Mycotoxins are toxic substances produced by fungi (molds) growing on crops in the field, during handling or in storage. It is estimated that up to 25% of the worlds crops may be contaminated with mycotoxins. While over 400 mycotoxins have been chemically identified, the impact of only a few mycotoxins are known. Common types of mycotoxins include aflatoxin, deoxynivalenol (also referred to as DON or vomitoxin), zearalenone, T-2 toxin and fumonsin. Of the thousands of mold species that can grow on feedstuffs, only a small proportion produce mycotoxins.
Mold Growth and Mycotoxin Formation
Some molds proliferate while the crop is growing while others propagate during handling and storage. Field mold spores propagate in both the grain and forage parts of plants. Weather related growing conditions contribute to the onset of molds that may produce mycotoxins. Storage fungi are soilborne mold spores brought into the silo with forages. Most molds identified in silage do not produce mycotoxins.
In the fall of 2011, some of the Fraser Valley corn silage harvest picked up mycotoxin contamination from soil in lodged corn plants. The limiting factor for mold growth in stored silage is pH. If silage is stored too dry, or is insufficiently packed and covered then air allows for microbial activity which depletes silage acids allowing pH to rise with subsequent mold growth and possible mycotoxin contamination. With hay, the limiting factor for mold growth is moisture where contamination is more likely in higher moisture hay.
Mycotoxin Effects
An excess of mycotoxins cause undesirable effects when animals are exposed. Sometimes mycotoxins occur at concentrations high enough to cause major losses in health and performance of animals. However, mycotoxins are usually at lower levels that result in interactions with other stressors to cause subclinical losses in performance, increases in incidence of disease and reduced reproductive performance.
The mode of action of mycotoxin ingestion within ruminants is not clearly defined nor well understood, however it is known that mycotoxins exert their effects through three primary mechanisms:
- Reduction in amount of nutrients available for use by animal
- Effects on hormonal systems and subsequent reproductive performance
- Suppression of immune system
In the past, ruminants were thought to tolerate the adverse effects of mycotoxins – perhaps due to the ability of rumen microbes to detoxify mycotoxins. However, with high producing dairy cattle increased rumen passage rates overwhelm the ability of the rumen to completely denature the toxins.
Mycotoxicosis Symptoms and Diagnosis
The diversity of symptoms make mycotoxicosis diagnosis confusing and difficult. Sypmtoms of mycotoxicosis may be vague or nonspecific but may include: reduced feed intake, feed refusal, unthriftiness, rough hair coat, poor body condition and reproductive problems. Mycotoxins have also been associated with increased transition cow problems including more substantive symptoms such as displaced abomasums, ketosis, retained placenta, metrtitis, mastitis, fatty livers and other infectious disease because of immune suppression.
A definitive diagnosis of mycotoxicosis can not usually be made from symptoms, tissue damage or feed analyses. Regardless of the difficulty of diagnosis, mycotoxins should be considered as a possible cause of production and health problems when symptoms exist and problems are not attributable to other typical causes. These same factors make it difficult to establish levels of safety for mycotoxin ingestion. Interactions with other stress factors make recommendations difficult as animals under environmental or production stress may show more pronounced symptoms. Also, partial degradation in the rumen complicates recommendations.
Mycotoxin Testing
The accurate determination of mycotoxin concentrations present in grain and forages depends on a number of factors. The largest source of error is due to sampling. Molds grow in hot spots and associated mycotoxins are not uniformly distributed within a feed making it difficult to obtain a representative sample. Analytical techniques for mycotoxins are improving but constraints exist with respect to cost of analyses and the practicality of testing for a multitude of toxins. For example, testing for one type of mycotoxin does not preclude that another mycotoxin may be prevalent. Moreover, commercial testing for some specific toxins may not be readily available. Analyzing for mold spore counts may not be useful and are only a gross indication of potential for toxicity as not all molds produce mycotoxins. Generally, where growing conditions were conducive for feeds with mycotoxin contamination it is advisable to forgo mycotoxin testing and include an economical mycotoxin binder in the ration.
Managing Mycotoxin Contamination
As we understand more about mycotoxin diagnosis, we need to determine methods for reducing or removing the adverse effects of theses compounds on livestock production. There probably are levels of mycotoxins where no adverse effects will be observed and these are probably greater for ruminants than non-ruminants. However, it is prudent to lower concentrations as much as possible and manage mycotoxins that are present.
Prevention of mycotoxicosis begins in the field with good agronomic practices as the most commonly diagnosed mycotoxins found in forages are produced in the field prior to ensiling. Irrigation, reduction of conditions conducive to mold growth and minimizing soil contamination are all practices that will reduce the incidence of mycotoxin contamination in the harvested crop.
Choosing varieties that have resistance to fungal disease and insect damage can reduce field produced mycotoxins. Crop stressors such as wind, bird, hail, flood or insect damage increase the chances for mold growth. Mycotoxins increase with delayed harvest, late season rain and cool periods. Mold spore levels may be higher with no-till management. Deep disking is advised to facilitate the degradation of crop debris that contribute to mold growth. Optimizing soil fertility to improve plant health can reduce mold activity. Producing healthy plants helps to diminish plant stresses such as stalk lodging. Anything that helps improve plant health will help reduce disease lesions and pest damage, thus suppressing mold invasion and mycotoxin production. Planting corn year after year on the same ground creates the opportunity for increasing mold levels. Most plant pathologists advise crop rotation in an attempt to break the cycle.
A timely harvest at proper moisture and maturity levels not only ensures that molds will be minimized in the field, but also that storage mold activity will be minimized in the silo. Other silo management considerations that promote optimal fermentation will also minimize molds and associated mycotoxins during storage. These include: filling the silo rapidly, proper chop length, packing the silo sufficiently, covering the silo completely to reduce exposure to oxygen. Ensuring a clean silo prior to filling and that mud and manure is eliminated during the ensiling process will minimize the mold spore load entering during filling of the silo.
Feeding practices that reduce deterioration of the feeding face and reduce heating in the feed bunk should be implemented. Silo size should be matched to herd size to ensure daily removal of silage at a rate faster than deterioration. In warm weather, remove at least 15 cm of silage daily from the feeding face. The feeding face of silos should be cleanly cut and disturbed as little as possible to prevent aeration into the silage mass. Silage and other wet feeds should be fed immediately after removal from storage. Feed bunks should be cleaned regularly.
Although silage inoculants do not detoxify mycotoxins, they are recommended to help ensure optimal fermentation, rapid pH drop and stable silage. Reducing silage pH rapidly will reduce mold growth and subsequent mycotoxin formation.
When feeding high moisture byproduct feeds (e.g. brew mash), handle in quantities that will allow them to be fed out within 7 to 10 days. Discard any spoilage. High moisture grains should be avoided. Most molds need free water activity to grow and produce toxins so storing grain below 15% moisture helps reduce the infection rate.
Mycotoxins reduce feed consumption therefore manage the feeding regime to maximize intake. Acidic diets intensify the effects of mycotoxins. For that reason, ensure that adequate dietary fibre and buffers are fed. Dry cows, springing heifers and calves should receive the cleanest feed possible. Specific transition rations can reduce stress in fresh cows and reduce effects of mycotoxins.
Adsorbents
When animals are exposed to mycotoxins, favourable results have been seen if adsorbent materials and complex indigestible carbohydrates are added to the ration. Responses to some of these products in dairy cattle have been encouraging.
The concept of mycotoxin adsorbents added to the diet is to bind mycotoxins to prevent toxicity in the gastro-intestinal tract and prevent absorption across the gut wall. For best results an adsorbent should: effectively adsorb mycotoxins, reduce mycotoxin availability and activity, reduce animal toxicity and tissue residues, not be detrimental to the animal or food product, be resistant to the physical effects of feed manufacturing and be cost effective.
There are several natural based mycotoxin adsorbents on the market. Contact your Hi-Pro Feeds representative for advice on what fits the best on your farm.
Conclusion
Mycotoxins and molds occur in many feed types including grain, hay and silage, and can be the root cause of animal disorders. Chronic symptoms such as low performance and reduced immune status can be the result of mycotoxin ingestion. Most molds do not produce known mycotoxins. Proper crop management from field to feedout will reduce opportunities for mold growth and subsequent toxin production. Even after implementing good agronomic and feeding management practices, it may not be possible to completely eliminate mycotoxins from the diet. Many adsorbent type products are used in the feed industry to help minimize the effects of mycotoxins.
Glenn Smith, Dairy Mill Nutritionist, Hi-Pro Feeds, Chilliwack
