Southern rust (Puccinia polysora) is a fungal disease that infects corn leaves, thriving in warm, humid conditions and typically blowing north from tropical regions each summer. It appears as orange-brown pustules clustered on the upper leaf surface, with yellow flecks beneath. Though the fungus doesn’t produce mycotoxins itself, it weakens the plant, often causing early dry down, reduced starch levels, and lower digestibility—and it can open the door for other fungi that do produce toxins.
How It Affects Cattle Health
Southern rust affects cattle indirectly through the feed.
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Lower energy and digestibility: Leaf damage and reduced starch mean silage and high-moisture corn deliver less usable energy. Cattle may need to eat more to maintain gains, and feed efficiency can slip.
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Poor fermentation: Early dry down makes silage harder to pack, allowing oxygen to stay trapped and favoring molds and yeasts that spoil feed and lower palatability.
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Mycotoxin risk: Secondary ear and stalk rots—such as Fusarium or Aspergillus—can flourish in weakened plants, introducing toxins that depress appetite, slow growth, and impair immunity.
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Ration variability: When starch or digestibility drops by more than 10%, the energy balance shifts. Without testing and ration adjustment, performance may suffer or digestive upset may occur if extra grain is added too quickly.
For beef producers, these changes typically appear as inconsistent gains, lower intake, and reduced feed efficiency—especially when affected silage or grain makes up a large part of the ration.
Recommended Steps
1. Test before feeding.
Run a mycotoxin screen and nitrate test on silage or grain from infected fields. If levels are high, dilute with clean feed or use a proven binder with guidance from a nutritionist.
2. Evaluate fermentation.
Review lab reports for silage quality:
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Lactic acid: 4–6%
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Butyric acid: <0.25%
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Ammonia-N: <12% of total N
Unfavorable results suggest poor packing or oxygen infiltration.
3. Adjust the ration.
Analyze for starch and NDF digestibility and reformulate to maintain target energy without increasing acidosis risk.
4. Improve management next season.
Scout earlier, harvest based on moisture content, not calendar date, and consider using a silage inoculant when visible rust appears before harvest.
Identifying Southern Rust
Southern rust pustules are smaller, denser, and more orange than those of common rust, which tends to be larger and darker and appears on both leaf surfaces. Because the two diseases often overlap, lab confirmation remains the most reliable way to distinguish them.
Practical Insight
Southern rust is a crop disease with feed consequences. It reduces energy, digestibility, and fermentation quality while creating conditions for other fungi to thrive. The best defense is proactive management—test, adjust, and feed strategically. With proper testing and ration balancing, beef producers can protect performance even when crop quality takes a weather-related hit.
