DON (vomitoxin) in corn and wheat: testing, limits and storage

Where DON comes from and when risk is high

Fusarium graminearum infects wheat florets during flowering and corn ears through the silks, and in both crops the trigger is moisture at that window. Cool, wet grain fill keeps the fungus working, and kernel damage from insects, birds or hail opens more entry points.

Drying and cooling stop the fungus from making more DON, but nothing practical on the farm removes what is already inside a kernel: Crop Protection Network states that heating, freezing, roasting and chemical treatments cannot reliably reduce mycotoxins in grain, and Ohio State notes that drying does not reduce DON.

Field signs in corn and wheat

In corn, Gibberella ear rot is a pink to reddish mould that usually starts at the ear tip and can glue silks and husks to the ear. Pull husks fully back on ears from several parts of the field before black layer; Crop Protection Network's threshold is that if 10 percent or more show mould, schedule the field for early harvest and test the grain. Ohio State adds that grain with more than 5 percent mouldy kernels should generally not be fed.

In wheat, heads or parts of heads bleach white before the crop ripens, and affected spikelets hold shrivelled, chalky white or pink kernels, the "tombstones" that graders count. Nebraska's warning applies to both crops: appearance does not predict toxin, and healthy-looking kernels can carry DON, so the only way to know a number is to test.

Advisory levels and limits

Levels are in parts per million (mg/kg). Check whether a result is as-fed or dry-matter before comparing it with a limit.

United States (FDA advisory levels, 2010 guidance):

They are advisory rather than tolerances, but buyers write them into contracts.

Canada: As of October 2026, CFIA's RG-8 lists 1 mg/kg DON in diets for swine, young calves and lactating dairy animals, 5 mg/kg in diets for cattle and poultry, and 2 mg/kg in uncleaned soft wheat for human consumption. CFIA notes the document is being updated under the Feeds Regulations, 2024, so confirm the current table before a feed decision. The Canadian swine figure is a whole-diet value; the US figure is for the grain ingredient at up to 20 percent of the diet, which works out to a similar exposure.

Sampling and testing: why the sample matters more than the kit

DON sits in a small share of kernels that are not spread evenly through a truck, bin or silage pile. The Canadian Grain Commission's work on bulk wheat near international limits found that sampling accounted for two-thirds of total variation in results and that sample size offered the greatest room for improvement; its lab now grinds twice the mass and extracts four times as much ground grain. A probe that hits a hot spot overstates the lot; one that misses it understates it.

Habits that cut that error:

• Take 5 to 10 probes per truck or bin in an X pattern through the full depth; from a moving stream, sample the whole width and depth of the flow at intervals, not just the start and end.
• Avoid air (suction) probes on ear-rotted corn; they pull light mouldy kernels and fines and overstate DON.
• Combine the probes, mix, and grind the whole composite to a flour-like consistency before taking the test portion.

Test methods trade speed for precision. Lateral flow strips are fast and useful for screening loads, but they are qualitative or semi-quantitative and can give false positives and negatives. ELISA kits, the elevator standard, are semi-quantitative and accurate when run correctly, but each covers one toxin and must be approved for the commodity. Chromatography at a lab gives the defensible number for a dispute or a ration. Black light is not a DON test; DON does not fluoresce. If a result looks unrepresentative, ask for a resample and retest.

Harvest, drying, cooling and segregation

The fungus keeps producing DON until the grain is dry. Crop Protection Network and Ohio State both set the line at 15 percent: dry corn below 15 percent, keep it there, then cool it; Ohio State gives 36 to 44°F (2 to 7°C) as the storage range for dried corn. For scabby wheat, Nebraska advises 12 percent or less and aerating down to about 25°F (−4°C) as fall temperatures allow. Check bins regularly, core them to remove fines, and clean them between lots.

At the combine, raising fan speed blows out the lightest, most infected kernels; Nebraska notes this for scabby wheat and Ontario harvest guidance says the same for mouldy corn. Check losses behind the machine so you do not also blow out sound grain.

Segregate. Harvest and store known-infected fields, and the worst parts of fields, apart from clean grain, and never top up a bin of clean corn with suspect corn. Blending must stay within what your buyer and the regulations allow, and only works if both lots were sampled well enough that you know their numbers. Move suspect grain first and keep the cleanest grain for long storage.

Cleaning and screening: what it can and cannot do

Cleaning works because DON concentrates in fines, cob pieces, broken kernels and light, shrivelled kernels. In Ontario work summarised by OMAFRA, screenings from one lot tested above 15 ppm against 2.7 ppm in the precleaned corn. The catch is mass: those fractions are a small share of the load, so removing them does not move the whole-lot number far. OMAFRA's 2018 on-farm trials with a rotary screen cleaner and a perforated auger cleaner cut DON by 0.5 to 2.0 ppm, not enough to change marketability, and full-sized infected kernels stayed in the clean stream. Nebraska says the same of wheat. Treat cleaning as a way to improve a borderline lot and make grain safer to store, not as a fix for a bad one.

Grading, discounts and a checklist

Elevators test incoming loads and apply discounts or rejections against their own DON schedules. Nebraska observes that discounts are usually steepest at harvest and climb as DON rises above 2 ppm, and that deferring delivery, within the contract, can leave room to clean or find a tolerant end use. In Canada, ask your elevator how Fusarium damage and DON are assessed for your crop and class, and check the Canadian Grain Commission's grading factors.

• Scout fields that flowered or silked in wet weather first; at 10 percent or more mouldy ears, plan early harvest and testing.
• Harvest infected fields early, fan up, and keep them in their own bin.
• Dry below 15 percent (12 percent for scabby wheat), cool, core the bin, monitor.
• Match the number to the right table: FDA grain levels with diet caps in the US, CFIA diet levels in Canada, as-fed versus dry matter.
• Decide on cleaning with a before-and-after test, not by eye.