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May 22, 2002

Crop expert: Agronomic reasons abound for not shucking corn

WEST LAFAYETTE, Ind. – Plant corn as originally intended, or scrap those plans and plant soybeans instead? The question weighs heavier on farmers' minds the longer fieldwork is delayed.

With the ideal planting period for corn now past, farmers may be inclined to move corn acres into soybeans as an insurance policy against yield loss. Producers could be making a big mistake, especially if the changeover interrupts a corn-soybean rotation, said a Purdue University agronomist.

The long-term cost of planting soybeans after soybeans is greater than any possible short-term economic benefit gained by dropping corn this year, said Tony Vyn, a cropping systems specialist. Farmers should not switch corn acres to soybeans any earlier than the first week of June, he said.

"Both corn and soybeans are losing yield potential every day," Vyn said. "That begins to occur with corn on about May 10 and with soybeans about May 20. Our plea with farmers is that they consider the agronomic factors quite strongly before they elect to plant soybeans after soybeans. They need to be aware of the implications for not only 2002, but also for the years to come."

Fields planted in soybeans in consecutive years are more susceptible to pests, soil erosion and crop yield reductions resulting from weather or management induced stress, Vyn said.

"In normal situations we expect soybeans to yield about 10 percent less following soybeans than they do following corn," Vyn said. "In stress situations, and if soilborne diseases are a problem – and particularly if soybean cyst nematode numbers build up – that yield loss could easily be 20 percent or more. We have concerns about the yield levels of soybeans in future years that are going to result from a rotation decision made today."

Potential yield loss this fall is among seven agronomic reasons Vyn listed for not planting soybeans on intended corn acres. The other six reasons include:

• Soybean yield loss in 2003 – Farmers who disrupt their normal corn-soybean acreage mix to convert more acres to soybeans might experience yield loss if they return to their regular crop rotation in 2003.

For example, if a farmer with 1,000 acres equally divided between corn and soybeans switches 200 acres of his 500 corn acres to soybeans, that 200 acres will have to remain in soybeans in 2003 in order to return to a 50/50 corn-soybean split. In addition to placing those soybean-after-soybean acres under added stress in 2003, the corn acres planted on those fields in 2004 will experience no yield benefit. Corn performs no better following two years of soybeans rather than one, Vyn said.

• Increased risk of soybean disease – "Yields in soybeans after soybeans go down primarily because of soilborne pathogens related to soybeans," Vyn said. "Those pathogens can include things like phytophthora, soybean cyst nematode, sudden death syndrome and white mold.

"Many of these pathogens that build up when a host crop is present will lead to more difficulty for proper root growth in a soybean crop in the following year. That leads to the additional stress that the second-year soybean crop faces."

• Additional complications in weed control – If residual corn herbicides were applied in the fall or earlier this spring, planting soybeans is not recommended until after the specific herbicide's rotation restriction period has ended. A second straight year of soybeans in a field increases the risk that weeds will develop resistance to glyphosate – often the only active ingredient in herbicides for the widely used soybean varieties that are glysophate-resistant.

• Lower yield levels of available soybean varieties – Farmers may have to settle for lower yielding soybean seed at this stage in the planting season.

"The corn yield loss of 1 bushel per acre per day of delayed planting in May of the elite hybrids will, in many cases, be a smaller economic sacrifice than planting potentially mediocre soybean varieties," Vyn said. "Though the odds diminish as the calendar advances, yields in the 130-200 bushel per acre range are still theoretically possible for corn planted after May 20."

• Wasted nitrogen fertilizer – Since soybeans can fix their own nitrogen, farmers are throwing away dollars by planting soybeans on previously fertilized acres.

• Reduced soil quality – This can occur three ways when soybean acres are bumped up in a crop rotation: 1) Decreased soil organic matter, since less biomass is returned from a soybean crop compared to corn; 2) Poorer soil structural stability and soil bonding, because soybean roots and shoots decompose quicker than corn; and 3) Soil residue cover left by soybeans is significantly less than that following a corn crop.

"Thus, the soil erosion potential in the spring after two years of soybean production is higher than that after a corn-soybean rotation," Vyn said.

The agronomic factors for staying with corn apply equally in late May and early June, Vyn said. The short-term economics might make a switch to soybeans imperative in mid-June, he said.

Writer: Steve Leer, (765) 494-8415, sleer@purdue.edu

Source: Tony Vyn, (765) 496-3757, tvyn@purdue.edu

Ag Communications: (765) 494-2722; Beth Forbes, bforbes@aes.purdue.edu; http://www.agriculture.purdue.edu/AgComm/public/agnews/

Related Web site:
Purdue Extension Delayed Planting Web site

Related story:
Economist: Numbers say it is time to switch from corn to soybeans

Purdue News Service: (765) 494-2096; purduenews@purdue.edu


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