Damon Smith, Extension Field Crops Pathologist, Department of Plant Pathology, University of Wisconsin-Madison
We are quickly approaching that time of year where we will see silage choppers working the 2025 corn crop. This means it is time to understand overall crop health and how diseases might be affecting the crop so that you can make the best silage product you can.
Foliar diseases such as southern rust, tar spot, gray leaf spot (GLS), and northern corn leaf blight (NCLB) are all VERY visible in corn fields in Wisconsin this year and can be problematic for silage production. Not only do these diseases lead to a reduction in overall silage quality, but they can also force the plant to scavenge carbohydrates in the stalk which can result in standability issues and lodging. These diseases can also influence whole plant moisture, making harvesting at optimal moisture difficult. If silage is harvested at sub-optimal moisture, then packing the bunker properly can be a challenge which can lead to slow fermentation and continued growth of aerobic organisms like fungi. This can indirectly lead to an increase in mycotoxins and “mold” issues from these aerobic fungi.
Tar spot and silage harvest
Tar spot of corn has been an issue on silage corn in Wisconsin since 2016. In fact, the first finding of tar spot was on corn for silage that year. Since 2016 the largest and most consistent impacts happen on corn for silage vs. corn for grain. Yes, tar spot can result in significant grain losses, but tar spot can also affect the overall plant in other ways such as loss in dry-matter yield (Fig. 1) and whole plant moisture contents well below optimal, making bunker-packing a significant issue. In recent work in Wisconsin, we see significant dry matter yield reductions when tar spot severity on the ear leaf reaches over 10% at the time of chopping. This can result in as much as 15% reduction in yield, with these impacts dramatically increasing at severity levels of 20% or more on the ear leaf (Fig. 1).
Figure 1. Impact of tar spot severity on the ear leaf at harvest time compared to dry-matter yield in whole-plant chopped silage corn.
Southern rust has been another important foliar disease of corn this season. Southern rust and tar spot function similarly on the plant. Thus, if any foliar disease is moving quickly in your silage crop, you might consider chopping a bit earlier to reduce dry matter yield losses and to try to optimize whole plant moisture, prioritizing good bunker management. If moisture at chopping is not prioritized, then subsequent storage issues such as mold and mycotoxin concerns can arise indirectly, due to poor bunker management. This brings us to Gibberella ear and stalk rot in silage corn.
Gibberella ear rot is caused by Fusarium graminearum (a.k.a. Gibberella zea). The same pathogen can cause Gibberella crown and stalk rot in corn. We have observed both diseases on silage corn in Wisconsin in recent years. Our environment here in the Great Lakes region makes a perfect place for this pathogen to cause these diseases. In addition to the damage that the pathogen can cause, the fungus can also produce various mycotoxins, most importantly deoxynivalenol (DON or Vomitoxin). Our laboratory has conducted quite a bit of research recently trying to understand where in the plant DON accumulates. In detached plant part experiments, we have noted that DON can accumulate in both the stalk and ear portions of the plant, AND that these two phases of accumulation are not linked to each other (Chibuogwu et al., 2024). The fungus can infect these parts separately at different times during the season and the subsequent accumulation of DON can happen differentially in the stalks vs. the ears. This is partially why you can go out to the field and scout for ear rot and not see a lot of infection (moldy ears) but still have high DON levels at chopping time. Some of that DON is likely accumulating in the stalks.
We have also been following the fate of DON in silage harvested and chopped from a brown midrib (BMR) hybrid and a dual-purpose hybrid that were grown in the field and treated with fungicides at white silk (R1). We chopped the plants in each plot and then used mini-silos (polyethylene bags vacuum-sealed using a commercial grade vacuum packer) to conduct a time-course experiment following DON levels in the mini-silos (Chibuogwu et al. 2025). In all cases we saw DON levels generally increasing in the first 30 days after chopping (Fig. 2). They then leveled off and became stable at 60, 90, and 120 days after chopping. Some of this increase could be due to oxygen still in the system during the first 30 days after chopping. DON-producing fungi are aerobic and continue to consume some of the minute levels of oxygen still in the system, thereby still producing DON. However, this likely only explains some of the DON levels we detected.
Figure 2. DON concentration of chopped field-grown silage corn over time of ensiling.
There are also “masked” or conjugated forms of DON that are detectable in routine DON analyses. One such conjugate is DON-3-glucoside (D3G). D3G can be produced by fungi or during a plant’s attempt to protect itself from the toxicity of DON. Either way, D3G is not detectable in routine test and must be tested for specifically. We investigated our samples further during the first 30 days of ensiling and found that the level of D3G at harvest, explained a significant level of the DON recorded in samples after 30 days of ensiling. This is to say that D3G present at harvest, is likely metabolized in the first 30 days of ensiling releasing DON and resulting in higher DON levels 30-days later (Fig 3). Again, this relationship only partially explains why DON increases in silage during the first 30 days of storage. The full explanation is likely due to both metabolization of D3G and continued fungal respiration leading to an increase in DON at feed out compared to when it was packed in the bunker.
Figure 3. Relationship of DON-3-Glucoside (log) at harvest and DON (log) concentration after 30 days of ensiling.
The Take Home (modified from Mycotoxins and Silage: How to Feed the Dairy)
Deoxynivalenol in Stored Corn Silage
Research has shown that DON levels in silage can increase during the first 30 days of storage, likely due to:
- Residual Oxygen:The fungi that produce DON thrive in aerobic conditions. If silage isn’t packed properly, oxygen pockets may allow continued fungal activity.
- Masked Mycotoxins:DON-3-glucoside (D3G) is a conjugated form of DON that is not detected in routine tests. During early ensiling, D3G may be converted back to DON, increasing its concentration later in storage.
To minimize DON buildup:
- Pack and seal silage tightly with oxygen-barrier plastic.
- Consider testing for both DON and its conjugates to get an accurate contamination assessment.
Managing Corn Diseases and Mycotoxins in the Bunker
Balancing foliar disease and mycotoxin management is key to producing high-quality silage. Here’s how farmers can manage corn diseases and reduce DON risk at harvest and in storage:
- Prioritize Diseased Fields:Harvest fields with foliar disease or ear rot first.
- Monitor Moisture:Chop at the optimal moisture level to ensure proper fermentation. Watch the milk line and whole plant moisture and adjust chopping time accordingly.
- Enhance Packing and Sealing:When disease pressures are high, take extra time to eliminate oxygen pockets.
- Alternative Harvesting Methods:If corn is too dry for silage, consider harvesting for high-moisture grain or snaplage instead.
- Frequent Testing:Regularly test silage for DON and other contaminants to make informed feed management decisions.
- Consider an Inoculant: A heterofermentative bacteria such as buchneri can help inhibit yeast and fungal contamination and help stabilize silage.
Resources
Chibuogwu, M.O., Groves, C.L., Mueller, B., and Smith, D.L. 2024. Effects of fungicide application and corn hybrid class on the presence of Fusarium graminearum and the concentration of deoxynivalenol in ear and stalk parts of corn (Zea mays) used for silage. Plant Disease. https://doi.org/10.1094/PDIS-12-23-2662-RE.
Chibuogwu, M.O., Reed, H., Groves, C.L., Mueller, B., Barrett-Wilt, G., Webster, R.W., Goeser, J., and Smith, D.L. 2025. Influence of hybrid class and ensiling duration on deoxynivalenol accumulation and its derivative deoxynivalenol-3-glucoside while ensiling corn for silage. Plant Disease. https://doi.org/10.1094/PDIS-06-24-1166-RE.
