Getting the measure of mycotoxins is no easy task especially in the wide range of cereal crop commodities providing feed grain and the main ingredients of finished feed. Many field fungal pathogens and storage molds synthesize mycotoxins. Each group of fungi, including the aflatoxin-producing Aspergillus molds or the large number of Fusarium fungal pathogens delivering a broadside of Deoxynivalenol (DON), Zearalenone (ZEA), T-2 and HT-2 mycotoxins, has its own environmental requirements.
Each mycotoxin is the signature chemical and calling card left by a specific fungus or group of related fungi. Many of these highly versatile microbes “start life” as disease causing pathogens on cereal crops and subsequently extend into grain spoilage and mycotoxin contamination at any stage along the supply chain from on-farm storage to bags of finished feed.
Mycotoxins in the field
Field factors including fungicide treatment (by seed dressings or foliar sprays), fertilizer treatments and irrigation regimes, and the inherent disease resistance of the corn or wheat variety grown will play a part in the nature and magnitude of mycotoxin contamination. And watching over all this cereal agronomy is the weather as a wild card. The majority of cereal pathogens, and certainly Fusarium spp such as Fusarium graminearum responsible for wheat head blight and stalk and ear rot in corn, respond well to cool, cloudy, moist and humid conditions
Other fungal molds prefer it hot and dry. Classic case is the aflatoxin producing Aspergillus fungi, mainly A. flavus and A. parasiticus. They responded to the unprecedented high temperatures and accompanying drought experienced across the U.S. Corn Belt in 2012 with rapid growth rates and aflatoxin contamination in maize grain. Aspergillus spp are predominantly storage fungi and do not generally contaminate cereal grain prior to harvest. However, drought stress and insect damage often high in dry weather may allow infection by Aspergillus fungi and therefore aflatoxins production prior to harvest.
Mycotoxins moving into store
Mycotoxin first appears on the panicles of standing cereal crops, but cleaned grain arriving at the farm silo or loaded onto trucks for off-farm shipment is the first opportunity to test for what and how much mycotoxin is there. Getting the measure of mycotoxins from now on is a matter of what to test for, at what stage and how often.
Farmers and traders generally know the range of mycotoxins they need to test for in relation to the type of cereal and where grown. However, the appearance of one mycotoxin can often act as a “marker” for others because both are produced by closely related molds enjoying similar field conditions for infection and mycotoxin production. For instance, DON and ZEA will often occur together. These two mycotoxins are produced by Fusarium graminearum and also by a number of other closely related Fusarium fungi which infect a range of cereal crops.
Proper grain cleaning to remove all crop debris and especially the glumes (integuments surrounding small grain cereals like wheat) go a long way in preventing mycotoxin and fungal molds responsible from entering the post-harvest and processing stage. The extent to which mycotoxin making molds become active in storage to contaminate grain, and subsequently animal feed, will depend on grain moisture content and the conditions of storage.
Maintaining the balance
Twelve percent grain moisture or below is generally the figure at which fungal mold activity ceases, but the situation is more complex. Moisture level within the grain and in the surrounding air is dependent on temperature because warmer air has a greater water holding capacity.
Moisture inside the kernels of stored grain establishes an equilibrium level (balance) with the air outside, and the resulting relative humidity (R.H.) may be sufficiently high to encourage growth of deteriorative organisms including mycotoxin making fungi. Bacteria, fungal molds (including mycotoxin producers) and mites require a minimum R.H. of 90%, 70% and 60%, respectively. Insects depending on species need an R.H. level between 30% to 50%.