Head Scab and the Relationship to Saved Seed and Vomitoxin Production

Bob Mulrooney, Extension Plant Pathologist; bobmul@udel.edu

Head scab has been observed in barley and wheat this season in varying amounts. Barley is just now arriving at the grain elevators. The amount of scab that occurs is dependent on the flowering time, the presence of the scab spores that infect the heads during flowering and the weather conditions during flowering. Most of the barley and wheat varieties that we grow have little or no resistance to head scab. The fungus can be present on old corn stover, and residues of old barley and wheat crops. What drives this disease is wet, warm weather during the flowering period. If the heads of barley or wheat are infected with the fungus (Fusarium graminearum) that cause head scab, that fungus can produce several toxins that can contaminate the grain. These toxins are often referred to as vomitoxins because they can cause feed refusal in non-ruminant animals. The most common vomitoxin that is produced by the head scab fungus is deoxynivalenol or DON for short. DON production by the fungus is extremely variable depending on environmental conditions. The presence of scab on the grain does not mean that the grain has to have DON nor does high or low levels of scab relate to the amount of DON present. A high level of scabby kernels in the harvested grain means that DON will likely be present.

What about the saving or using seed from scab infected fields? As much scabby wheat kernels as possible should be removed from good seed during combining and seed cleaning. This is not easily done with barley or may not be possible because barley does not get as light as wheat. Saved seed kernels can be infected with Fusarium, and seed treatments can reduce the effects of Fusarium on seed. Fusarium on seed can cause a seedling blight of barley and wheat but the seedling infections do not result in head scab or DON in fields that might be planted with infected seed. In fact some studies have shown a reduction of scab infections in seed during storage. Low levels of scab infected wheat or barley can be saved for seed if properly handled and treated without any risk of scab occurring in the crop from that seed.

Another issue for barley producers is that the threshold levels of DON in wheat may not be the same compared to barley presuming that the barley is not intended for human consumption. The DON threshold for wheat is 1 ppm because of human consumption concerns. Barley for feed can have up to 10 ppm without harmful effects depending on the animals being fed and the proportion of infected grain being fed. In my opinion barley should not be held to the same threshold as wheat depending on its destination or final use. See the following information on DON levels in food and feed.

What are the critical levels of DON for use in food and feed?
The concentrations of DON in grain are expressed as parts per million (ppm). One ppm is equivalent to 1 pound in 1 million pounds, 1 penny in $10,000, 1 minute in two years, or 1 wheat kernel in 80 pounds of wheat. The U.S. Food and Drug Administration (FDA) has established DON advisory levels to provide safe food and feed. Unlike aflatoxin in corn, DON is not a known carcinogen. Furthermore, grain with DON would have to be ingested in very high amounts to pose a health risk to humans, but it can affect flavors in foods and processing performance. Human food products are restricted to a 1-ppm level established by the FDA. This level is considered safe for human consumption. The food industry often sets standards that are more restrictive. DON causes feed refusal and poor weight gain in some livestock if fed above the advisory levels. FDA advisory levels are as follows:

1 ppm: Finished wheat products, such as flour, bran and germ that potentially may be consumed by humans. The FDA does not set an advisory level for raw grain intended for milling because normal manufacturing practices and additional technology available to millers can substantially reduce DON levels in the finished wheat product. However, individual millers or food industries may have stricter requirements than 1 ppm.

10 ppm: Grains and byproducts destined for ruminating beef and feedlot cattle older than 4 months and for poultry, providing that these ingredients don’t exceed 50 percent of the diet.

5 ppm: Grains and grain byproducts destined for swine, providing that these ingredients don’t exceed 20 percent of the diet.

5 ppm: Grains and grain byproducts destined for all other animals, providing that these ingredients don’t exceed 40 percent of the diet.

Taken from NDSU Fact sheet PP-1302, DON (Vomitoxin) in Wheat. http://www.ag.ndsu.edu/pubs/plantsci/pests/pp1302.pdf


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