With the warm and wet conditions this season, southern rust is on the rise in Midwest corn crops. It may be time to start considering the impact that could have on corn silage and preparing to adjust rations accordingly. While southern rust is not a direct threat to herd health, it has been shown to lower the nutritional value of silage and can compromise feed quality.
Southern rust, a fast-developing fungal disease caused by Puccinia polysora, does not itself produce toxins, but it weakens the plant and provides the opportunity for other diseases to move in. These opportunists include various Furasium species, which produce mycotoxins (fumonisin and deoxynivalenol) that can be harmful in feed.
Southern Rust and Corn Silage Quality
Southern rust is known to impact corn silage quality. A study from the University of Florida showed increasing rust infestation resulted in increased dry matter and fiber fractions, but that dry matter digestibility decreased by 13%. Further, high rust silages had lower neutral detergent fiber digestibilities than medium and no rust silages. Southern rust also affected the concentrations of lactate and volatile fatty acids, causing both to decrease with increasing infestation. These results indicate decreased nutritive value.
The observed increased dry matter also reduced silo packing effectiveness. If moisture levels are too low at harvest, it is difficult to achieve adequate packing, which leads to poor fermentation and an increased risk of mold growth.
Because southern rust coverage reduces the photosynthetic area of the leaf, grain fill is often hindered, leading to a lower energy and protein content in the silage.
Southern Rust Silage Management
There are a handful of strategies producers can apply to counteract the effects of southern rust:
- Adjust harvest time based on moisture content. Southern rust can cause corn to dry down faster than normal. Monitor moisture levels closely to ensure the proper fermentation of silage.
- Consider a silage inoculant. Inoculants improve fermentation, and the rapid pH drop can inhibit mold and yeast growth.
- Ensure good packing and storage. Pack silage well to limit oxygen exposure and prevent mold growth. Cover bunkers immediately and weigh down coverings thoroughly.
Feeding Southern Rust Silage
To counter the nutritional challenges of feeding southern rust-infected corn silage, dietary supplementation may be necessary.
Prior to inclusion, test all potentially infected silage for mycotoxins. This will allow you to determine the safety of the feed and avoid potential health issues. If mycotoxins are high, the incorporation of a mycotoxin binding agent into the ration will help reduce toxin absorption in the animal’s digestive tract. Additionally, supplementation with antioxidants, such as vitamin E and selenium, could help animals by countering oxidative stress caused by mycotoxins and supporting immune function.
If grain fill was affected and starch levels are low, you may need to incorporate an additional energy course to compensate. Further, poor grain fill could reduce the already low protein content of corn silage, and protein supplementation may be required.
When incorporating infected silage, ensure it is thoroughly mixed into the TMR to dilute potential ‘hot spots’. Inclusion levels of contaminated silage in the feed may need to be limited or removed entirely for sensitive animals, including lactating or breeding animals. Livestock should be monitored closely for symptoms of mycotoxin toxicity, such as reduced intake, weight loss, digestive issues or reproductive challenges. Be prepared to respond if issues arise.
When feeding corn silage infected with southern rust, caution is essential to protect livestock health and performance. The thoughtful use of compromised silage can help minimize risk while maintaining efficiency and animal well-being.
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