage facility, testing for mycotoxins is critical. In developing a testing plan, being aware is key.
“You can’t test every load for every mycotoxin,” Bowers said. “But what you can do is be aware and know what your risk is based on what crop you’re testing and what the weather conditions have been and your location. You can predict the risk and direct the testing to that.”
Several different types of tests are available from something as simple as a black light to test for Aspergillus flavus to immunochromatographic assays and High Performance Liquid Chromatography. During the testing time, it’s important to minimize the wet holding time, Bowers said, because mycotoxin levels can increase rapidly in high-moisture grain.
One important factor to remember is that mycotoxins are not distributed evenly in grains, and typically occur in hot spots of contamination.
This may be a result of actual heating that occurred in a certain area of stored grain, which promoted fungal spread and mycotoxin production in that isolated region, Bowers said. It is more likely due to the localization of grain harvested from an affected area of the field in one section of a storage bin.
Aflatoxin is an especially tricky mycotoxin as it takes only a few kernels to contaminate a lot of grain at an unacceptable level. In the case of corn, it has been estimated that as few as 6 aflatoxin-contaminated kernels in 10,000 can cause grain to be contaminated at the regulatory limit of 20 parts per billion. For grain at 15.5% moisture, this is 6 kernels in approximately six-and-a-quarter pounds of grain.
“If you don’t detect those accurately, it can cause problems in categorizing that grain and putting it in the right market,” Bowers said.
The United Nations’ Food and Agriculture Organization (FAO) recently released an online tool (www.fstools.org/mycotoxins/) to help in assessing the effectiveness of mycotoxin sampling plans, she said. After entering in sampling specifics, the tool will show the risk to the seller of being falsely rejected as well as the risk that a purchaser will wrongly accept grain that is contaminated above the regulatory limit.
The tool shows that sample size is a key factor in risk to the buyer. There is significantly higher risk of wrongly accepting a lot of contaminated grain when the sample size is small (about a pound) compared with a larger sample size (10 pounds).
“Increasing sample size is one of the cheapest ways to increase your accuracy in detecting contaminated lots as long as the sample taken is represenattive of the entire lot,” Bowers said.
During storage, drying is a critical point where contamination can occur. The more time that passes between harvesting and drying, the shorter the allowable storage time for that grain, Bowers said. Drying too quickly or at high temperatures can cause stress cracks in the grain, which are prime entry points for storage fungi.
Strategies like dryeration and combination drying are good options which reduce the risk for kernel damage that might promote fungal activity, she said. Aspergillus and Penicillium fungi, which produce aflatoxin and ochratoxin, are the main mycotoxin-producing fungi to be wary of in stored grains as they can be active at moistures lower than many other fungi.
Temperature is another important factor during storage. At cool temperatures (below 60 degrees F), fungi grow very slowly and many storage insects and mites are dormant. Aeration can be used to reduce temperature and minimize moisture. Maintaining a uniform temperature throughout grain is important to avoid hot spots of contamination.
Pests can cause kernel damage and create hot spots, so it’s important to control the presence and activity of rodents, birds, insects and mites. Insects can transport spores from an initial contamination site to other sites they contact. Pests can cause cross-contamination through their interaction among grain storage containing contaminated and uncontaminated grain, Bowers said.
Between grain loads, bins should be thoroughly cleaned of residual grain and fines which may contain contamination from the previous lot. Any cracks or openings that could allow entry of water, insects or rodents should be fixed. The aeration system should be cleaned and checked to ensure it is working properly, Bowers said.
Stored grain should be checked often for signs of spoilage. Twice a week is not unreasonable, especially during seasons with high-risk of warming or moisture accumulation and day/night temperature extremes.
Signs to look for are an off-odor; crusted, colored or wet grain on the top surface of the bin; signs of pest activity; and temperature changes.
If a facility accepts damaged or contaminated grain, there are a few extra considerations, Bowers said. For one, the grain will spoil faster than sound grain under the same storage conditions. Storage moistures should reflect this difference, and be reduced 1%-2% compared to what is typical for sound grain.
Cleaning is a good option, she said, because cracked, broken or fine kernels are more likely to contain mycotoxins. If those can be separated out, it will help reduce some of the contamination levels. A 5% increase in broken or damaged kernels decreases the shelf life of grain by about one order of magnitude. For example, this would be a reduction from 90 days of safe storage to only nine days, Bowers said.
Grain that is damaged or known to have contamination should not be stored with clean grain. This promotes cross contamination, and could be considered illegal if aflatoxin-contaminated grain is involved.
There are appropriate feed uses for aflatoxin-contaminated grain testing up to 300 parts per billion, but blending contaminated grain with uncontaminated grain to achieve a reduced level of contamination is illegal, Bowers said. The typical option for using mycotoxin-contaminated corn is to send it to its appropriate livestock feed use.
Mycotoxin-contaminated grain should be labeled thoroughly, especially if its level or type of contamination limits the food and feed markets where it can safely be utilized, she said.