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College of Food, Agricultural, and Environmental Sciences


Transgenics Could Pose Planting Challenges This Season

February 21, 2008

COLUMBUS, Ohio -- The percentage of transgenic corn hybrids increasing in Ohio may prove to make this season's planting more of a challenge.


According to the U.S. Department of Agriculture, over 40 percent of Ohio's corn acreage was planted to transgenic hybrids in 2007, up from 26 percent the previous year. Peter Thomison, an Ohio State University Extension agronomist, said that number is expected to increase this year, and could play a role in the dynamics of corn planting.

"How growers will be approaching the way they plant corn this year will be more important than ever, given the increasing acres of transgenic corn," said Thomison.

Transgenics is the science of introducing a gene from one organism or plant into the genome of another organism or plant. In crop production, Bt corn to control European corn borer and rootworm, and Round-Up Ready corn and soybeans for enhanced weed control would be examples of transgenics.

Thomison, who also holds an Ohio Agricultural Research and Development Center appointment, said that corn growers face the challenge of pollen contamination and planting refuges when dealing with transgenic hybrids.

"With the number of transgenic plantings increasing, there are implications for farmers who are growing non-transgenic varieties. One of those implications is preventing pollen contamination from transgenic hybrids," said Thomison.

Given the close proximity of non-transgenic fields to potential transgenic fields, Thomison said that growers might have a difficult time ensuring that their non-transgenic corn remains that way.

"Go out and talk to neighbors and find out when they are planting. Stagger planting dates and plant hybrids of various maturities," recommends Thomison. "Take steps now to make sure those non-transgenic fields are isolated."

Another challenge growers face is adhering to the concept of planting a 20 percent refuge within a transgenic field, specifically when using Bt corn hybrids.

"A refuge is simply a block or strip of corn planted with a hybrid that does not have the Bt gene. The primary purpose is to maintain a pest population that is not exposed to the Bt toxin, allowing susceptible insects to remain within the population and mate with any resistant insects that survive in the transgenic area," said Ron Hammond, an OSU Extension entomologist. "This allows any offspring to remain susceptible to the Bt hybrid."

Thomison said that planting a refuge is extremely important, especially with more and more growers jumping from non-transgenic hybrids right to triple-stack hybrids.

"It we are going to hold on to the value of these traits in the future, it's going to behoove growers to try and make sure they use a refuge," said Thomison.

According to the USDA, Ohio farmers planted more stacked gene varieties last year than any other type of transgenic hybrid available. Thomison said that affordable packages and incentives from seed companies are driving the interest.

When managing a refuge, growers are urged to adhere to the following guidelines:

• In terms of distance of the refuge from the Bt corn for corn borer, the non-Bt refuge can be within, adjacent, or near the Bt field, but it has to be within a half-mile -- preferably within a quarter mile. For corn rootworm, the non-Bt refuge has to be within the same field as the Bt corn, or adjacent to it.

• There are several planting options, including a separate field refuge, mainly used to control corn borer; an adjacent field; a separate, but adjacent block next to the Bt corn; a block refuge within the Bt field; a refuge planted along the perimeter; or a split-planter refuge. "If planting the refuge using a split planter, the strip width must be at least four rows, preferably six rows," said Hammond.

• Both transgenic and non-transgenic areas should be managed in a similar manner. Growers should plant both hybrid types close to or at the same time, and select Bt and non-Bt hybrids that have similar growth and development characteristics. "If planting a refuge for corn rootworm, for example, the cropping history must be the same. That is, if Bt corn is planted following corn, then the refuge must be planted following corn," said Hammond.

• When using insecticides, there are procedures for controlling the target insect and secondary insects. When controlling for European corn borer, the non-Bt corn refuge may be treated with conventional insecticides only if the target pest reaches economic threshold. "A foliar Bt-based insecticide cannot be used within the refuge," said Hammond. When controlling for Western corn rootworm, a soil-, seed-, or foliar-applied insecticide is allowed for the refuge. If an aerial insecticide is applied to the refuge for control of rootworm adults, the same treatment must be applied at the same time to Bt corn.

• If a refuge is planted in a rotated field, the Bt field must also be planted on rotated ground. However, if the refuge is corn following corn, the Bt field can be planted on either a continuous or rotated field.

Guidelines also exist when planting a hybrid with stacked traits that contain both Bt types.

"Growers have two management approaches. The first choice is to plant separate refuges for each target pest," said Hammond. "We recommend the second choice -- the common refuge approach where corn without any Bt technology is planted. In this case, a 20 percent refuge must be planted within or adjacent to the transgenic field."

Hammond said that growers should consult their seed dealers for information regarding properly managing for transgenic corn hybrids, or for any additional IRM (insect resistant management) requirements related to single and stacked-trait corn hybrids.

Looking beyond transgenic hybrids, Thomison recommends that growers follow proven cultural practices when planting corn this season. Some of those tips include:

• Perform tillage operations only when necessary and under the proper soil conditions. "Avoid working wet soil, and reduce secondary tillage passes," said Thomison. "Shallow compaction created by excessive secondary tillage can reduce crop yields." He added that deep tillage should only be used when a compacted area has been identified and soil is relatively dry.

• Complete planting by mid-May. "The record corn yields of recent years owe much to timely planting and good seedbed conditions. If soil conditions are dry, begin planting before the optimum date," said Thomison. "The recommended time for planting corn in northern Ohio is April 15 to May 10 and in southern Ohio, April 10 to May 10." Thomison recommends avoiding early planting on poorly drained soils or those prone to ponding. "Yield reductions resulting from 'mudding the seed in' may be much greater than those resulting from a slight planting delay."

• Adjust seeding depth according to soil conditions to avoid poor root development, shoot uptake of soil-applied herbicides, and uneven plant emergence. "Plant between 1.5 inches to 2 inches deep to provide for frost protection and adequate root development. In April, when the soil is usually moist and evaporation rate is low, seed should be planted no deeper than 1.5 inches," said Thomison. "When soils are warm and dry, corn may be seeded more deeply, up to 2 inches on non-crusting soils."

• Adjust seeding rates on a field-by-field basis. "Adjust planting rates by using the yield potential of a site as a major criterion for determining the appropriate plant population," said Thomison. "Most research suggests that planting a hybrid at suboptimal seeding rates is more likely to cause yield loss than planting above recommended rates." On productive soils, with long-term average yields of 160 bu/acre or more, final stands of 30,000 plants/acre or more may be required to maximize yields. On soils that average 140 to 150 bu/acre, a final stand of 28,000 plants per acre may optimize yields. On soils that average 120 bu/acre or less, final stands of 20,000 to 22,000 plants/acre are adequate for optimal yields.

• Calculate appropriate seeding rates. "The number of plants per acre at harvest is always less than the number of seeds planted. Planting date, tillage practices, pest problems, chemical injury, planter performance, and seed quality can affect final corn populations obtained in the field," said Thomison. "To compensate for these losses, a corn grower needs to plant more seed than the desired population at harvest. When early planting is likely to create stressful conditions for corn during emergence, consider seeding rates 10 percent to 15 percent higher than the desired harvest population."

Finally, Thomison said that growers planting no-till and reduced-tillage continuous corn should take steps to prevent uneven stands.

"Last year we seemed to see more problems with uneven corn in fields with reduced-tillage continuous corn production. If growers haven't used a corn after corn cropping system regularly, there could be unevenness issues," said Thomison. "They might want to put more effort in planter adjustments and checking seeding depths on a regular basis in continuous corn production."

For more information on corn management, log on to OSU Extension's Agronomic Crops Team Web site at


Candace Pollock
Peter Thomison, Ron Hammond