Article by Larry Roth, Ph.D., PAS – Cargill Animal Nutrition
Imagine the scene: Your nutritionist walks into your office, sits down across the desk from you, and hands you assays of your forages that indicate you have high levels of molds and mycotoxins. “Where did these evil organisms come from?”, you ask. It is helpful to be aware of the many factors at play when forages are affected by molds and mycotoxins.
A mycotoxin is a secondary metabolite produced by mold. One mold species may produce many different mycotoxins, and several mold species may produce the same mycotoxin. High mold levels do not necessarily mean mycotoxin levels will be high. Different micro-environmental conditions can trigger production of varying levels of various mycotoxins by a given mold species. The exact environmental triggers for particular mycotoxins and levels are not precisely clear, but understanding and managing the factors contributing to mold growth can lessen mycotoxin presence in forages.
There are favorable and optimum temperature ranges for growth of the Aspergillus, Penicillium, and Fusarium molds (Figure 1). Certain Aspergillus and Penicillium molds prefer warm, Summer conditions, while Fusarium generally thrive in cooler temperatures, as in the Fall, for optimal growth. Aspergillus molds are the primary aflatoxin producers; aflatoxin is most commonly observed in heat and drought conditions. Although a few Penicillium molds produce aflatoxin, most members of this genus produce mycotoxins of lesser importance. The Fusarium genus is perhaps of greatest concern in the Upper Midwest due to the production of vomitoxin (DON), zearalenone, and fumonisin mycotoxins. Fusarium species like the cool, wet conditions which have come to typify Fall in the Midwest.
Figure 1. Favorable and Optimum Temperature Ranges For Aspergillus, Penicillium, and Fusarium molds
While forage producers cannot control temperature, rainfall, and humidity, they can influence other factors involved in mold growth and the potential for mycotoxin production.
Soil Health: A healthy and diverse microbial population in the soil will enhance crop residue degradation, which could otherwise serve as a mold reservoir. A young corn plant pushing its first green leaf through last year’s crop residue could become infected with harmful molds. Healthy soil will also have greater water-holding capacity to sustain growing plants during dry spells and allow water to readily drain during excessive rainfall. Proper tillage can maintain some surface residue to reduce soil erosion, while excessive tillage may reduce soil aggregation, lowering nutrient and water-holding capabilities.
Agronomic Practices: Any stressors on the plant can tip plant health in favor of mold production. Proper planting time, plant population, plant genetics, insect control, and fungicide use are among the many agronomic practices that influence plant health and can bolster the plant in its battle against harmful molds.
Harvest Conditions: Timing harvest before the appearance of cool, wet conditions favorable for Fusarium growth may reduce molds and mycotoxins brought from the field to the silage storage structure. Chopping at a proper moisture, reducing chop length as the crop dries, and proper packing can reduce mold growth during ensiling. Segregating mold-infected crops from clean forages during ensiling may pose logistical challenges, but could pay-off in animal health and production by keeping clean forages clean and enabling feeding strategies based on animal susceptibility to mycotoxins.
Storage and Feed-Out Conditions: Proper packing to exclude oxygen is essential in controlling the growth of aerobic molds, such as Fusarium and most Aspergillus. Recent research suggests P. roqueforti may continue growing in well-packed silage, however. Mold spores can survive ensiling, and then start growing with the reintroduction of oxygen and proper temperature at feed-out. Proper feed-out practices are always essential when managing ensiled feeds, and even more critical with forages infected with mold at harvest.
High mold levels do not necessarily mean high mycotoxin concentrations in forage. On the other hand, mycotoxin concentrations could be high in forage with low mold counts due to toxin production pre-ensiling. Viewing a forage assay with high mold counts or high mycotoxin levels is never a pleasant experience for a forage producer. Taking the time to review the production practices that resulted in the particular forage can help change the course for future forage crops, however.