Ear rots and mycotoxins
Ear rots are beginning to show up in pockets across the state, leading to concerns about mycotoxin contamination of grain. So far, we have received images and samples with Gibberella, Diplodia, Fusarium, and Trichoderma ear rots, four of the most common ear rots in the state. Of these, Gibberella (GER) and Fusarium ear rots are of greatest concerns, since grain harvested from affected fields will be contaminated with mycotoxins, particularly vomitoxin in the case of GER.
Vomitoxin is a concern for the livestock and ethanol industries. Feed made from heavily contaminated grain may lead to vomiting and low weight gain in animals; pigs are particularly sensitive. Vomitoxin is not destroyed during ethanol production, nor is it removed in the ethanol fraction, but rather becomes concentrated in the grain fraction.
This leads to three-fold higher levels of the toxin in DDGS, a nutrient-rich co-product of ethanol production that is commonly sold as an ingredient for animal feed. Consequently, ethanol plants may reject GER-affect grain with high levels of vomitoxin. Slow grain dry-down, late-season rainfall, and delayed harvest will increase ear rot severity and mycotoxin contamination.
Severely diseased and toxin-contaminated grain are usually smaller than healthy grain, and are covered with fungal mycelium (mold). Compared to healthy grain, diseased grain break easily during harvest, transport, and other forms of grain handling, increasing the number of fine particles and the amount of dust in the grain lot. Fines and dust usually consist of pieces of cobs that are often more contaminated with mycotoxins than the grain itself.
Fields with ear rot problems should be harvested as soon as possible and handled separately from healthy fields, even if it means harvesting those field at a higher-than-usual moisture content. Adjusting the combine to minimize damage to the grain and increasing the fan speed will help to remove lightweight grain, fines, and dust particles, and as a result, reduce the overall level of mycotoxin contamination of the grain lot.
First consider using combine settings that are as gentil on the cobs as possible to minimize cob breakage. To do this, start at the concaves and rotor speed. You want to set your concave clearance to match your average cob diameter. On newer machines that are zeroed correctly, take a few shelled ears and find the metric wrench that just fits over the cob; this will be your concave setting in mm.
For other machines, measure the diameter of the average cob and the distance from your rasp bars to the concave. Once your concave setting is correct, run your rotor as slowly as you can to get the grain shelled off. If you are still having problems with cob breakage, consider changing the type of concaves you are using.
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In general, round bar concaves shell wet corn and corn with rotten cobs better than large wire concaves. If you change concaves to round bar, be sure to recheck the clearance to rasp bars.
Once your threshing system is set correctly, turn your attention to air speed and separator. Run your air speed as fast as you can. Some producers even install rubber flaps and special kits to stop the air from escaping in places it should not. Next open your bottom sieve all the way to allow maximum air to reach the top sieve/chaffer. For wet or moldy corn, do all of your separation and cleaning on the top sieve.
Generally, the initial top sieve setting is 5/8 inch open on the front and middle sections, open it more if you have corn coming out the back and close it to clean up your grain tank sample. Another option is to reconsider how open the back foot that goes directly into the reclean system is; often we open this more than the main separation area, but with moldy corn, this may not be the best practice.
Using the same distance opening as the main sieve or less, down to even closing this section all the way, helps the air float the fines out instead of being lost. When vomitoxin levels are unacceptably high, close your reclean all the way and even block it off so that no air escapes. This helps with cleaning.
The last step is to put perforated screens under the augers and elevator doors on the combine. These perforated screens are often used for dry beans, and from our research, help to separate the fines. One quick test this fall showed vomitoxin levels around 26 ppm in samples of the screenings coming from our perforated screens, while the grain tank sample was 5 ppm.
We are still running tests to determine how effective this approach is for reducing vomitoxin, but some work out of Canada showed a 40% vomitoxin reduction through perforated screens on all combine auger and elevator bottoms. One important thing to keep in mind is that this works best once corn is below 25% moisture.
Above these levels, the perforated screens can create more fines as they cause a grinding action on the wet corn.
There is a YouTube Video form Pete Johnson that has be circulating which shows many of these adjustments (see below). However, remember that adjusting the combine to reduce vomitoxin is a careful balancing act between reduce the vomitoxin levels and not loosing good corn from the machine.
Screening, cleaning, and harvesting strategies will reduce vomitoxin contamination by removing lightweight kernels and fines, but will not get rid of vomitoxin that is inside the grain.
Sampling, Testing, and storage
Grain samples should be pulled from field suspected of having ear rots problems and tested for mycotoxins. As we mentioned in our factsheet (here), moldy kernels are usually not evenly distributed in a grain lot, and as a result, toxin-contaminated grain are usually found in pockets (hot spots). Consequently, poor sampling and/or testing technique may lead to incorrect estimates of vomitoxin in the grain lot.
For instance, a sample pulled from a hot spot may lead to an overestimated of the overall level of contamination on the load. To obtain better estimates of contamination, pull multiple samples from the grain stream during harvest or from the grain truck/wagon. When testing at grain elevators, you may request a second sample be drawn if you feel the first sample was not representative of the entire lot.
Following vomitoxin testing, you have the right to reject the grain buyers’ results and ask the handler to send the second sample to a federally licensed grain inspector for a re-test. Refer to Ohio Code 926.31 for details.
Grain should be dried down to below 15% moisture and storage in a clean dry bin. If multiple bins are available, consider testing and segregating different fields based on vomitoxin test levels. After drying there are usually more fines that may separate out.
These can be removed using rotary screen separators and slant screen separators. We are still studying to determine the level of vomitoxin reduction you can expect to see with screening methods, but we have seen it significantly reduce very high vomitoxin numbers.