Trait Decisions In Corn – Part I: Don’t Forget About Performance
Mark Loux, Ron Hammond, Bruce Eisley, Peter Thomison

Corn hybrid selection is becoming more complicated, as companies increasingly introduce specific traits that are contained in the seed and incorporated into seed prices.

Hybrids are available that contain traits for herbicide resistance (Roundup Ready, Liberty Link, and Clearfield) or insect resistance (Bt rootworm, Bt corn borer), or combinations of these (“stacked hybrids”). While corn growers at one time selected hybrids based primarily on performance – yield, standability, and dry down – they must now decide whether they need and are willing to pay for hybrids with transgenic insect and herbicide resistance. In addition, several corn diseases have become more prevalent in the past few years, and choosing hybrids with the best resistance to key diseases remains important.

Similar to conventional hybrids, there is a range in the performance of hybrids with transgenic traits. Growers should not have to sacrifice performance in order to use certain insect or herbicide resistance traits. Use of a hybrid because it has a certain trait, without consideration of performance in comparison to other hybrids, places growers at risk of lost income. Where possible, decisions on performance should continue to be based on historically reliable yield information from replicated trials at multiple locations conducted over multiple years. Data from a limited number of non-replicated strip trials do not allow the statistical comparisons necessary to accurately determine differences in yield or other factors (see http://corn.osu.edu/story.php?issueID=118&layout=1&storyID=659)

It appears that there can be quite a range in the price of traits, based on the company selling the hybrid, how many traits are in the hybrid, and geography. The price of insect resistance traits is lower in Ohio and Michigan than in states further west, because we have fewer problems with corn borer and first-year corn rootworm, and consequently have less need for these traits. The price of each trait can be substantially lower when the hybrid contains two or more traits (“stacked” hybrids). For example, the cost of a Roundup Ready or Bt rootworm hybrid may be $12 to $15 per acre higher than a conventional hybrid, but the cost of a hybrid with both traits may only be $17 per acre higher. Where both traits are needed and will improve income, it can make sense to buy a stacked trait hybrid in order to reduce the price of each trait. However, growers should be cautious about paying more for a stacked trait hybrid in situations where the second trait is unnecessary and will not increase income.

Part II: Insect Resistance
Mark Loux, Ron Hammond, Bruce Eisley, Peter Thomison

Transgenic (or “GMO”) crops are very effective for management of insects when the situation calls for the use of a preventive tactic. Growers should be aware that any use of these transgenic traits is a priori, that is, prior to knowing an economic problem will occur. They are not therapeutic in nature, which is often the case with use of insecticides. The insects that these traits control, corn borer and rootworm, are existing concerns, but there is presently little or no evidence that suggests that problems with these insects are increasing in Ohio. Recommendations for planting transgenic hybrids thus continue to follow the recommendations for other management tactics – use them only when necessary.

The first insect-related transgenic corn introduced was targeted at European corn borer. Although this transgenic trait is very effective for management of corn borers, growers should ask themselves whether the insect in fact warrants control in their fields. Has the corn borer been a problem in the past? Have you scouted and then treated for this insect in previous years? If you have never had this problem, there is really no reason to begin using a transgenic corn hybrid for its control. However, if the European corn borer has been an occasional or frequent problem in your fields, transgenic corn hybrids resistant to the European corn borer are good alternatives. If you have had problems with other insects that these corn borer transgenic hybrids control, such as black cutworm, fall armyworm, you also might consider using them. Later planting dates (after May 25) generally increase the possibility of damage from European corn borer, and warrant the planting of corn borer Bt hybrids if suitable maturities are available.

The situation with corn rootworms is slightly different, because this insect pest has caused more consistent problem in Ohio, but primarily in continuous corn. However, growers should still ask how often rootworms have caused sufficient injury in their fields to warrant a preventive treatment, especially in first-year corn. There are two circumstances that justify a preventive treatment against rootworm. If a grower chooses to grow continuous corn, a preventive treatment of either a soil insecticide, a high rate of a seed treatment, or a transgenic corn hybrid resistant to rootworms is recommended in Ohio. The second condition that might require a preventive treatment is when a potential for a problem with the western corn rootworm variant exists. This variant is currently a concern in the western portion of Ohio. Our sampling over the past few years continues to suggest that this is NOT presently a widespread problem. Recommendations call for treating first-year corn only if you have sampled the previous year’s soybean crop for the presence of adult western corn rootworms. Fields are sampled by using yellow-sticky traps over a 4 to 6 week period. Treatment for the western corn rootworm variant the following year should be used only when the number of adult beetles on the traps reaches the threshold of 5 adult rootworms/trap/day during any weekly trapping period. Sampling conducted in 2005 by OSU extension personnel showed that only a single field out of over 90 fields sampled reached this threshold. Most fields remained below 3 adult beetles/trap/day. These findings suggest that widespread problems should not occur in 2006.

Another aspect with transgenic corn for management of rootworms (YieldGard Rootworm and YieldGard Plus, and Herculex RW and Herculex Xtra) is that the seed comes treated with a commercially applied seed treatment, either Poncho 250 or Cruiser. These seed treatments are for the management of secondary soil pests, such as wireworms, grubs, and seedcorn maggot. While soil insecticides also control these soil pests, the Bt rootworm trait will not. Of note, the seed of transgenic corn hybrids for corn borer control (YieldGard Corn Borer and Herculex I) are not treated with insecticide unless specifically requested by the purchaser.

When comparing the cost of corn hybrids with Bt rootworm resistance to that of conventional hybrids, it should be noted that the cost of the insecticide treatment on the transgenic seed is included in the seed cost. Hybrid price comparisons shown by seed companies may add the cost for an insecticide seed treatment to the price of the conventional seed, which makes the latter appear to be higher in price. Based on OSU evaluations and those of neighboring states, we continue to believe that insecticide seed treatments are probably not needed on a widespread basis. While fungicide seed treatments are very important, we continue to see very low levels of secondary soil pest problems. But again, growers who have experienced problems with insects such as wireworms, grubs, or seedcorn maggot should consider the use of seed treatments. We recommend that, when comparing costs of using transgenic hybrids with those of conventional hybrids, growers do not automatically include a seed treatment in the cost of the conventional seed. This additional cost should only be included if the grower would normally intend to use an insecticide seed treatment.

In summary, use of hybrids with transgenic Bt traits is a very effective management tactic for growers requiring control of corn borer or rootworm. However, transgenic hybrids should be considered to be only one part of an overall IPM approach in the management of insect pests, and should therefore be used only when and where they are appropriate. Growers should also be reminded that if choosing to use a transgenic corn hybrid, whether based on insect management concerns or because of a price advantage, that they MUST follow insect resistant management (IRM) guidelines. At the very least, growers will have to plant a refuge of at least 20% to a hybrid that does not have the Bt trait. This refuge can be within mile of the transgenic field if using a transgenic corn borer hybrid. However, the refuge for a transgenic rootworm hybrid, including any hybrid with the stacked traits, must be within or adjacent to the transgenic field. Growers should check with their seed dealers for other IRM requirements, because there are additional guidelines. The point is that the use of any transgenic hybrid will increase crop management concerns that a grower will have to follow because of the need for a refuge.

When growers select transgenic hybrids, they should first focus on the agronomic performance of the hybrids across as many test locations as possible. Some of the first herbicide resistant and corn borer Bt corn hybrids (introduced about 10 years ago) were associated with “yield drag” or “yield lag”. As these traits have been widely incorporated in elite genetic backgrounds, differences in yield between Bt/herbicide resistant and their normal counterparts have become less evident, and many of these hybrids are now among the top yielding entries in state performance tests. However, relatively little information is available in university variety trials comparing the agronomic performance of Bt rootworm hybrids corns with their non-rootworm resistant counterparts. Similarly little information is available in university trials on the performance of hybrids with stacked traits. While hybrids with rootworm resistance and stacked traits may produce yields comparable to their conventional counterparts, recently introduced elite conventional hybrids may still offer higher yield potential in environments where insect problems are not present.

Growers also need to consider the time and cost associated with channeling transgenic corn grain approved in the U.S. but not globally. Because of the likelihood of pollen drift and cross pollination, grain from conventional corn hybrids planted in refuge areas next to transgenic Bt corn, not approved for export, may also have to be channeled. Grain facilities that purchase and handle transgenic grain not yet approved in foreign markets can be located on a web based grain handlers database (http://asta.farmprogress.com/) created by the by the American Seed Trade Association.

Trait Decisions in Corn – Part III: Roundup Ready or Not?
Mark Loux, Ron Hammond, Bruce Eisley, Peter Thomison

Growers are familiar with the effectiveness of glyphosate-based weed management programs in Roundup Ready soybeans, and the advantages that glyphosate can have over other postemergence herbicides. One key difference between corn and soybeans is that corn is subject to competition from weeds sooner after crop emergence, and timing of the postemergence glyphosate application is therefore more critical. For this reason, using a combination of preemergence herbicides applied at the time of planting and a postemergence glyphosate application tends to result in less impact of early-emerging weeds on corn and protects Roundup Ready corn yields. This type of program can also help delay the onset of glyphosate resistance compared to programs consisting of glyphosate exclusively.

It can be difficult to put an exact price on the Roundup Ready corn program, since the price of the Roundup Ready trait depends upon whether it is bundled with Bt traits. If we assume that the cost of the Roundup Ready trait is $12 per acre, and that a preemergence plus postemergence herbicide program will be used, the cost of the entire program could range from about $25 to $32 per acre (not including application costs). The higher price is for the type of herbicide program recommended by many companies, in which a preemergence atrazine premix product (Bicep II Magnum, Guardsman Max, etc) is applied at approximately 2/3 the typical full labeled rate, and followed with the postemergence glyphosate application. A number of other effective preemergence plus postemergence programs are available for conventional corn in the $30 per acre price range. The cost of a preemergence plus postemergence program, even Roundup Ready, can be considerably more than the cost of a preemergence herbicide program, which may be more like $18 to $25 per acre. Growers who need a Bt trait may be able to purchase a stacked trait hybrid containing the Roundup Ready trait for only a few dollars more per acre. This will reduce the cost of the weed management program, making it more competitive with all corn herbicide programs. In addition, several companies are offering growers incentives to use certain preemergence herbicides on Roundup Ready corn, and these incentives can reduce the overall cost.

One of the benefits of the Roundup Ready corn system is, of course, the effectiveness of glyphosate on a number of more problematic weeds. Use of Roundup Ready corn should certainly be considered by those growers with weed problems that often or always require a postemergence treatment, and especially where a grower tends to apply postemergence herbicides when corn is past the V5-V6 stage of growth. Herbicide choice on large conventional corn hybrids can be limited, due to the risk of injury, but glyphosate can be applied to Roundup Ready corn that is 30 inches tall or larger, depending upon whether it is RR Corn or RR Corn 2 (although the crop canopy can prevent spray from reaching small weeds in tall corn). Weeds that always or frequently require a postemergence treatment include shattercane, johnsongrass, giant ragweed, triazine-resistant lambsquarters, Canada thistle, burcucumber, waterhemp, and other perennial weeds. Producers who routinely apply postemergence herbicides in corn may find that they can reduce their herbicide costs by using Roundup Ready corn and glyphosate. However, there are fields in the Ohio where these weeds are not present or where their populations are low, and the other weed species are adequately controlled by preemergence herbicides. Producers who have maintained consistently effective weed control with total preemergence programs may find that the Roundup Ready program unnecessarily increases their weed control costs and their postemergence application workload.

Growers should keep in mind that continuous use of Roundup Ready crops and glyphosate-based weed management programs is likely to increase the rate of development of glyphosate-resistant weed populations, especially if glyphosate is the only herbicide used. It is essential that growers integrate other herbicides with glyphosate in both Roundup Ready corn and soybeans, and properly manage postemergence glyphosate applications (small weeds, the “right” rate). Growers using Roundup Ready corn can also expect to occasionally see volunteer Roundup Ready corn in the following year’s soybeans. Where the soybeans are Roundup Ready, growers will need to include grass herbicides (Assure II, Select Max) with glyphosate in postemergence treatments, at an additional cost of about $6 per acre.

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