Mycotoxins represent a broad category of toxic agents produced by various naturally occurring fungi, mostly soil borne and environmentally dependent. Three types – aflatoxins, fumonisins and zearalenone – cause most mycotoxins in cattle, says Jim Simpson, a consulting beef cattle nutritionist who operates Simpson Nutritional Services, LLC, based in Canyon, Texas.
Simpson presented information on mycotoxins during the Academy of Veterinary Consultants (AVC) 2018 spring conference in Fort Worth, Texas.
Effects of mycotoxins in cattle often remain subclinical, causing performance losses but not clinical disease or death loss.
Aflatoxin, produced by Aspergillus fungus, is the most common mycotoxin in corn and other grains used in cattle feed. The fungus can grow on standing crops or in storage, depending on environmental conditions. Stressed crops, such as those grown under drought conditions, become susceptible to aflatoxins, as can grain damaged by insects. Aflatoxins are known carcinogens, and the FDA lists “action levels” for the toxins in feedstuffs, including 300 parts per billion (PPB) in corn for feedlot cattle and 100 PPB in corn for breeding cattle. The action level for aflatoxins in corn for dairy cattle is much lower, at 20 PPB, because the toxin can be excreted in milk.
At lower levels, aflatoxins in cattle feed inhibit growth and feed efficiency. At higher levels, the toxins can cause chronic liver disease and acute poisoning, and also are associated with a higher incidence of bovine respiratory disease.
Pre-harvest treatments, such as Afla-Guard and Aspergillus flavus AF36, show promise for reducing aflatoxins in grain through competitive exclusion, Simpson says. Also, Bt corn hybrids appear less susceptible to aflatoxins than conventional varieties, probably because of less insect damage to grains.
Simpson offers several suggestions for minimizing aflatoxin risk in cattle operations.
- Watch growing conditions in your area for signs of crop stress. Also utilize historic information as the soil-borne fungus tends to recur in certain fields when environmental conditions favor it.
- Test grain from each new crop, and test stored grain randomly through the year, as the fungus can spread during storage.
- Work with your analytical laboratory to ensure accurate sampling of feed stores.
- Ultraviolet “black light” screening can identify Aspergillus fungus in feeds, but produces around 50% false-positive results.
- Quick test kits can detect aflatoxins in feed as deliveries arrive, but the kits require significant financial investment and can slow the grain-receiving process.
- Removing fines from delivered feeds can reduce aflatoxin levels significantly.
- Purchase grain from reputable sources, ideally with aflatoxin guarantees.
While aflatoxins present the most common risk in cattle feeds, Simpson notes that fumonisins, produced by Fusarium fungus, can emerge as a problem in cattle, such as during a 2016 outbreak in Texas. The fungus can grow in fields or in stored grains, depending on environmental conditions. Hot, dry weather during the mid-growth stage in corn, and cooler, wet weather during pollination tend to favor Fusarium and fumonisins. The toxins are associated with leaky gut syndrome, immune suppression and oxidative stress in cattle, and are deadly to horses.
Zearalenone, also associated with Fusarium fungi, is most common in corn-corn rotations, especially with wet weather during silking and harvest. Cattle consuming grain contaminated with zearalenone can secrete zeranol, a nonsteroidal estrogen, in their urine, which can disqualify animals from programs specifying no hormone treatments.