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Agronomic Crops Network

Ohio State University Extension


C.O.R.N. Newsletter 2008-15

Dates Covered: 
May 27, 2008 - June 2, 2008
Todd Mangen

Sawflies in Wheat

Authors: Ron Hammond, Andy Michel, Bruce Eisley

This past week we received a photo of a caterpillar-looking insect collected from a wheat field. At first glance it tends to resemble an armyworm based on the general “caterpillar” appearance, feeding injury, and time of year. However, these caterpillars are a sawfly species, which are wasps (Hymenoptera), not moths or butterflies (Lepidoptera). The main characteristics that indicate a sawfly and not an armyworm larva are the yellowish/greenish color, the 7 prolegs with a pair on each body segment, and the typical 3 pairs of legs. Sawflies are rarely seen as economic concerns in wheat fields in Ohio. Currently, this is the only report we have of them. Wheat growers should be aware of the potential for these insects to be in their fields, however, unlikely. Of importance is not confusing these sawflies with armyworm larvae, which will be a different color, more brownish with stripes along its length, and without all the prolegs.

Replant Issues in Corn

Authors: Peter Thomison

I’m receiving reports of farmers replanting some of their early planted corn. Most of these replants are related to excessively wet soil conditions and insect and seedling blights damage. Replant decisions in corn should be based on strong evidence that the returns to replanting will not only cover replant costs but also net enough to make it worth the effort. Don’t make a final assessment on the extent of damage and stand loss too quickly. The following are some guidelines to consider when making a replant decision.

If the crop damage assessment indicates that a replant decision is called for, some specific information will be needed, including:
Original target plant population/Intended plant stand
Plant stand after damage
Uniformity of plant stand after damage
Original planting date
Possible replanting date
Likely replanting pest control and seed costs

To estimate after damage plant population per acre, count the number of viable plants in a length of row that equals 1/1000 of an acre and multiply by 1000. Make several counts in different rows in different parts of the field. Six to eight counts per 20 acres should be sufficient. Table 4-12 in the OSU Agronomy Guide ( shows row lengths required to equal 1/1000 acre when corn is planted at various row widths.

A major consideration in making a replant decision is the potential yield at the new planting date and possibly a different planting rate; this can vary depending on the hybrid used, soil fertility and moisture availability. Table 4-15 in the OSU Agronomy Guide is a chart developed by Dr. Emerson Nafziger at the University of Illinois ( that show effects of planting date and plant population on final grain yield for the central Corn Belt. Dr. Bob Nielsen, Purdue University, has modified this table to provide estimates of potential yield losses for planting dates in early June (
Grain yields for varying dates and populations in both tables are expressed as a percentage of the yield obtained at the optimum planting date and population.

Here's how the table from the OSU Agronomy Guide ( can be used to arrive at a replant decision Let's assume that a farmer planted on April 30 at a seeding rate sufficient to attain a harvest population of 30,000 plants per acre. The farmer determined on May 28 that his stand was reduced to 15,000 plants per acre as a result of saturated soil conditions and ponding. According to Table 4-15, the expected yield for the existing stand would be 82% of the optimum. If the corn crop was planted the next day on May 29, and produced a full stand of 30,000 plants per acre, the expected yield would be 81% of the optimum. The difference expected from replanting would be negative (81 minus 82, or minus 1 percentage point) and indicate no advantage to replanting.

However, it’s also important to note plant distribution within the row. Remember that values in replant charts like Table 4-15 from the OSU Agronomy Guide are based on a uniform distribution of plants within the row! Add a 5% yield loss penalty if the field assessment reveals several gaps of 4 6 feet within rows and a 2% penalty for gaps of 1 3 feet. Yield loss due to stand reduction results not only from the outright loss of plants but also from an uneven distribution of the remaining ones. The more numerous and longer the gaps between plants within the row, the greater the yield reduction. It’s also important to consider the condition of the existing corn. Some of the corn in these early stands has not developed beyond V1-2 due to slow emergence and development (limited GDD accumulation) and are exhibiting poor uneven growth to sidewall compaction. Some of the corn planted 3 to 4 weeks ago is just beginning to emerge. As Dr. Emerson Nafziger notes in his recent newsletter article ( “This slow growth is not a good thing, but compared to a more favorable spring, such as the one we had a year ago, it means that replanted corn will not start out as far behind early-planted corn as it often would. So in relative terms, replanting is "favored" this year…. Up until the end of May, it is likely that even uniform stands of less than 15,000 per acre will benefit from replanting.”

When making the replant decision, seed and pest control costs must not be overlooked. Depending on the seed company and the cause of stand loss, expense for seed can range from none to full cost. As for the correct hybrid maturity to use in a late planting situation, continue to use adapted hybrids switching to early/mid maturities, if necessary, depending on your location in Ohio.

You also need to review herbicide and insecticide programs under late planting conditions. For instance, it may be necessary to reapply herbicides, especially if deep tillage is used. However, try to avoid such tillage depending instead on post-emergence chemicals or cultivation for weed control. Concerning insect control, if soil insecticides were applied in the row at initial planting, check insecticide label restrictions before re application. Also remember that later May and June planting dates generally increase the possibility of damage from European corn borer (ECB) and warrant selection of ECB Bt hybrids (if suitable maturities are available). In OSU studies conducted in 2004 and 2005, short season (104 day or less) Bt hybrids planted after the first week of June consistently out-yielded their non-Bt counterparts and usually produced yields comparable to commonly grown hybrid maturities (108 day or greater).

The cost of replanting will differ depending on the need for tillage and chemical application. The cost and availability of acceptable seed will also be considerations. These factors must be weighed against expected replanting yield gains. If after considering all the factors there is still doubt as to whether or not a field should be replanted, you will perhaps be correct more often if the field is left as is.

The following are additional on-line sources of information on making replant decisions and late planting issues.

Nafziger, E. 2008. Corn with Issues: Keep, Replant, or Change Crops? . The Bulletin, Univ. of Illinois IPM. [On-Line]. Available at (URL verified 5/27/08).

Nafziger, E. 2005. University of Illinois Interactive Agronomy Handbook - See Corn Chapter, “Replant Decision Aid” On-line at [URL verified 5/27/08]
(select “Corn”, then “Decision Tools” and “Replant Decision Aid”)

Nielsen, R.L. 2008. More thoughts on late corn planting. Corny News Network, Purdue Univ. [On-Line]. Available at (URL verified 5/27/08).

Nielsen, R.L. 2002. Delayed Planting & Hybrid Maturity Decisions. Purdue Univ. Cooperative Extension Service publication AY-312-W. Available at [URL verified 5/27/08].

Armyworm on Corn

Authors: Ron Hammond, Andy Michel, Bruce Eisley

We received our first report of true armyworms on corn that had been planted into a rye cover crop, the situation most likely to produce armyworm problems. Although larvae were still small, so was the corn. With the large adult armyworm flight this past spring, the potential for widespread problems are high. Thus, corn planted into rye cover crops should be scouted for potential outbreaks. Remember that Bt transgenic hybrids are not effective against this armyworm, including Herculex I that while having activity against black cutworm in the spring and fall armyworm later in the summer, has no activity against the true armyworm at this time. Thus, all transgenic corn hybrids should be scouted if planted into a grass cover crop.

Slug Update

Authors: Ron Hammond, Andy Michel, Bruce Eisley

With the critical time for slug feeding being now and over the next few weeks, we are getting more reports on slug feeding on field crops throughout Ohio. The slugs have finally reached the size where feeding becomes very noticeable, and they will only grow larger and feed more. Thus, it is critical that growers begin their monitoring for slug injury. A concern that we have is that the cool and wet weather is resulting in crops that are not growing at a normal pace. Corn and soybean will be in a more susceptible stage of growth (that is, smaller plants) for a longer period of time. When determining the need for control, take special note of the newer leaf tissue, either the inner most leaves in the corn whorl, or the unifoliates or new first triofoliates on soybeans. If this new leaf tissue is being severely fed on, treatment is perhaps warranted.

When treating, growers only have a few choices of molluscicide baits. The first and most common are baits containing metaldehyde. There are two of these baits available, 1) Deadline MPs and 2) Orcal Snail and Slug Bait, which are similar materials that should be applied at 10 lb/acre. The other toxin also available is iron phosphate, which is in bait known as Sluggo. While this bait is a natural product and is considered “safer”, our thought is that the metaldehyde baits, when used as directed, are also very safe. Most problems with metaldehyde baits come from pets getting into open bags or from spills. When broadcast over fields, all the molluscicide baits are considered safe, including to the environment. The availability of these baits with the exception of Deadline MPs in Ohio is unknown, and can only be determined by checking with your dealers. Although we know these treatments at the suggested rates are expensive, ranging from $16-18 per acre, we would point out this is still a better option compared to the amount of damage that can result, often a significant stand reduction or a yield loss greater than 20%. And with the current prices of corn and soybean, stopping a serious slug outbreak with one of these baits will give you a good return. A point to consider: compared to the cost of transgenic traits for insect control, and some of the new fungicides for rust control or when applied for “plant health”, molluscicide baits perhaps are bargains!

Although some growers swear by the spraying of 28% N or other concoctions, our experience suggests highly variable control. It seems like that for every grower who says they work, we talk to 3-4 growers who says they do not. Although we do not recommend these sprays, growers might choose to try them. If you do, use them at night with no wind when the slugs are climbing on the plants. Check the crop prior to spraying to make sure that slugs are out of the ground and climbing on the plants. As with any type of treatment, come back in a few days to a week to determine the level of control. The best way to determine level of control with N spraying, or with molluscicide baits, is to either examine plants for slugs at dusk, or to look at the new growth, the inner most leaves in the corn whorl or the unifoliates or new first triofoliates on soybeans. Continued feeding on new leaf material implies limited control, while leaves with little feeding injury suggest good control.

Killing Failed Glyphosate-Resistant Corn Stands

Authors: Mark Loux

This is essentially a reprint of last year’s article on this same subject. In fields where the first corn planting is not viable and needs to be killed so that corn can be replanted, there are still no ideal herbicide options for control of glyphosate-resistant (Roundup Ready/Agrisure GT) corn. Current available options include the following:

1. Select Max in now labeled for control of failed stands of glyphosate-resistant corn (this could be used under a section 18 last year), and is probably the most consistently effective option. Apply Select Max at the rate of 6 oz/A with nonionic surfactant (0.25% v/v) and AMS (2.5 to 4 lbs/A) when corn is up to 12 inches tall. Do not replant corn until at least 6 days after Select Max application. The 6-day waiting period is necessary due to the short period of Select Max soil residual activity. Failure to allow 6 days between application and planting could result in injury and problems with establishment of a new corn stand.

2. The application of Liberty or a combination of Gramoxone Inteon plus metribuzin can control emerged glyphosate-resistant corn without the need for a waiting period between application and replanting. However, in university trials, control from these treatments has been more variable than with Select Max.

Ohio State University (OSU) and several other universities conducted studies in 2006 to determine the effectiveness of Liberty, Gramoxone, and Gramoxone plus metribuzin for control of glyphosate-resistant corn. The rate of metribuzin in these treatments was limited to no more than 3 oz/A, the maximum rate that can be applied preemergence to corn. The following is a summary of the results from selected studies:

* OSU - We were able to obtain near complete control of corn treated at the 3-inch stage (V1-V2) with Liberty (32 oz), Gramoxone Inteon (18 oz/A), or Gramoxone Inteon + Sencor (18 + 3 oz/A). Here and in the following statements, “near complete” indicates 97 to 100% control. We observed generally less effective control when treating 11-inch corn (V5), but still obtained near complete control with Gramoxone Inteon (36 oz/A) or Gramoxone Inteon + Sencor (18 + 3 oz/A).

* University of Illinois – the only treatment that provided over 80% control of 5-inch corn was Gramoxone plus Sencor (41 + 3 oz/A), at 82% control. Control of 3-inch corn did not exceed 65% for any treatment.

* Penn State University – Gramoxone Inteon (24 oz/A) and Gramoxone plus metribuzin (16 + 2 oz/A) provided 91 and 97% control of 6-inch corn, respectively, and 77 and 87% control of 9-inch corn.

* Purdue University – Control of 5-inch corn ranged from 80 to 84% for Gramoxone and Gramoxone plus metribuzin treatments. Gramoxone plus metribuzin and Liberty were the most effective treatments on 3-inch corn, but control was fair at best, ranging from 72 to 75%.

The variable control of glyphosate-resistant corn in university trials with Liberty, Gramoxone, and Gramoxone plus metribuzin makes it difficult for us to know exactly what to say about their use. Select Max is probably the most consistently effective of all the treatments discussed here, but requires a wait of 6 days until planting corn. Where it is not possible to wait 6 days to replant, the best option may be a mixture of Gramoxone plus metribuzin, which should generally control small corn (less than about 5 inches?). In the trial OSU conducted in 2006, Liberty effectively controlled 3-inch corn, but was much less effective at other universities. Consequently, we find it difficult to recommend Liberty to control a failed corn stand.

Central Ohio Agronomy In-Field Twilight Program

Authors: Howard Siegrist

The Central Ohio Agronomy In-Field Twilight Program will be held on Thursday, June 5, 2008 from 6:00 p.m. until 9:00 p.m. The in-field program will be held at the David Miller farm, 10750 Millersport Road, Millersport, Ohio 43046. Concurrent sessions will address timely management concerns including: Nitrogen Sidedress Considerations by Robert Mullen, OSU Soils Specialist; Plant Health Fungicides ¬In Corn, Soybeans and Wheat provided by Dennis Mills, OSU Extension Program Specialist; Staging Post Sprays and What to Mix as Tank Partners with Glyphosate in Corn, led by Mark Loux, OSU Weed Specialist. Current establishment issues in corn and soybean development will be assessed by Wesley Haun of OSU Extension. No reservations needed.

Cost is $10.00 per person for the materials and a light supper that will be served at 8:30 p.m. Free pocket guides on row crop management will be distributed while the supply permits. Two hours of continuing education credits will be available for certified crop advisors. The event is sponsored by OSU Extension offices in Fairfield, Perry, Pickaway and Licking counties along with the Ohio Soybean Council. Individuals traveling to the event from the north will need to take S.R. 79 south through Buckeye Lake from I 70 to Millersport. Millersport Road runs south from Millersport. S.R. 37 is closed for realignment. For additional information contact Howard Siegrist at

Archive Issue Contributors: 

Contributors: Pierce Paul (Plant Pathology), Ron Hammond, Andy Michel and Bruce Eisley (Entomology), Peter Thomison (Corn Production), Florian Diekmann (Ag Subject Matter Specialist) and Mark Loux (Weed Science). Extension Agents and Associates: Roger Bender (Shelby), Howard Siegrist (Licking), Glen Arnold (Putnam), Harold Watters (Champaign), Mike Gastier (Huron), Wes Haun (Logan), Jonah Johnson (Clark), Bruce Clevenger (Defiance), Les Ober (Geauga), Steve Bartels (Butler), Alan Sundermeier (Wood), Gary Wilson (Hancock) and Tim Fine (Miami)

Crop Observation and Recommendation Network

C.O.R.N. Newsletter is a summary of crop observations, related information, and appropriate recommendations for Ohio crop producers and industry. C.O.R.N. Newsletter is produced by the Ohio State University Extension Agronomy Team, state specialists at The Ohio State University and the Ohio Agricultural Research and Development Center (OARDC). C.O.R.N. Newsletter questions are directed to Extension and OARDC state specialists and associates at Ohio State.