The 2008 corn harvest was complicated by a wet growing season and wet harvest. Some corn was not adequately dried for long-term storage because of the cost of fuel to dry the corn. As a result, some corn was damaged in the field and some corn has continued to deteriorate in the bin.
Eleven of 94 (11%) cases submitted to the Iowa State Veterinary Diagnostic Laboratory (ISU VDL) from August 2008 through May 2009 had corn that contained a mycotoxin. Aflatoxin ranged from trace amounts up to 640 ppb (parts per billion), vomitoxin varied from 0.5 ppm (parts per million) up to 17 ppm, zearalenone varied from 1-38 ppm, zearalenol from 0-2 ppm, and T2 toxin varied from 0-0.6 ppm.
Vomitoxin (DON) and zearalenone are mycotoxins that are poorly tolerated by swine. DON can cause nearly complete feed refusal at high concentrations (> 5 ppm), and zearalenone is a hormonal toxin similar to the effects of estrogen, causing infertility in sows when fed at high concentrations (3-10 ppm or more). DON, T2 and DAS (diacetoxyscripenol) are all in the trichothecene class of mycotoxins and the effects can be additive.
In cases submitted to the ISU VDL where mycotoxins have been demonstrated, the most common clinical effects reported were decreased growth rates, some degree of feed refusal, vulvar swelling in prepuberal gilts, and decreased reproductive efficiency in females. Effects observed in swine are included in Table 1.
The same fungi that produce vomitoxin may also produce zearalenol and zearalenone. The presence of one indicates there is a high likelihood that the other mycotoxin is present as well.
Sampling is Critical
Sampling is the key to accurately determining the presence of mycotoxins in a feed sample. It is best to collect samples from moving grain to get a random sample. To achieve a more accurate estimate, it is critical that the collected grain sample be representative of an entire truckload or bin of grain. Grain and other particles, such as weed seeds, separate based on particle size and density as they flow and settle into a truck or bin. Smaller, denser material often may be found in the center of the truck or bin, and this is the material that is often higher in mycotoxin content. The probe is the only sampling method approved by the U.S. Department of Agriculture’s Grain Inspection Packers & Stockyards Administration for stationary lots. Multiple (5-10) probe samples are generally recommended to obtain the best representative samples.
DDGS Confounds Feed Quality
The addition of distiller’s dried grains with solubles (DDGS) to feed containing corn contaminated with fungi can complicate the mycotoxin picture. Mycotoxins are reported to be concentrated in DDGS as much as three times that of the original contaminated corn. Besides the potential for concentrated amounts of mycotoxin, DDGS are also a good substrate for fungal growth whereby additional mycotoxin may form before consumption.
The particle size of DDGS varies around 200-300 microns and this can contribute to bridging in bins. If bridging occurs and fungi are present, you can expect growth of fungi in the bin. Feed bins need to be periodically inspected for bridging, especially as the inclusion rate of DDGS increases. If buildup has occurred in feed bins and feeders, thorough cleaning and spraying 10% bleach on bin surfaces before refilling the bin is recommended to reduce fungal contamination. However, Clorox is not likely to destroy existing mycotoxins.
Sample with Care
Analyses for mycotoxins and fungi in feedstuffs are confusing. Spore counts are sometimes used to estimate overall feed quality but are not useful to estimate mycotoxin presence in feed or feedstuffs.
There are various mycotoxin test kits on the market. Some are extremely sensitive and will detect low levels of toxin that are not of concern. Positive results from test kits should be confirmed and quantified. Quantitative assays are preferred to help determine the biological risk (Table 2).
A variety of “mycotoxin binders” are marketed based on detection of low levels of mycotoxin. The value of these binders is questionable when mycotoxins are present at low levels, with the possible exception of aflatoxin. Aluminosilicates and other adsorption binders are effective against aflatoxins, but efficacy for vomitoxin, zearlenone and fumonisins is not well documented. The Veterinary Diagnostic Lab at Iowa State University performs these mycotoxin analyses on a confirmatory and quantitative basis. A 200-gram sample is suggested for submission after sub-sampling is performed.
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Steve Ensley, DVM, PhD
Iowa State University Veterinary Diagnostic and Production Animal Medicine
sensley@iastate.edu