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Agronomic Crops Network

Ohio State University Extension


C.O.R.N. Newsletter 2007-13

Dates Covered: 
May 14, 2007 - May 21, 2007
Keith Diedrick

Replanting Considerations for Corn

Authors: Peter Thomison

Replant decisions in corn should be based on strong evidence that the returns from 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. (Table 1 shows row length needed for various row widths.) 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 (on-line at 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 different planting rate; this can vary depending on the hybrid used, soil fertility and moisture availability. Tables 4-14 and 4-15 in the OSU Agronomy Guide (on-line at show effects of planting date and plant population on final grain yield for the central Corn Belt. Table 4-15 is a newer chart developed by Dr. Emerson Nafziger at the University of Illinois that includes earlier planting dates and higher optimum plant populations. Table 4-14 is based on older data from the 1970's, but it still provides a reasonable assessment of potential yield losses, especially for planting dates in 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 is how the tables from the OSU Agronomy Guide can be used to arrive at a replant decision (Table 4-15 will be used in this example). Let's assume that a farmer planted on May 9 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 79% 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 is 81 minus 79, or 2 percentage points. At a yield level of 150 bushels per acre, this increase would amount to three bushels per acre which would probably not justify replanting costs.

It's also important to note plant distribution within the row. Remember that the values in Tables 4-14 and 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.

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 nothing 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, 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. Try to avoid such tillage, depending instead on postemergence chemicals or cultivation for weed control. If soil insecticides were applied in the row at initial planting, check insecticide label restrictions before re application. Also remember that later planting dates generally increase the possibility of damage from European corn borer (ECB) and may warrant selection of ECB Bt hybrids. 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).

Replanting itself does not guarantee the expected harvest population. Corn replant decisions early in the growing season are based mainly on plant stand and plant distribution. As the season progresses and yields begin to decline rapidly because of delayed planting, the calendar date becomes more important. 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 than not if the field is left as-is.

The following are additional on-line sources of information on making replant decisions:

Nielsen, Bob. 2006 Corn Replant Decision-Making. Corny News Network. Online at

Nielsen, Bob and Peter Thomison. 2002. Delayed Planting & Hybrid Maturity Decisions. Purdue Univ. Cooperative Extension Service publication AY-312-W. Online at

Nafziger, E. 2005. University of Illinois Interactive Agronomy Handbook - See Corn Chapter, "Replant Decision Aid" On-line at: (select "Corn", then "Decision Tools" and "Replant Decision Aid")

Killing Failed Roundup Ready Corn Stands

Authors: Mark Loux

Killing a failed corn stand for the purposes of replanting corn used to be pretty straightforward. Apply glyphosate to the failed corn, and plant the new corn stand. Or, plant the new corn stand and then apply glyphosate to the failed stand before the new corn emerges. This process remains unchanged for those fields not planted with Roundup Ready corn. However, killing the failed stand has become somewhat more complicated in fields where Roundup Ready corn was planted initially.

There are several options for the control of Roundup Ready or GT corn in fields where corn will be replanted:

1. ODA (with Ohio State University support) submitted for and obtained a Section 18 Crisis Exemption label for the use of Select Max to control failed stands of Roundup Ready or GT corn. This temporary label for Select Max is valid until May 24. Apply Select Max at the rate of 4 oz/A with nonionic surfactant (0.25% v/v) and AMS (2.5 to 4 lbs/A) when corn is up to 6 inches tall. Corn should have at least two fully emerged leaves at the time of application. Use a spray volume of 15 gpa or higher. Do not replant corn until at least 5 days after Select Max application. The 5-day waiting period is necessary due to the short period of Select Max soil residual activity. Failure to allow 5 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 Roundup Ready or GT 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. Another problem with the use of the Gramoxone/metribuzin treatment is that, for reasons unknown to us, metribuzin is currently in very short supply.

Ohio State University and several other universities conducted studies in 2006 to determine the effectiveness of Liberty, Gramoxone, and Gramoxone plus Sencor for control of Roundup Ready corn. A summary of the results from some of these 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 Sencor (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 Sencor treatments. Gramoxone plus Sencor 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 Roundup Ready corn observed among university trials with Liberty, Gramoxone, and Gramoxone + Sencor 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 5 days until planting corn. Where it is not possible to wait 5 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). However, it may not be possible to obtain metribuzin, an essential component of this treatment. Application of Gramoxone Inteon alone may be the next best option, but control may be more variable compared to Gramoxone plus metribuzin. In the trial OSU conducted in 2006, Liberty effectively controlled 3-inch corn, but was much less effective at other universities.

More on Dry Weather and Preemergence Herbicides: What to Do Now?

Authors: Mark Loux

It is our hope that by the time you read this, we will either have a solid forecast for rain tonight, or will already have received an inch or so of rain. Much of the state is in desperate need of a soaking rain to promote the activity of preemergence herbicides. Herbicides were applied to many fields over 10 days ago, and have received no rain since then. The only real good news is that weed emergence has been slower than normal due to the dry soil conditions.

We use the general rule that a half to one inch of rain must fall within 7 to 10 days after preemergence herbicide application, in order for herbicide activity to be maximized. Preemergence herbicides need to be moved an inch or two into the soil when the shoots of germinating weeds are still below the soil surface, so that herbicide uptake into the shoot and roots can occur. Failure for this to occur within 7 to 10 days after application usually results in weed emergence, and most herbicides are much less effective once the weed shoots are above ground. It is still possible for later rains to move herbicide into the soil, so that they can control later-emerging weeds, and this usually occurs at some point. The real problem is that the first flush of weeds that emerges within 2 weeks after planting is often not well controlled and requires postemergence treatment.

Some manufacturers promote "reachback activity" for an herbicide. This can be loosely described by the following sequence of events - herbicide is moved by rain into the soil where it is taken up by the roots of small emerged weeds, followed by movement of herbicide from the roots to the shoot of the weed, where herbicidal activity is expressed and the weed dies (hopefully). Herbicides that have at least some "reachback activity" on small weeds include Balance/Radius, Callisto, atrazine, and some ALS inhibitors. "Reachback activity" is most likely to be expressed in very small weeds (an inch or less), and it is debatable whether it provides enough control of weeds escaping preemergence herbicides that postemergence rescue treatments are not required. A common question this week has been how long after rain we could expect to see symptoms of "reachback activity" from Balance. Our sources at Bayer tell us that if substantial reachback is going to occur with Balance, we can expect to see symptoms on shoots (bleaching) within 3 to 5 days after rain. Our best suggestion regarding the whole issue of "reachback activity"- assuming we get a significant amount of rain Tuesday night, allow no more than 5 days for the appearance of herbicide symptoms. Decisions on the need for postemergence herbicides can be based on what occurs within those 5 days.

Probably the most difficult question to answer is-when the activity of preemergence herbicides is inadequate due to a lack of rain and weeds have emerged when should postemergence herbicides be applied? Early postemergence applications when weeds are very small can reduce short-term herbicide costs, but waiting until weeds are somewhat larger can result in more effective mid- to late-season control. Keep in mind also that under continued dry conditions, weeds will be easier to control with postemergence herbicides when small. Control of large, drought-stressed weeds can be somewhat variable. In addition, none of the suggestions provided below supercedes the basic principles of weed crop competition, which apply to any situation where preemergence herbicides have either partially failed or have not been used: 1) in corn, postemergence herbicides should be applied when weeds are no more than about 3 inches tall to avoid crop yield loss; and 2) in soybeans, postemergence glyphosate applications should be made when weeds are no more than about 6 to 8 inches tall to avoid crop yield loss. A closer look at strategies for several situations where preemergence herbicides have at least partially failed and weeds have emerged:

Roundup Ready crops provide for much flexibility in dealing with this situation due to the low cost of glyphosate. One strategy is to apply a low rate of glyphosate soon when weeds are very small, and if necessary, make a second postemergence glyphosate application in 3 to 4 weeks to control later-emerging weeds. The alternate strategy is to delay the initial glyphosate application until weeds reach a size of about 3 inches in corn, or about 6 to 8 inches in soybeans. The later application in the second strategy can improve control of later-emerging weeds, but does not necessarily eliminate the need for a second glyphosate application. Similar strategies can be used in Liberty Link corn, but the cost of Liberty is about twice that of glyphosate, and the multiple postemergence application approach can be expensive.

Conventional corn (not Roundup Ready or Liberty Link)- the strategies are similar to the those listed for Roundup Ready corn, but postemergence herbicides for conventional corn are more costly than glyphosate. As a result, there can be considerable difference in cost between the two strategies. Waiting until weeds, especially grasses, are 3 inches tall results in the need for full rates of postemergence grass herbicides, such as Steadfast, Steadfast ATZ, Option, and Equip. These are usually combined with broadleaf herbicides, such as dicamba, Callisto, Distinct, or Hornet. This approach will provide the most effective control of later-emerging weeds, but the treatment cost can be in excess of $15 to $20. The alternative strategy is to apply a lower-cost treatment soon when weeds are very small, with the hope that we will get enough rain for preemergence herbicide activity to kick in within a week or so and control later emerging weeds. There are a number of low cost options for control of small broadleaf weeds 2,4-D, dicamba, and reduced rates of other broadleaf herbicides should be effective on weeds that are an inch or two tall. Some broadleaf herbicides can also control small grasses, although probably not at reduced rates. Herbicides in this category include atrazine, NorthStar and Impact. Activity on emerged grasses is a primary difference between Impact and Callisto, because the two products have similar activity on broadleaf weeds. Callisto has activity on primarily crabgrass, whereas Impact can provide effective control of foxtails, crabgrass, and barnyardgrass that are less than 2 inches tall.

Conventional soybeans (not Roundup Ready)-the only real option where preemergence herbicides have failed is to apply a broad-spectrum postemergence herbicide treatment when weeds are 3 to 6 inches tall. Follow with a second postemergence application if necessary (likely in fields with giant ragweed).

Postemergence Control of Dandelion in Corn

Authors: Mark Loux

Dandelions can be extremely competitive with corn and soybeans under dry conditions. Fields where tillage or preplant herbicide treatments failed to adequately control dandelions should be treated with postemergence herbicides soon to prevent reductions in crop stand and yield potential. Based on OSU research, the most effective postemergence herbicide treatments for dandelion control include: Equip; Callisto + atrazine; Steadfast/Steadfast ATZ + Distinct or Callisto; and Distinct. Status can be used instead of Distinct in any of these combinations. Treatments that include Callisto often provide the most rapid control of dandelion, which can be important under dry conditions. We do not have data on control of dandelion with Impact, but it may be similar to that of Callisto based on the other similarities between these two products.

Conditions Not Favorable for Foliar Disease Development

Authors: Pierce Paul, Dennis Mills

Over the past few weeks, conditions have been very dry across most of the state, resulting in very little or no foliar disease development (most of Ohio with less than 0.8" of rainfall). Low levels of diseases like powdery mildew, Septoria, and Stagonospora leaf spot have been observed on the lower leaves in some fields, but due to the dry conditions, these diseases have not spread up the plant. Spores of Septoria and Stagonospora move from one leaf to another by rain splashing them up the plant (common during frequent rains that can occur in May and June). The wheat plant is generally susceptible to Stagonospora during and after heading and Septoria from stem elongation to flag leaf emergence. Septoria favors temperatures between 50 and 68 F, while Stagonospora flourishes between 68 and 81 F.

Powdery mildew develops best at temperatures between 59 and 71 F with high relative humidity, and tends to be most severe as wheat grows rapidly between Feekes stages 7 and 10. Mildew is usually more severe in dense stands of heavily fertilized wheat. Cool and moist conditions are needed for spores to germinate and infect. Spores are produced on the surface of the leaves and are easily spread by the wind. High humidity is needed for infection to occur. If conditions become too warm (temperatures above 77 F) and dry, the development of powdery mildew is slowed.

The dry weather we are experiencing is preventing infection and halting the development of powdery mildew lesions, Septoria and Stagonospora. Rain is forecasted mid-week, and if it falls, growers should walk fields 7 to 10 days after to check disease progress. Remember to scout fields before making a decision to apply a fungicide. If a susceptible variety is planted, a fungicide application is usually recommended for powdery mildew control when 2 to 3 pustules are detected on the leaf below the flag leaf (the top-most leaf) anytime from Feekes 8 (flag leaf emergence) and 10 (boot), and for Stagonospora leaf and glume blotch if 1 to 2 lesions are detected on the leaf below the flag leaf.

Soybean Aphid Update

Authors: Ron Hammond, Bruce Eisley

Last fall we reported on the large populations of winged soybean aphids on buckthorn, and this spring regarding the large number of soybean aphid eggs and the subsequent egg hatch. However, following the extreme cold spell we had in early April, questions arose on the survivorship of the aphids during that time period and effects on the predicted soybean aphid population this summer.

Within the past few weeks, two colleagues of ours, David Voegtlin (University of Illinois) and Bob O'Neil (Purdue University), surveyed soybean aphids on buckthorn in northern locations from Illinois into northwest Ohio. They found lower-than-expected numbers of winged aphids, and both believe that the cold weather in early April may have killed aphids that hatched from eggs in late March. The other possibility is that the buckthorn foliage was killed by the hard freeze, leaving the aphids without any nutritional source. Even though overall aphid densities are lower than expected, aphid colonies were abundant.

Voegtlin and O'Neil agree that soybean aphid densities this summer are still likely to be high in some areas of the Midwest, probably including Ohio. Whether we observe the predicted population remains to be seen. All growers will benefit from carefully monitoring soybeans this summer, and treat at the established thresholds. Stay tuned to the C.O.R.N. newsletter for updates as we get into the summer growing season.

Bean Leaf Beetle

Authors: Bruce Eisley, Ron Hammond

Last week we scouted a rust sentinel plot South Charleston, and saw evidence of significant bean leaf beetle feeding in patches throughout the field. With soybean planting at full speed now, growers should also monitor their emerging fields for bean leaf beetles. If early defoliation reaches 50%, or cotyledons are being destroyed and plants appear stunted, an insecticide treatment is recommended. Keep in mind that bean leaf beetles will tend to be most numerous in early-planted fields. If populations are deemed sufficient to warrant treatment, a list of labeled insecticides is available at

As in past years, there is concern among some growers with the beetle's ability to vector bean pod mottle virus (BPMV). The virus is transmitted during early season feeding by the over-wintering beetles. Concern with BPMV tends to be greatest on food-grade soybeans and those soybeans grown for seed (where seed quality is an important issue). If growers choose to treat the bean leaf beetle for virus control, they should spray the bean leaf beetle during the VC-V1 stage after the soybeans emerge from the soil and when beetles begin to appear in the field. Recommendations also suggest that a second spray be made in July at the beginning of the first beetle generation.

Black Cutworm Update

Authors: Ron Hammond, Bruce Eisley

Last week in this C.O.R.N. newsletter, we mentioned that black cutworm (BCW) moths were being captured in Ohio. In the past few days, we are starting to hear reports and see evidence of cutworm activity. Growers should begin scouting their corn fields as the plants emerge from the soil. If a rescue treatment is necessary, a list of insecticides labeled for cutworm rescue treatment can be found at:


Authors: Ron Hammond, Bruce Eisley

We expect that gray garden juvenile slugs are now hatching throughout central Ohio, and expect to begin seeing them in fields in northern counties soon. Although it is currently dry, we anticipate slug injury on both corn and soybeans in central locations, and northward throughout May and into June. No-till growers who have experienced slug problems in past years should be prepared to monitor their slug situation over the next month and a half.

Soybean Rust Find in Louisiana

Authors: Anne Dorrance

Soybean rust was identified in a kudzu patch west of New Orleans last week. Louisiana also had soybean rust during 2006, but this year's find is 53 days earlier. This is a concern for southern soybean producers, but the impact on Ohio's production is still an open question. The number of kudzu patches with soybean rust and subsequent buildup over the next 1 to 2 months will be key to answering this question. No soybean diseases have been reported in Ohio for the 2007 season.

Archive Issue Contributors: 

State Specialists: Anne Dorrance, Pierce Paul, and Dennis Mills (Plant Pathology), Ron Hammond and Bruce Eisley (Entomology), Peter Thomison (Corn Production), Mark Loux (Weed Science). Extension Educators: Roger Bender (Shelby), Todd Mangen (Mercer), Howard Siegrist (Licking), Glen Arnold (Putnam), Bruce Clevenger (Defiance), Wes Haun (Logan), Jonah Johnson (Clark), Keith Diedrick (Wayne), Steve Prochaska (Crawford), Mark Koenig (Sandusky), Harold Watters (Champaign), Mike Gastier (Huron), Greg LaBarge (Fulton), Steve Foster (Darke), Ed Lentz (Seneca), Alan Sundermeier (Wood), and Jim Lopshire (Paulding).

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.