Thus, it is important to regularly analyze these feeds to maintain dietary consistency as much as possible. Compounding the variable nutritional characteristics of forage crops, the dominant populations of natural micro-organisms that colonized the standing plants in the field also vary considerably and will dictate the fermentation and preservation characteristics of the resultant silages.
These microbes play various roles during the different stages of the ensiling process, but the goal is to have a dominant army of efficient lactic acid bacteria (LAB) to prevail in the ensiling battles and win the microbial war, maximizing the feed value of the silage.
In this microbial war, molds are one of the main pathogenic spoilage organisms that can cause significant issues in ensiled feeds.
Molds are ubiquitous fungi that have a fuzzy or dusty appearance due to their characteristic filamentous growth and production of masses of spores on structures sticking up out of the filamentous “mat.”
Although molds do not always pose a threat to healthy animals with proper immune function, they can infect animals that have suppressed immunity (e.g., under stressful situations, during the transition period, etc.), typically by producing spores that cause respiratory problems.
Some molds can also produce mycotoxins under certain circumstances, causing issues beyond the more obvious mold infection (mycosis).
Mycotoxins affect animals by reducing intake, impacting nutrient absorption and metabolism, impairing the immune system, damaging organs and altering their exocrine and endocrine systems.
Ruminants may be protected from immediate or acute health problems given that mycotoxins may be inactivated to various extents in the rumen. However, this can hide chronic problems potentially caused by these compounds.
Because they are harvested at a more mature growth stage, whole-plant corn, sorghum or small-grain cereals are more prone to mold growth than grasses or legumes.
Molds include a wide range of genera (e.g., Fusarium, Aspergillus, Mucor, Penicillium). Some molds proliferate while the crop is growing in the field prior to harvest while others propagate during the storage phase.
At the time of ensiling or during fermentation, mold growth can continue while there is still air trapped in the forage mass. Therefore, mold growth is inhibited in well-preserved silages characterized by low pH, presence of short-chain fatty acids and the absence of oxygen.
Because mold growth occurs where anaerobic conditions have not been achieved or maintained, contamination is usually seen in the surface layers of the ensiled forage and often indicates poor sealing or compaction. Mold can also develop during advanced stages of aerobic deterioration in silages, which is initiated by lactate-assimilating yeasts.
Molds cannot only use sugars and lactic acid for growth via respiratory pathways but can also metabolize cellulose and other cell-wall components, increasing the already considerable losses of nutrients and dry matter due to yeast growth.
The presence of molds is easily observed, but their surrounding areas likely contain mycelium not visible to the naked eye, other spoilage organisms like yeasts and aerobic bacteria, and potentially, mycotoxins. Thus, it is easy to underestimate the compounding detrimental effects of these microbes.
Mycotoxins represent a conundrum to all involved in animal feeding. There are likely many more toxins produced by molds than those we know of which, in turn, is a considerably higher number than those we can detect through analysis.
Even then, we have only very limited knowledge of the effects in animals of an even smaller number and almost no knowledge of the potential interactions when more than one toxin is present, which in nature will almost always be the case.
Mycotoxins can accumulate on the plant in the field, during harvest and silo filling, while in storage or during feedout and tend to occur in hot spots in the feed, where molds have been able to grow due to the presence of air, though not necessarily right where the visible mold growth is.
Aerobic spoilage of silages is almost always initiated by the growth of yeasts, so preventing their growth can help in eliminating the chance for that “secondary” growth of the opportunistic molds during feedout. However, Penicillium molds are more resistant to acidic pH levels and low oxygen concentrations.
As a result, they are commonly found in corn silages. Wet, cool conditions are favorable for these typically blue/green molds, species of which can produce patulin and PR toxin, mycotoxins harmful to cattle. In particular, the presence of PR toxin has been highly correlated with moldy silages and herd health issues.
Another frequently observed mycotoxin is deoxynivalenol (DON), which is produced by Fusarium species. Besides DON, the pink/red/purple Fusarium molds can also produce zearalenone, T-2 toxin and fumonisin.
Aspergillus molds are also an important source of mycotoxins potentially detrimental to cattle. They typically have a green-grey coloration and like dry, warm conditions.
Aspergillus fumigatus may cause abortions, mastitis and mycotic pneumonia, has been associated with hemorrhagic bowel syndrome and also can cause a severe infection known as “farmer’s lung” in humans.
Aspergillus species may produce gliotoxin and, most importantly, aflatoxin (Aspergillus flavus), which is the only toxin the FDA has imposed statutory limitations on (20 ppb for grain and feed products and 0.5 ppb for milk).
An exception to the characteristic appearance described above is Aspergillus ochraceus, which produces white hyphal filaments and bright yellow spores. It also produces ochratoxin which, while a cause of kidney damage in monogastrics, is fortunately broken down readily in the rumen.
Just like we won’t eat moldy food ourselves, discarding moldy silage is a good practice. While ruminants can generally tolerate and eat a little mold without problems, remember that by the time we can see molds in the feed, the feed value has been highly compromised.
Some types of mold may cause reproductive and production problems, so why take any risks? Also, people handling the feed are also at risk, not least to the previously mentioned farmer’s lung; inhaled spores lead to mold growth in the lung tissue.
If molds are present and contamination with mycotoxins occurs, preferably discard but at least dilute the contaminated silage.
Clay-based materials such as bentonite, zeolite and calcium aluminosilicate (50 to 225 grams per cow per day) have been shown to reduce the effects due to aflatoxin contamination, while yeast cell-wall extracts (10 to 20 grams per cow per day) – also called MOS and glucomannans – can be effective in reducing toxicity due to T-2 toxins, DON and zearalenone.
Prevention of molds is vital to reduce spoilage losses and the presence of mycotoxins in silage. As molds are opportunistic pathogens that colonize dead or decaying plant material, all areas of agronomic and silage management need to be addressed, such as:
- Minimize stubble left in the field.
- Plant insect-, disease-resistant varieties and rotate crops.
- Monitor plants in the field for animal damages, diseases and climate stressors such as drought, lodging and hail damage.
- Check and clean storage structures and size the amount of silage removed daily with the herd requirements to maintain a feeding rate faster than the rate of deterioration.
- Keeping the air out of the silo is mandatory to start the ensiling process correctly. Mature, dry forages are harder to pack well, so ensile the crop at the recommended stage of maturity and moisture level.
- Use silage additives at effective dose levels. A high dose rate of Lactobacillus buchneri has been positively reviewed by the FDA for increasing aerobic stability, reducing opportunistic secondary mold growth at feedout.
As a caution against cutting dose rates, even with chemical additives, it has been reported that the use of a low rate of propionic acid not only did not inhibit Aspergillus flavus growth but actually stimulated aflatoxin production.
- After packing, seal the silo effectively, make sure seams are overlapped by 6 feet or more, apply extra weight on seams and edges and monitor plastic integrity during storage (repair holes and tears immediately).
- During feedout, adopt a removal rate that reduces heating and spoilage (minimum of 6 inches per day), keep the face clean and straight, and avoid leaving silage in drop piles (or compost heaps) for extended periods.
Molds and mycotoxins are widely prevalent and are a major source of economic losses. Adopting good ensiling practices helps to inhibit mold development and reduce feed losses and the potential of mycotoxin production.
The use of additives along with dilution of the feed can help reduce the effects on animal productivity. FG
Bob Charley is Forage Products Manager at Lallemand Animal Nutrition.
PHOTO 1: Fusarium mold in corn silage.
PHOTO 2: Penicillium mold in corn silage. Photos courtesy of Renato Schmidt.
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Renato Schmidt
- Forage Products Specialist
- Lallemand Animal Nutrition
- Email Renato Schmidt
