Southern rust showed up early and crept deeper into the Midwest this year, reducing yields and causing a slew of headaches that could extend well beyond harvest. Livestock producers in particular are concerned about feed quality as the fungal disease can open the door to higher mold and yeast populations in infected corn feed. Below is more information about southern rust and what the experts are saying about cattle health and feed quality.
What is Southern Rust? Southern rust is a fungal foliar disease that flourishes in hot and humid conditions and requires a living host to reproduce. It is identified by orange to brown raised lesions on the top leaf surface, and yellow flecking on the leaf underside. The spores are typically blown from the south by wind and rain. Weather conditions across much of the Midwest were ideal for the fungus to take hold. “June was warm, but July was warmer and one of the wettest on record in the north-central Corn Belt. Those are the conditions you need for southern rust to thrive,” said Carl Joern, Pioneer field agronomist in Indiana. At the same time, much of the country experienced stronger-than-normal southwesterly winds. As several agronomists have pointed out, the northward progression of southern rust follows the wind patterns through the Midwest this summer. The good news is that the fungus does not overwinter in cold conditions or survive in crop residue.
Mycotoxin Concerns: Early dry down caused by southern rust can cause problems with proper silage packing, which increases risk of poor fermentation and higher mold and yeast populations. Optimal moisture content for corn silage is generally between 60% and 70%. Southern rust may lead to silage that is drier than expected based on calendar date, according to MU Extension state beef nutrition specialist Eric Bailey. When silage is too dry, oxygen cannot be kept out of the pile and silage does not pack to the proper density of less than 15 pounds of dry matter per cubic foot. Southern rust itself does not produce mycotoxins but it opens the door for other stalk and ear rot diseases that produce mycotoxins in corn-based feedstuffs, according to MU Extension veterinary toxicologist Tim Evans.
Corn Feed Quality Impacts: Yield losses in corn are due to the pathogen feeding off plant photosynthates (glucose) and reductions in total plant photosynthetic leaf area. Yield losses begin even before 5% of the ear leaf is covered with lesions. Reduction in grain yield also means a reduction in starch (grain contributes most of the starch in an ensiled corn feedstuff) and a reduction in energy. Also, digestibility can have declines of more than -10%.
Dairy vs Beef: Corn infected with southern rust may be more of a concern for the dairy industry than the beef industry, says Bailey. That’s because of the dairy industry relies heavily on corn silage, and milk production is highly sensitive to subtle changes in nutritive value. MU Extension veterinary toxicologist Tim Evans recommends running a mycotoxin panel screening before feeding forage or grain infected with southern rust. Producers also should consider diluting with clean feedstuff and using other mycotoxin mitigation strategies as needed. If you have concerns about southern rust’s effect on your feedstuff, consider a fermentation quality analysis, says Bailey. Examine results closely: Lactic acid should be between 4% and 6%; butyric acid should be less than 0.25%; and ammonia should be less than 12%. If you receive an unfavorable fermentation profile, contact your local MU Extension livestock specialist for help in developing a plan to use that silage. To learn more about southern rust, visit the Crop Protection Network website HERE. (Sources: CPN, MU Extension, Michigan Farm Bureau, Reuters)
