Mould spores are virtually everywhere in the field and easily survive over winter in soil and plant residues. The most common method of fungal 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 spp.) are capable of producing recognizable toxins including aflatoxin, vomitoxin (DON), fumonisin, zearalonone and T-2. Estimates are that 70-90% of all mycotoxins are already on the plant prior to harvest and ensiling. However, the presence of visible ear moulds does not correlate well with mycotoxin contamination. It should be noted that no silage acid or inoculant product is capable of degrading these preformed, field-produced toxins.
Practical approaches to minimizing field produced toxins are:
1. Reduce fungal populations and access sites by planting hybrids with insect, stalk rot and ear mould 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.
4. Consider traditional tillage methods to reduce fungal spore loads in crop residue.
The field fungi described previously 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 aerobically-challenged storage conditions caused by low harvest moisture, poor compaction or improper feedout techniques. Crops heavily laden with Candida and Hansula yeast species are of particular concern because these lactate consumers can elevate silage pH. If excess oxygen penetrates high pH silages, conditions are conducive for additional growth of the field fungi in the storage structure.
Mould species isolated from silage and high-moisture grains primarily include Fusarium, Mucor and Penicillium with a much smaller incidence of Aspergillus and Monila.
Storage fungi like 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-greyish in colour and do not produce any known mycotoxins. Their primary concern is reducing silage nutritional quality, bunklife and palatability.
Based on a technical service sample summary conducted by Pioneer in the US, the most common moulds found in silage and HM grains were:
- Mucor (white/grey fluffy) – 45%
- Penicillium (green/blue) 45%
- Aspergillus (yellow/green) – 7%
- Monilia (white/yellow) – 3%
Most experts agree that Penicillium (typically green-bluish in colour) and their toxins (primarily PR, but also patulin, citrinin, ochratoxin, mycophenolic acid and roquefortine C.) are of greatest concern in ensiled forages because of their resistance to low pH. Nutritionists may lack awareness of PR toxin because no laboratory, to date, has developed an economical screen to detect this toxin. 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 opportunistic diseases associated with compromised immune systems. It is important not to totally rule out a toxin issue, even in normal-appearing silage, because toxins can be present in silages lacking visible spoilage or fungal growth. Conversely, mouldy silage may be completely free of detectable toxin loads. It is often difficult to confirm mycotoxins 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 approach is to compare the analysis of spoiled/moldy samples to normal-looking silage. The best approach for estimating actual toxin intake, from questionable forage or grain, is to sample the feed after being blended in a TMR mixer. This provides a more homogeneous sample compared to traditional methods of sub-sampling composited, random samples taken from across the face of the storage structure.
Bill Mananna, Ph.D., Bill Seglar, DVM., Fred Owens., Ph.D., Scott Denis, Ph.D., Robin Newell, M.S.