February 22, 2005

Consider risks when altering crop rotations to avoid soybean rust

WEST LAFAYETTE, Ind. - Because of the potential threat of soybean rust, farmers considering planting fields of corn again rather than rotating to soybeans should weigh all the factors before digging in, said Purdue University experts.

With the uncertainty of soybean rust, some farmers may figure that returning to corn for a second year will be more profitable than planting soybeans. While this specific one-year comparison may prove true, the longer term corn-to-soybean rotation remains more profitable, said Bob Nielsen, Purdue Extension corn specialist.

"When planting second-year corn, there is always a 5 to 18 percent reduction in yields," said Tony Vyn, agronomy professor at Purdue.

Planting corn for the second year poses many disadvantages and hidden costs, said Nielsen and Vyn. For one, a corn-after-corn rotation requires additional nitrogen fertilizer applications, which could hike nitrogen fertilizer input costs by nearly one-third.

"Nitrogen fertilizer rates for corn following corn are significantly higher than corn following soybeans," Nielsen said. "Second-year corn typically requires an additional 40 to 50 pounds per acre of nitrogen fertilizer, which would be a 30 percent increase in those fertility costs."

The risk of disease also is greater when planting corn a second year. Certain leaf blights and ear rot fungi can be common with back-to-back corn crops. Second-year corn is also prone to increased pressure from certain insects such as corn rootworm.

"The potential for corn rootworm is higher when switching to second-year corn, especially in those areas of Indiana not yet afflicted with the first-year variant of this pest," Nielsen said. "This would require farmers to apply more insecticides to fields, a likely increase of $18 per acre."

Tillage also becomes a concern. Corn following corn requires more tillage than corn following soybeans. It's important to select the best tillage practice on fields where corn is planted a second year because of the need to leave enough surface residue cover to prevent soil erosion but not so much residue that second-year corn yields will suffer..

"For corn on corn, no-till systems are the best option for fields with sandy soils," Vyn said. "Strip tillage works best on intermediate soils since it will help preserve some residue cover and will allow faster soil drying and warmer soil temperatures in the spring. Full-width tillage is the best choice for clay soils with poor drainage."

Farmers choosing to plant continuous corn should take actions to minimize the risks involved. Farmers should concentrate on planting corn hybrids with high-quality early seedling vigor, disease resistance and good overall plant health, Vyn said.

"Farmers should be very cautious with corn on corn," Vyn said. "I'd recommend switching only a portion of their acreage to second-year corn. In addition, it may be beneficial to implement field-length test strips to monitor any yield reduction."

For more information on how to mitigate the downside risks of corn following corn, view Nielsen's online newsletter article on the topic.

Writer: Molly Brock, 765-496-2761, brockml@purdue.edu

Sources: Bob Nielsen, 765-494-4802, rnielsen@purdue.edu

Tony Vyn, 765-496-3757, tvyn@purdue.edu

Ag Communications: (765) 494-2722;
Beth Forbes, forbes@purdue.edu
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