By Bill Mahanna
As we start feeding new-crop forage and grains, there will undoubtedly be mold and mycotoxin concerns, especially in regions where crops have suffered environmental or disease stress.
There have been many excellent reviews of the more than 400 known mycotoxins, including one by L.W. Whitlow and W.M. Hagler Jr. in the recently released 2008 Feedstuffs Reference Issue & Buyers Guide (Whitlow and Hagler, 2007). This article will focus on the practical, applied aspects of mycotoxin testing, prevention and treatment drawn from recent discussions with some of the leading authorities in the industry.
In the Field
Mold spores are virtually everywhere and easily survive over winter in soil and plant residues. The most common method of fungi entry in corn is through the roots during the seedling stage, down silk channels during pollination and plant wounds from environmental or insect injury (Seglar, 2007; Munkvold, 2003). Common field fungi (primarily Aspergillus and Fusarium spp.) are capable of producing recognizable toxins, including aflatoxin, vomitoxin (deoxynivalenol), fumonisin, zearalonone and T-2.
Estimates are that 70-90% of all mycotoxins are already on the plant prior to harvest and ensiling, although the presence of visible ear molds does not correlate well with mycotoxin contamination (Seglar, 2001).
Practical approaches to minimizing field-produced toxins are: (1) reduce fungi populations and access sites by planting hybrids with insect, stalk rot and ear mold resistance, (2) harvest in a timely manner with particular attention to proper moisture levels, (3) isolate silages from crops exposed to severe drought or hail damage and (4) consider traditional tillage methods to reduce fungal spore loads in crop residues. No silage acid or inoculant product is capable of degrading these preformed, field-produced toxins.
The field fungi described above do not typically grow in the anaerobic, low-pH environment found in well-managed silages. However, it is possible for these fungi to produce additional toxins in the storage structure, but only in aerobically challenged silages resulting from low harvest moisture, poor initial compaction or improper feed-out techniques.
Crops heavily laden with Candida and Hansula yeast species are of particular concern because these lactate consumers can elevate silage pH. Conditions then become conducive for growth of field fungi in the storage structure should excess oxygen penetrate high-pH silages.
Mold species isolated from silage and high-moisture grains primarily include Fusarium, Mucor and Penicillium with a much smaller incidence of Aspergillus and Monila (Seglar, 2007; Mansfield et al., 2007). Storage fungi, such as Penicillium, Mucor and Monila, do not typically invade the crop prior to harvest, but their soil-borne spores are on the forage crop when they are being ensiled.
Mucor and Monila are typically white to grayish in color and do not produce any known mycotoxins. Their primary concern is reducing silage nutritional quality, bunk life and palatability.
Most experts agree that Penicillium (typically bluish-green in color) and their toxins (primarily PR toxin, but also patulin, citrinin, ochratoxin, mycophenolic acid and roquefortine C) are of greatest concern in ensiled forages because they are very resistant to low pH (Whitlow, 2007; Kuldau, 2007).
Nutritionists may lack awareness of PR toxin because no laboratory, to date, has developed a screen to detect this Penicillium -produced toxin. However, Trilogy Analytical Laboratory is very close to commercializing a thin layer chromatography screen for PR toxin (Malone, 2007).
The only practical approach to preventing growth of storage fungi is implementing silage management practices that create and maintain anaerobic silage environments.
Nutritionists usually begin to suspect mycotoxin issues after linking observations of spoiled silage, digestive upsets and erratic intake with symptoms of opportunistic diseases that seem to be the result of compromised immune systems.
It is important not to rule out a toxin issue, even in normal-appearing silage, because it is well documented that toxins can be present in silages lacking visible spoilage or fungal growth (Whitlow, 2007). Conversely, moldy silage may be completely free of detectable toxin loads.
It is often difficult to confirm mycotoxin as the culprit responsible for production and health problems. The first obstacle is obtaining a representative sample from the contaminated portion of the crop. One might consider analyzing moldy samples for comparison with visibly clean areas.
The best approach for estimating actual toxin intake from questionable forage or grain is to sample the feed after being blended in a total mixed ration mixer. This is a safer approach that provides a more homogeneous sample compared to traditional methods of subsampling composited, random samples taken from across the face of the storage structure.
As an industry, we may be severely underestimating the contribution of toxins to production problems because they can often exist in conjugated forms (primarily with sugars) that escape laboratory detection (Whitlow, 2007). These undetected toxins can then exert their toxic and immunosuppressive effects when disassociated in the digestive tract (Kendra, 2005).
Enzyme-linked immunosorbant assay tests are designed as rapid and inexpensive toxin screens for grain, but they are prone to many false positives when used on forage samples (Malone, 2007). It is best to utilize a laboratory providing chromatography approaches such as high-pressure liquid chromatography, gas chromatography or thin layer chromatography.
Some nutritionists contract with a mycology laboratory to have silage fungi isolated and identified. If the isolated fungi are from a mycotoxigenic species, then toxins become a plausible causative agent, whether or not random feed sampling detected the actual presence of a toxin.
Once toxins are detected, or are highly suspected from fungi identification, nutritionists must decide on a practical approach to remediation. Unfortunately, the options are few, other than segregating obviously spoiled feed and "shotgun" approaches to neutralizing toxins' effects or stimulating the immune system by increasing ration energy, protein, vitamins (A, E, B1) and minerals (selenium, zinc, copper, manganese).
The most effective remedy may be the tried and true adage of "dilution is the solution." This is much easier to accomplish on farms that have multiple storage options for isolating problem silages rather than ensiling all of the forage in one or two large bunkers. The concept of dilution has several implications regarding mycotoxicosis.
It has been proposed that there are no more feed toxins today than in the past. It may be that animals today are consuming significantly more dry matter coupled with increased detection capabilities. Dilution also becomes an important consideration as producers feed more and more of a single feedstuff that may be susceptible to toxin issues.
A practical example might be the increase in corn silage inclusion rates in the West to offset higher grain prices. It would be prudent for nutritionists to communicate with growers, custom harvesters and feeders to ensure that silage is harvested, ensiled and managed in a manner that minimizes the potential for storage-produced toxins.
It has also been shown that binding agents are capable of reducing toxin levels in feed. However, while many of these products have generally recognized as safe status, the Food & Drug Administration does not allow the addition of these products to the ration specifically for the purpose of mycotoxin reduction. Obviously, more public funding of research in this area is warranted, along with appropriate regulatory standards.
The Bottom Line
There are field-produced toxins over which nutritionists have little control. Nutritionists can begin to exert influence by helping ensure proper harvest moisture, silage compaction and feed-out methods to reduce aerobic environments conducive to the growth of storage-produced toxins.
Confirming toxin presence is challenging from a sampling perspective and because toxins often exist in a non-detectable, conjugated form. If a laboratory analysis is conducted, chromotography methods should be used to test forage samples. New laboratory screens are becoming available to detect the presence of some of the common, storage-produced Penicillium toxins.
Kuldau, G. 2007. Personal communication.
Malone, B. 2007. Personal communication.
Mansfield, M.M., and G.A. Kuldau. 2007. Microbiological and molecular determination of mycobiota in fresh and ensiled maize silage. Mycologia, 99(2):269-278. The Mycological Society of America, Lawrence, Kan.
Munkvold, G. 2003. Agronomic factors related to mycotoxin contamination of corn. Pioneer Seeds Animal Nutrition Symposium. 64th Minnesota Nutrition Conference. Sept. 16. St. Paul, Minn.
Seglar, B. 2001. Mycotoxin effects on dairy cattle. Proceedings of the 25th Forage Production & Use Symposium. Wisconsin Forage Council meeting. Jan. 23-24. Eau Claire, Wis.
Seglar, B. 2007. Personal communication. firstname.lastname@example.org.
Whitlow, L. 2007. Personal communication.
Whitlow, L.H., and W.M. Hagler Jr. 2007. Mycotoxins in Feeds. Reference Issue & Buyers Guide. Feedstuffs. Vol. 79, No. 38. Sept. 12. Miller Publishing. Minnetonka, Minn.
This article was originally published in October 2007 Feedstuffs issue, and is reproduced with their permission.