“Mycotoxin” is a general term used for secondary metabolites, which have been shown to have some associated toxicity issues and are produced quite naturally by some fungi (molds). Mycotoxin production is sometimes stimulated by certain stressors.
Fungal disease events in field crops are mostly characterized by yield and quality losses, but there is also the potential for mycotoxin production and contamination. The spoilage molds responsible are naturally present in the environment, and mycotoxins may be produced during all phases of production, from the crop in the field all the way through to livestock feeding.
Mycotoxins are climate-dependent with plant- and storage-associated problems. Their growth is impacted by factors such as bioavailability of micronutrients, tissue damage and other attacks from different pests, which are also influenced by climate. Mycotoxin incidence and intensity vary by year, so it is important to analyze at-risk feeds regularly.
Mycotoxins can be formed prior to or after harvest in the case of either denominated field mycotoxins or storage mycotoxins (Table 1). This article focuses on management and prevention of mycotoxins in the field.
Field mycotoxins and producer molds
Molds are opportunistic pathogens growing on damaged or stressed plants. General risk factors for mycotoxin production in the field are:
- Humid weather at silk emergence
- Drought
- High temperatures during grain maturation
- Insect, hail or other mechanical damage to plants
- Delayed maturation/harvest
Corn and small-grain cereals are more susceptible to mycotoxin accumulation in their seed tissues, although they can also be found on the stem portion of those crops.
Fusarium molds range in color from white to deep red (Figure 1) and can produce mycotoxins in the field and post-harvest.
The presence of Fusarium mycotoxins is often associated with diseases such as ear and stalk rot in corn, head scab in small grains, red ear rot and pink ear rot. Gibberella ear rot is often associated with deoxynivalenol (DON or vomitoxin) and sometimes zearalenone, while Fusarium ear rot is associated with fumonisin mycotoxins.
Mycotoxin production favors temperatures between 45ºF and 75ºF, while other favorable conditions include excessive moisture during grain fill of wheat; a cool, wet growing season with insect damage; and dry conditions mid-season followed by wet weather.
DON is produced mainly by F. graminearum and also affects cereal crops. Temperature plays an important role. Small changes may influence the incidence and severity of contamination.
Zearalenone can occur in most cereal (kernels) such as corn, wheat, barley, oats and rye. The concentration reached depends on factors like substrate, temperature and humidity, duration of mold growth and strain of the fungus.
Fumonisins are commonly found in corn grown in North America. Contamination is favored by drought stress prior to and during silking. Production is also associated with kernels damaged by birds or insects. Fusarium ear rots that produce fumonisins are referred to as pink ear rots.
Aspergillus molds range from a yellow-greenish coloration (Figure 2) to black. Some species produce aflatoxins under warm conditions and drought stress pre-harvest and in warm, humid conditions post-harvest.
Aspergillus ear rot is commonly associated with aflatoxin production and is a serious problem in the southern U.S., where the disease is more prevalent. The level of aflatoxin in feeds is regulated by the FDA since it is a powerful carcinogen and can be passed through into milk and into human consumption.
Prevention is the best option
Prevention of the growth of molds on crops in the field is vital to reduce both spoilage losses and the presence of mycotoxins. As molds are opportunistic pathogens, all areas of agronomic and silage management need to be addressed.
To begin with, select locally adapted varieties with proven disease resistance. Be aware that late-planted corn is at higher risk because of greater chances of moisture stress during flowering and grain-filling phases.
Desirable traits are general adaptation to high temperatures and drought, resistance to ear- and root-feeding insects, hard endosperm and tight husks. Hybrids with transgenic insect resistance such as Bt-hybrids have shown reduced mycotoxin production in corn compared to conventional counterparts.
Fertilize the field in accordance to the soil analysis and avoid high plant populations. Proper management of residues in the soil reduces “fungal refuges” and so the risk of mold and mycotoxin contamination. Crop rotation can also alleviate fungal buildup in addition to reducing soil erosion and increasing crop yield.
Weeds may harbor insect and disease organisms that attack crops. The most economical and reliable way to control weeds, pests and diseases is to anticipate them. Walk fields and scout for weeds and pests to enable timely and targeted spraying decisions resulting in better control and reduced costs.
In addition to animal and insect damage, mechanical damage allows mold spores to disperse from the plant to the soil, resulting in an inoculum for the crop in the following year. When harvesting, avoid lodged or fallen material, since soil contact may increase the risk of mycotoxin contamination.
There are Aspergillus strains that naturally do not produce aflatoxins (atoxigenic strains) and can be applied to competitively exclude aflatoxin producers. Application should be done before silking, and the field should be treated uniformly.
Sampling guidelines for mycotoxins analysis
Sampling is a crucial step that involves special handling procedures for accurate determination of molds and mycotoxins. Microbiological changes can happen easily if the samples are not properly handled.
Monitor the field and consider if there were conditions that would stimulate the growth of molds and, possibly, the production of mycotoxins. Next, check if there are pockets in the field with problems, such as disease or pest infestation, which could lead to hot spots of contamination – or if the whole field could be affected (e.g., physical plant damage by a storm).
The best time to sample the forage is during harvest. To accurately sample, take a 2- to 4-pound sample per load of forage as it is being delivered to the silo. Make sure that multiple samples are taken from the core or middle of each load. Do not take a single sample from the most convenient place.
When finished collecting the samples, mix them well and dump on the ground in piles. Divide piles in quarters and discard opposite corners to reduce the size of the sample. Then repeat this procedure until you reach the required sample size.
After finishing subsampling, place the samples in plastic bags and exclude as much air as possible before sealing. If possible, use vacuum heated-sealed bags. Refrigerate immediately and keep the samples cool. This will minimize microbial activity, and the results will accurately reflect the level present at the time of sampling.
Do not freeze samples if you are also requesting mold analyses. Label samples accurately and place them in an insulated container and ship, expedited, on ice to the lab. Do not ship samples late in the week to minimize transit time.
Summary
Molds and mycotoxins are ubiquitous and a real and increasing concern in agriculture and livestock systems. Adopting good management of crop production can inhibit mold development and reduce the incidence of mycotoxins. PD
Bob Charley is forage products manager at Lallemand Animal Nutrition.
-
Renato Schmidt
- Forage Products Specialist
- Lallemand Animal Nutrition
- Email Renato Schmidt
