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Addressing bugs, nematodes, nutrients

By Staff | Sep 24, 2010

AN ISU Professor of plant pathology, Greg Tylka talks to a group of farmers about the latest information and control technologies for corn nematodes. More than two dozen farmers attended the field day which took place Sept. 9 at the ISU research farm near Kanawha.

KANAWHA – More than two dozen farmers gathered in Kanawha Sept. 9 boarded a convoy of hayracks for expert updates on a handful of issues that promise to challenge farmers for seasons to come.

At different stations Iowa State University Extension experts Erin Hodgson, Greg Tylka, Antonio Mallarino and John Sawyer offered presentations on observed and expected insect pressures, growing concerns over corn nematode activity, manure application and effectiveness and nitrogen application and efficacy.

Insect pressures

Hodgson said frequent heavy rains during the growing season greatly dissipated populations of soybean aphids across the state, but opened the door for other less common pests to pressure some areas.

Damaging populations of multiple defoliating species, including grasshoppers, leaf tiers and green cloverworms, were seen in some areas, but remain difficult, and often less rewarding, to control because of their rapid migratory movements.

Where significant aphid pressure was seen, Hodgson said Bt hybrids provide the surest protection against damage, but less aggressive aphid resistance traits still allow farmers to scout ahead of the problem.

“Host plant resistance gives you time,” Hodgson said. “Those fields will eventually see a yield loss, but the resistance allows you to treat those fields later without an effect to yields.”

Corn nematodes

Another insect threat, corn nematodes are, according to Tylka, not new to Iowa fields, but are significantly a more spottier and difficult to scout challenge to corn yields.

“If you took a sample from almost every field in the state you would find some corn nematodes,” Tylka said. “But just because you have them doesn’t mean you have damage.

“You need to know which ones you have and you have to know what the numbers are.”

Interest in corn feeding species of the tiny rootworms has increased in recent years with the introduction of commercial seed treatments and sprays to control the pest.

The significantly variable biology of corn nematodes makes threshold populations vary greatly between species and overall populations difficult to judge from year to year.

Tylka said that while corn nematodes are not as hearty as soybean feeding species, able to survive in the soil for only about a season without a food source, certain species can create significant pressure and produce symptoms which are often mistaken for other stress factors.

“There’s nothing amazing or spectacular about the damage,” Tylka said. “You see smaller plants with smaller ears.”

Because corn nematodes attack and sometimes crawl inside roots sampling for the pests must be done mid-season and include root material. Currently no corn hybrids offer natural resistance to nematodes, but two control products – Avicta and Votivo – are available as additives to the Cruzer and Poncho pest control systems.

Nutrient applications

Mallarino and Sawyer presented two common fertilizer sources, animal manure and applied nitrogen.

According to Mallarino the greatest challenges with all forms of manure remain the difficulty of spreading the product evenly and keeping nutrients in the soil once they are spread.

Drier products offer more resistance to runoff, but can have other disadvantages. Chicken manure is generally resistant to runoff, but delivers too much potassium for a desired amount of phosphorus.

The effectiveness of the different forms of key nutrients remains a subject of much research, but Mallarino said an important point to consider is that conventional wisdom on availability of nutrients is not always correct.

“A fertilizer or manure nutrient with 100 percent availability may not be all absorbed by a crop,” Mallarino said. “Availability is not equal to supply.”

In his talk Sawyer said that optimum nitrogen levels remain a moving target for each growing season.

Records from the Kanawha farm show that the best rate of nitrogen application varied as much as 85 pounds per acre for corn-soybean rotations and more than 100 pounds per acre for corn-on-corn rotations.

Data from the Kanawha farm and other research plots is collected yearly and fed into a Corn Nitrogen Rate Calculator provided by the ISU department of agronomy, Sawyer said. The calculator recommends nitrogen rates for corn-on-corn and corn-soybean rotations based on a range of possible commodity prices.

It is available at extension.agron.iastate.edu/soilfertility/nrate.aspx.

Contact Kevin Stillman at stillman.kw@gmail.com.

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