Fusarium head blight (FHB) has threatened this year’s wheat crop in Kentucky, causing concern among growers, crop advisors and grain buyers. While the impact of this disease is yet to be determined, it more than likely will vary from farm to farm and region to region. Knowing how the disease impacts seed quality can help growers deal with it in their operation.
Wheat kernels that are infected early (while the head is maturing) will likely be smaller in size, have a shrunken appearance and slightly discolored. However, if wheat heads are infected during field dry down, kernels that may contain mycotoxins may look normal to the naked eye resulting in the possibility of leading one to a false sense of security.
Prolonged conditions in the right combination can result in elevated levels of FHB (and other diseases). Molds responsible for mycotoxin production generally prefer warm, humid conditions (temperatures > 60º F and relative humidities > 70 percent) for extended periods of time to actually produce vomitoxin, (zearalenone or fumonisin). However, under ideal conditions that are specific to each type of mold, mycotoxins can be produced in a matter of hours!
For this reason, diligence is essential when managing any crop that is suspected of being contaminated with mycotoxins. Extra care must be taken when harvesting, handling, drying and storing the crop to minimize the cost penalties associated with discounts from excessive toxin levels or poor wheat quality. Test kits are available to determine mycotoxin levels and should be used to screen any suspected fields for damage.
Consider harvesting wheat early if sufficient drying capacity is available on the farm or commercially. Early harvest may help reduce the spread of head scab and other diseases within individual fields, and can also prevent field sprouting, boost test weight and perhaps most importantly, increase soybean yield when double-cropping. Also consider segregating wheat by
field or variety to prevent mixing sound wheat with diseased wheat. It may be best to harvest diseased wheat last to avoid the time consuming task of cleaning out the combine, carts/wagons, trucks, conveyors and other handling equipment between fields.
Proper adjustments to the sieving and cleaning section on a combine are critical when dealing with a contaminated wheat crop. Most wheat diseases turn plump healthy kernels into small, shriveled “tombstone” kernels, so typical recommendations for a conventional combine are to increase fan speed and manage the load of straw, chaff, weeds and foreign material on the sieves. If the fan is set too low, the walkers/sieves will fill up with straw and all wheat kernels will ride out the back on a mat of chaff. If fan speed is too high it will blow sound and shriveled kernels out the back.
Under normal harvest, fan speed should be set to provide good separation between sound kernels and straw or chaff. With “scabby” wheat that may contain vomitoxin, fan speed should be increased to remove light weight kernels. Operators should monitor grain in the tank often to determine machine settings for best performance, and recall that ground speed should be adjusted to match yield within fields so that a nearconstant feed rate is achieved through the combine.
Wheat fields that are suspected of having high levels of vomitoxin or fumonisin should be scouted or monitored prior to harvest to determine if segregation or early harvest is needed. With the right combine adjustments, wheat can be harvested above 20 percent moisture if sufficient drying capacity is available on the farm or commercially. Contaminated wheat should be dried to 13 percent within 24 hours and held separately from the rest of the crop. High temperature bin dryers or standalone automatic batch or continuous flow grain dryers are all adequate for drying high moisture wheat quickly. (For bin drying, grain depth must be managed to provide a minimum airflow rate of 5 cubic feet per minute (cfm) per bushel. A general rule of thumb for a given amount of airflow is that doubling the depth of grain requires 10 times more fan horsepower. For example, delivering 5 cfm per bushel (cfm/bu) in a 30-foot diameter bin filled with 3 feet of wheat (1700 bu) requires a 2.5 HP fan–but a depth of 6 feet in the same bin (3400 bu) requires 25 HP! Thus, bin dryers are limited to small batches for timely processing with limited heat to prevent over-drying the bottom layer of grain. Several bins are needed to add drying capacity.)
Table 1 shows the moisture content that soft winter wheat will approach with sufficient exposure to the temperature and relative humidity conditions shown. Daily average air conditions during late June and early July are generally near 75 degrees with 65 percent humidity. Under these conditions, wheat will approach 12.5 percent moisture which is safe for storage.
In contrast to bin dryers, self-contained automatic batch or continuous flow dryers have inherently high airflow rates (50 to 125 cfm/bu) so drying wheat in these units is often done with little or no additional heat. If heat is used, limit drying temperatures to 120º F (110 for seed wheat).
It is best to store sound wheat separately from diseased wheat and clean handling equipment between loads to avoid contamination. The next best option is to clean mixed wheat as it is moved into storage to remove shriveled kernels. If this isn’t practical for your farm, keep in mind that lighter wheat kernels will tend to collect in the center of a storage bin during filling, which restricts airflow in this region. For wheat that isn’t cleaned, core the bin soon after filling to remove trash and smaller kernels to improve airflow. Also, don’t fill a bin past the top ring to allow room for adequate ventilation in the head space and for grain inspections. Always be aware of the entrapment hazards associated with flowing grain and wear dust protection masks when
working inside bins and other enclosed spaces where grain is stored. More information on harvesting, drying and storing wheat is available at county extension offices and online www.uky.edu or http://www.bae.uky.edu/ext/Grain_Storage/PDFs/Wheat_HDS_ID-125.pdf