By Pierce Paul, Cereal Pathologist
More and more reports are coming in of corn testing positive for vomitoxin, with levels as high as 6-10 ppm in some cases. Some of these numbers are taking growers by surprise.
Although the weather has been favorable for ear rot development, and consequently, grain contamination with vomitoxin, test results could be misleading in some cases, and may even be incorrect. Since there is not a lot you can do about grain contaminated with mycotoxins, you should at least check to make sure that you got a fair test. Get a second opinion if you have to.
There are several things about the mycotoxin testing process that could lead to inaccurate results, including how samples are drawn and handled. Remember, the number of ears infected within a field and the number of kernels infected on a given ear are highly variable. As a result, moldy grain and vomitoxin levels vary considerably within the grain lot. There are always “hot spots” and these may affect the accuracy of sampling and testing for vomitoxin.
For instance, if a single sample is drawn and the location from which it is drawn happens to be a hot-spot, then the level of contamination of the lot will be overestimated. Conversely, if the sample misses the hot spots completely, vomitoxin contamination may be underestimated. Poor sampling may result in considerable variation in test results, and could be the cause of some grain rejections.
If you have questions and concerns about how the samples were drawn and the test results, pull another sample and get a second opinion from an independent lab. Before pulling samples for toxin analysis, grain handlers should first protect themselves from dust and toxin exposure by wearing a mask, goggles and gloves. Careful attention to sampling, extraction and testing protocol should be followed to accurately measure vomitoxin accumulation in grain.
To collect a representative sample from the bin or truckload of grain, 5-10 subsamples should be randomly collected from multiple locations. Samples taken only from the central or outer portions of the load or from the beginning and end of the grain stream will not provide an accurate estimate of toxin contamination. For end-gate sampling, sample from the entire width and depth of the stream. For probe sampling, use hand or mechanical probes to sample from the entire bin, in an X-shaped pattern, for example.
The use of suction or air probes is not recommended when sampling grain for mycotoxins. Once subsamples are obtained, bulked and cleaned, the grain must be ground uniformly, in a clean grinding apparatus, to resemble flour. Finer particle size increases surface area of the grain and allows for more efficient extraction of vomitoxin.
The most common test for vomitoxin is an ELISA test. These kits are very specific for the toxin being tested (vomitoxin in this case) and will not provide estimates of other toxins in the sample. There are separate kits for each toxin.
ELISA-based tests are generally qualitative, providing a yes/no answer for the presence of DON, or semi-quantitative, giving an estimate of DON above certain levels or within a given range. However, quantitative estimates can also be obtained using some ELISA-based test. A color-change will be indicative of vomitoxin presence in the sample. To quantify toxin concentration, an additional step of assessing color quality through a well reader or spectrophotometer is required. There is a relationship between the intensity of color in the sample cup and vomitoxin, as determined by a standard curve included in the kit.
ELISAs are easy, quick and affordable, but must be performed carefully to ensure quantifiable and accurate results. Due to the test’s specificity, you must use an ELISA kit specifically designed to detect vomitoxin. In addition, the ELISA kit must be approved for the substrate to be tested (corn, DDGs, wheat, etc.).
In addition to resampling and retesting, here are a few guidelines to help minimize further mold development and toxin accumulation in storage:
1. Dry harvested grain to 15% moisture and below to prevent further mold development in storage.
2. Store dried grain at cool temperatures (36-44 F) in clean, dry bins. Moderate to high temperatures are favorable for fungal growth and toxin production.
3. Periodically check grain for mold, insects and temperature, and send grain samples for mycotoxin analysis.
4. Avoid storing severely affected grain for too long, especially since it becomes much more difficult to keep grain cool and dry as outside temperatures increase in the spring and summer.
See Fact Sheet PLPATH-CER-04 for more on sampling, testing, and storage for vomitoxin in corn.