A multilevel approach can reduce mycotoxins
|By Mike Rankin|
Producers who have concerns about molds or mycotoxins should begin preventative measures when looking through seed catalogs. Most seed companies provide resistance ratings for Fusarium, Gibberella and Diplodia ear rot. Unfortunately, resistance ratings are not available for Aspergillus (produces aflatoxin) due to the difficulty of obtaining locations or methods which provide consistent visual or mycotoxin results necessary for rating hybrids.
Some molds are more commonly found in the field while others predominate in feeds or silages during storage. Field mold spores are virtually everywhere and easily survive over winter in soil and plant residues. These molds can enter corn through the roots during the seedling stage, travel down silk channels during pollination, or via plant wounds from environmental or insect injury.
The presence of visible ear molds in the field does not correlate well with actual mycotoxin contamination levels. However, estimates are that 70 to 90 percent of all mycotoxins are already on the plant prior to harvest and ensiling.
The most common field molds are Aspergillus, Fusarium and Gibberella, capable of producing toxins including aflatoxin, fumonisin, vomitoxin (DON), zearalenone and T-2. Aflatoxin is the only mycotoxin regulated by the United States Food and Drug Administration with an upper legal limit in milk of 0.5 ppb (parts per billion). Aflatoxin can be detected in milk within 12 to 24 hours of consuming contaminated feeds, but the good news is that when these feeds are removed from the ration, milk aflatoxin levels fall back to acceptable levels within one to four days.
To minimize the problem
Practical approaches to reducing field-produced toxins are:
1. Selecting hybrids with insect, stalk rot and ear mold resistance
2. Timely harvest with particular attention to proper moisture levels
3. Isolating silages from crops exposed to severe drought or hail damage
4. Using traditional tillage methods to reduce mold spores in crop residues
Also, it’s important to note that no silage acid or inoculant product is capable of degrading preformed, field-produced toxins.
Field molds described above do not typically grow in the low oxygen, low pH environment found in well-managed silages. However, these molds can grow and produce additional toxins in silages that have low harvest moisture, poor compaction or experience poor feed-out methods.
Crops heavily laden with yeast (such as corn silage and high-moisture corn grain) are of particular concern because yeast can elevate silage pH, creating conditions more conducive for the growth of field molds when excess oxygen is allowed to 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. Most experts agree that Penicillium (typically green-bluish in color) and their toxins (primarily PR [Penicillium roqueforti], 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. Many may lack awareness of PR toxin because no laboratory, to date, has developed a commercial screen to detect this toxin. The only practical approach to preventing growth of storage molds is implementing silage management practices that create and maintain anaerobic silage environments.
Elusive to confirm
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. The best approach for estimating actual mycotoxin intake is to sample the feed after being blended in the TMR mixer. This is a safer approach providing a more homogeneous sample compared to traditional methods of sub-sampling composited, random samples taken from across the face of the storage structure.
ELISA (enzyme-linked immunosorbent 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 utilize a diagnostic laboratory providing chromatography methods such as HPLC (high pressure liquid chromatography), GC (gas chromatography) or TLC (thin layer chromatography).
Once toxins are detected, or highly suspected, producers must decide on a practical approach to remediation. Unfortunately, there are few options with guaranteed success other than segregating obviously spoiled feed and implementing perhaps the most effective remedy of “dilution as the best solution.” This is much easier to accomplish on farms that have multiple storage options for isolating problem silages, compared to those ensiling all the forage in one or two large bunkers.