As producers 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.
Mold and mycotoxin issues start when the crop is in the field. The most common method of fungi entry in corn is through the roots during the seedling stage, down silk channels during pollination and via plant wounds from environmental or insect injury. Common field fungi (primarily Aspergillus and Fusarium species) are capable of producing recognizable toxins including aflatoxin, vomitoxin (DON), fumonisin, zearalonone and T-2. Estimates are that 70-90% of all mycotoxin are already on the plant prior to harvest and ensiling, although presence of visible ear molds does not correlate well with mycotoxin contamination. Mold spores are virtually everywhere and easily survive over winter in soil and plant residues.
No silage acid or inoculant product is capable of degrading these preformed, field-produced toxins but the following practical approaches will minimize them:
- Fungi populations and access sites by planting hybrids with insect, stalk rot and ear mold resistance
- Harvest in a timely manner with particular attention to proper moisture levels
- Isolate silages from crops exposed to severe drought or hail damage
- Consider traditional tillage methods to reduce fungal spore loads in crop residues
Turning now to the silage: 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 feedout techniques. Crops heavily laden with yeast (Candida and Hanensula) are of particular concern because these yeast can consume lactic acid and thus elevate silage pH. Should excess oxygen penetrate the high pH silage, conditions become conducive for field fungi growth in the storage structure.
Most experts agree that Penicillium (typically green-bluish in color) and their toxins (primarily PR 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. 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. Conversely, moldy silage may be completely free of detectable toxin loads.
It often is 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 it has been blended in a TMR mixer. This is a safer approach that provides a more homogeneous sample compared with traditional methods of sub-sampling composited, random samples taken from across the face of the storage structure.
ELISA (enzyme-linked immune stimulant 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. It is best to use a laboratory providing chromatography approaches such as HPLC (high pressure liquid chromatography), GC (gas chromatography) or TLC (thin layer chromatography). Some nutritionists contract with a mycology laboratory to have silage fungi isolated and identified. If the isolated fungi are from a toxin-producing 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 highly suspected from fungi identification, nutritionists must decide on a practical approach to remediation. Unfortunately, the options are few: segregate obviously spoiled feed; use "shotgun" approaches to neutralize toxin effects; or stimulate the immune system by increasing ration energy, protein, vitamins (A, E, B1) and minerals (Se, Zn, Cu, Mn). 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 the forage in one or two large bunkers.
It also has been shown that binding agents are capable of reducing toxin levels in feed. However, while many of these products have GRAS (generally recognized as safe) status, the Food and Drug Administration (FDA) does not allow 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.