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Dealing With Scabby Grain, Vomitoxin Marcia McMullen, Ken Hellevang, Charlie Stoltenow, George Flaskerud Plant Pathologist, Agricultural Engineer, Veterinarian, Marketing Specialist Excessive and continuous rains in portions of North Dakota during grain flowering and fill resulted in a high incidence of scab in small grains during the 2001 growing season. Following are answers to frequently-asked questions on scab and vomitoxin. What is scab? Scab is caused by a fungus, Fusarium graminearum, that may invade developing small grain heads from flowering through kernel development. The scab disease also is called Fusarium head blight or FHB. Scab infection occurs when long periods of dew, cloudy and foggy weather, and temperatures of 70 to 85 degrees all coincide with flowering and early grain fill. However, infection does occur at cooler temperatures when moisture or dews persist for 72 hours or longer. A crop may be susceptible to scab infection from flowering through dough stages of kernel development. What crops are susceptible to scab? The scab fungus can attack small grains, including wheat, barley, oats, and rye, and can also cause stalk rot of corn. Certain grassy weeds, such as foxtail, also may become infected. Scab does not affect broadleaf crops. What does scab look like? Symptoms of scab are premature bleaching of individual spikelets, partial heads, or whole heads. Florets may be bleached tan to white within green healthy portions of the head. Infected kernels are chalky white to pink and shriveled. A peach-orange mass of fungus spores may be visible at the base of infected glumes. The disease reduces yields and test weight. Scabby kernels are considered part of dockage at the grain elevator and high scab levels result in a lower grain grade and price discounts. Swath or straight cut fields with scab? When crop diseases are present, straight combining should be considered to minimize further disease risks. Severely infected fields or portions of field should be harvested and stored separately if at all possible. Swathing may be beneficial if moisture levels of grain can be brought down faster than with straight cutting. However, swathing onto wet ground or leaving infected grain in a swath that may get additional rains, could cause molding and further grain deterioration. Consider increasing fan speed on the combine to blow out some of the shriveled, diseased kernels and other light material that causes problems in marketing and storage. What is vomitoxin? Vomitoxin, also called deoxynivalenol or DON, is a toxin that may be produced from scab. The occurrence of scab does NOT automatically mean that vomitoxin is present, but in recent years, high levels of scabby kernels in the harvested grain generally meant high levels of vomitoxin also were present. Vomitoxin can affect flavors in foods and baking quality. Humans and animals potentially can get sick from vomitoxin, depending on toxin levels consumed. What are safe levels of vomitoxin? The Food and Drug Administration has established vomitoxin advisory levels as follows:
Research at North Dakota State University and in Canada suggests that ruminants are able to tolerate higher levels of vomitoxin than advised by the FDA. Studies on sheep and cattle, both fed and gestating, indicate that these species are not adversely affected by consuming complete rations containing up to 25 ppm vomitoxin. Contact a veterinarian or feed specialist for further information on safe feeding levels for specific animal feed uses. The phone number for the NDSU Veterinary Diagnostic Laboratory is (701) 231-8307. How does vomitoxin affect livestock? Vomitoxin causes feed refusal and poor weight gain in livestock. Hogs are most sensitive to vomitoxin, even at one parts per million contamination of hog feed. The toxin can also cause problems in horses, breeding and lactating animals, but at higher concentrations. Cattle, sheep and poultry are more tolerant of vomitoxin. Diluting scabby wheat with normal quality grain may be a logical method of feeding wheat safely to livestock. Straw from scab-affected fields should be suitable for livestock bedding (except hogs), but affected fields should not be grazed in lieu of harvesting because grazing animals likely would consume mostly grain heads that may be unsuitable for feeding. Again, contact a veterinarian or feed specialist before feeding grain infected with vomitoxin to livestock. Can I get sick from grain with vomitoxin? Grain with vomitoxin would have to be ingested in very high amounts to pose a health risk to humans. Human food products are restricted to the one ppm level established by the FDA, a level considered safe for human consumption. Moldy grain can cause allergy and breathing problems, however, so a good quality dust mask should be worn when working around grain with high amounts of dust and molds. What about moisture content of infected grain? Moisture content of scabby grain going into storage should be about 12 percent. Damaged grain is more susceptible to storage problems. Light, thin kernels caused by scab tend to accumulate in one location in a storage bin and higher moisture levels cause hot spots. Test grain for vomitoxin at a moisture contents of 14 to 15 percent or lower for accurate readings. Does drying or storage affect infection levels? Drying won't reduce scab or vomitoxin levels, but drying prevents further fungus development. Drying does not increase vomitoxin levels either. There is no known evidence of scab or vomitoxin increasing in storage, if grain is stored at appropriate moisture levels. Evidence also has shown that survival of the Fusarium fungus decreases with time in storage under proper storage moisture. The fungus requires 22 to 25 percent moisture content to grow. At that moisture level in storage, it is likely that other bacteria and fungi will out-compete the Fusarium fungus. The only exception is crib-stored ear corn. When storing scabby grain, try not to co-mingle it with grain from previous years. If possible, store this year's grain from scabby fields separately from grain with no signs of scab. How should I market my scabby grain? Producers may find it to their advantage to store and sell it later in the marketing year. Probably the worst strategy would be to sell grain directly off the combine. Discounts for quality are generally the most severe at harvest. Discounts generally decrease after harvest as the marketing system is able to assimilate the lower quality grain. Low quality durum that was $.75 at harvest in 2000 brought $1.50 in July 2001. Taking the loan deficiency payment at harvest has generally been profitable. A less risky, but likely less profitable, strategy is to take out a government loan and then monitor the elevator price net of discounts for a profitable opportunity to repay the loan and sell. As a last resort, the grain could be held until loan maturity and delivered to the Commodity Credit Corporation (CCC), in which case you will be billed for discounts at settlement. In pursuing alternative strategies, make sure that the grain of different qualities are segregated. It may be profitable for some or all of the grain to be cleaned and or blended. Once the grain is in the bin, check quality by getting a representative sample for each bin by using a probe. To get an idea of the value of your grain, obtain several bids on the samples from elevators. Take the grades on your grain samples to your local Farm Service Agency for their estimate of the loan settlement value on each sample. Keep in mind that an estimate of loan settlement value is just that, an estimate, and is based on your sample. A schedule of CCC discount factors can be found on the internet at www.fsa.usda.gov/dafp/psd/loan_premiums.htm. Can infected grain be cleaned? Scabby wheat can generally be cleaned for about 30 cents or more per bushel. Cleaning (air and screens or gravity table) will reduce scab and vomitoxin levels, but cleaning will result in a loss of some non-scabby grain as well. Compare the price you would receive for scabby grain with a cleaned-grain price, minus cleaning costs. Gravity table cleaning may not be suitable for severely infected grain because of the degree of clean out needed. Cleaning may pay for some producers. The scabby portion may be sold for livestock feed if it does not contain prohibitive levels of vomitoxin. If one field has low vomitoxin will other fields? Not necessarily. Production of vomitoxin from scab can vary and the environmental factors that favor vomitoxin formation are not well understood. Crop, variety, weed infestation level and localized environment all may affect vomitoxin level. How can I determine if grain has vomitoxin? The presence and amount of vomitoxin from scabby grain can only be verified through laboratory analysis. Tests must be done on samples that are representative of the field, bin, or grain wagon. Use proper sampling methods to obtain representative samples. Quantitative analysis for the presence of vomitoxin is done by the :
North Dakota Grain Inspection Services in Fargo, Grand Forks, Minot and Jamestown also perform vomitoxin tests for similar fees. Call the laboratory first to obtain specific details on how the sample should be collected, quantity needed for analysis and packaging instructions. Since kernels separate based on size and density in a grain load, be sure to collect a sample that is representative of the entire load. Your local elevator may also have a vomitoxin testing kit. Many elevators are using Neogen immunoassay test kits. The Neogen test has been approved for use by the Federal Grain Inspection Service. How can I minimize scab problems next year? The scab fungus overwinters on wheat, barley, oat and corn residue, but the disease generally is more severe if wheat or barley is planted into corn ground. The fungus that causes head scab in small grains also causes corn stalk rot. Corn stalks are an excellent reservoir of the fungus, which can survive in the stalks for several years. Crop rotations are always one of the best ways to reduce diseases. Crop rotation won't eliminate diseases, but planting into ground where the previous crops grown were not susceptible to diseases does reduce infection risks. Studies in Ontario, Canada showed that the number of wheat heads infected with scab when corn was the previous crop was five to 10 times higher than when wheat was the previous crop. The number of wheat heads infected with scab when wheat was the previous crop was about 1.5 times higher than if the previous crop was soybeans. In recent studies in Morris, MN, researchers showed that average scab severity in wheat was 1.5x higher when corn was the previous crop than when soybean was the previous crop. Are any varieties resistant to scab? No wheat varieties are totally resistant to scab, but some varieties have shown more tolerance, including recent releases of Alsen and McVey hard red spring wheats and Belzer durum. Planting at least two or three tolerant wheat varieties with differing maturity dates will help minimize the risk of scab infection. However, in a wet year large quantities of the fungus which causes scab are produced, subjecting virtually any wheat field to some degree of scab infection. Check variety trial results from this year's plots to see which varieties performed the best in the presence of scab. Currently grown six-row malting barleys are susceptible to scab. Conlon, a two-row barley variety accepted for malting, has more tolerance to the disease than current six-row varieties. Do fungicides help reduce scab severities and vomitoxin levels? Yes. Reductions of scab severities and vomitoxin levels of 50-70% have been common with certain fungicide applications in North Dakota. Success has been greatest when fungicides were applied to moderately susceptible spring wheat varieties. Under severe epidemics, fungicides have not reduced scab or vomitoxin levels sufficiently on durum or barley cultivars to achieve top market grade. The systemic fungicide Tilt currently (2001) has a state label for heading application to wheat, while Folicur fungicide has had Section 18 Emergency exemptions for heading application to wheat and barley since 1998. Fungicides should be applied at early flowering for wheat or early heading for barley, if weather conditions are favorable for scab infection. Non-ionic surfactants increase fundicide performance, as do nozzles directed at an angle towards the grain head rather than oriented vertically. Should I till infected fields? Tillage buries disease-carrying debris, allowing for microbial degradation of the disease organism and lowering the chance of dispersal of the disease organism. However, consult with your local NRCS about tilling scab-infected fields designated as highly erodible. An Ontario scab study showed that the incidence of scab in subsequent wheat crops was reduced by two to 10 times with fall plowing of corn fields. A Morris, MN study showed that moldboard plowing of corn residue reduced the disease severity by 1.5x over chisel plowing or no-till. Bear in mind that in a very wet year, scab may occur in fields even when planted on plowed ground, because of very abundant spore production and movement to nearby fields. What about burning scab-infected fields? Burning crop residue to reduce disease levels is generally not recommended. Fire would destroy disease organisms in straw but would not affect soilborne diseases. In addition to short-term smoke and fire hazards, burning results in a loss of soil nitrogen and increases wind erosion susceptibility. Consult first with your local NRCD office before burning fields designated as highly erodible. Also, verify failed acreage with your local FSA office. You may need to leave unburned check strips for FSA verification. Make a tillage pass or two around the field area to be burned to help contain the fire. Recognize that you are liable for crop and property damages suffered by others should your blaze become out of control. Are there seed considerations for next year? Definitely. Line up seed early for the next growing season. Quality problems several years in a row means good seed will be in demand. A 56-pound test weight and 85-percent germination should be minimum standards for saved seed. A seed quality and germination test of saved seed is recommended. The North Dakota State Seed Department analyzes seed for germination percentage. Contact the department at (701) 239-7210. Clean disease-infected grain before using for seed. Saved seed with germination of 80 to 90 percent should be treated with a fungicide seed treatment before planting. Some standard seed treatment fungicides significantly improved germination and emergence of scab infected wheat in previous years. Will a seed treatment prevent scab? No. A seed treatment before planting will improve seed germination and seed vigor, but it will not prevent subsequent head scab infection. Wet weather during flowering and grain fill is the primary factor behind scab infection. If next year's growing season should prove dry, diseases may not be a problem, regardless of rotation or tillage practices used. Does vomitoxin increase in storage? No. Vomitoxin is formed in the fields as a result of fungal growth in the kernel and in the glumes under conditions of very high humidities. Once grain dries below moisture levels of about 22 to 25 percent, fungal growth and vomitoxin production is stopped. Grain stored at appropriate storage moisture contents will not have further vomitoxin development. Does scabby wheat require changes in natural air/low temperature drying? Yes. The scabby wheat will be more susceptible to deterioration than good quality wheat. Therefore, recommended airflow rates to dry scabby wheat should be increased a little above recommendations for good quality wheat. Should scabby wheat be stored at a lower moisture content? Yes. Anytime the kernel is damaged there is more potential for grain deterioration during storage. Damaged wheat should be stored at or below about 12 percent moisture content, and should be cooled by aeration as quickly as possible. Does high temperature grain drying decrease vomitoxin levels? No. Normal grain drying will not decrease vomitoxin levels. Grain can be detoxified only at temperatures of about 450 degrees Fahrenheit, a temperature so hot that the grain is roasted. Does the vomitoxin or scab fungus pose a risk to grain handlers at harvest or in storage? Yes. Mold spores, including those of the Fusarium fungus, will cause allergic reactions and breathing problems if inhaled. Appropriate personal protective gear such as masks designed to keep out mold spores and grain dust should be used. Generally, these masks are rated as N95 type masks. Can the scabby grain be removed with a grain cleaner? Yes. The scab affects the kernel in a variety of ways that permit it to be removed. The kernels may be deformed enough that they can be removed by screening. The kernels will have a lighter test weight so the very light kernels can be removed by airflow. If screening and aspiration are not adequate the wheat can be sorted by density with a gravity table. The cost of cleaning will typically be about 25 to 30 cents per bushel. The amount of scabby grain and the amount of wheat lost during cleaning will need to be evaluated in making the decision to clean the grain.
NDSU Extension Service September 2001 Revised |