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Agronomic Crops Network

Ohio State University Extension


C.O.R.N. Newsletter 2006-16

Dates Covered: 
June 6, 2006 - June 13, 2006
Greg LaBarge

Replanting Roundup Ready Corn – How to Kill the First Planting

Authors: Mark Loux

(Thanks to Bill Johnson and Glenn Nice,Extension Weed Scientist, Purdue University for their input into this article)

Every management decision has consequences, and the decision to plant Roundup Ready corn is no exception. Among the positive consequences of Roundup Ready corn that include improved control of certain weeds and less risk of crop injury due to herbicides, there are potential negative consequences such as an increased risk of glyphosate-resistant weeds and volunteer Roundup Ready corn in Roundup Ready soybeans. As the acreage of Roundup Ready corn increased, it is natural that more producers would be faced with the situation where poor weather resulted in an inadequate initial Roundup Ready corn stand, just as they have faced this with conventional corn hybrids. Glyphosate has typically been used to control the first planting of conventional corn in replant situations, but will obviously not control Roundup Ready corn. So, how do we effectively and legally control a failed stand of Roundup Ready corn?

It appears that hundreds if not thousands of acres have been treated with postemergence grass soybean herbicides (Select, Assure II, etc) this year in an attempt to kill the existing Roundup Ready corn stand. Information from the WSSA’s Herbicide Handbook indicates that these herbicides have at least some soil residual activity, and thus will have the potential to injure replanted corn if the radical or coleoptile come into contact with a high enough concentration of herbicide. The soil half-lives of these herbicides are as follows: quizalofop (Assure II) - 60 days; sethoxydim (Poast) - 5 days; clethodim (Select) - 3 days; and for Fusion components, fluazifop -15 days and fenoxaprop – 9 to 30 days. While they can effectively control emerged corn, the labels for these herbicides do not support their use as a preplant treatment in corn, Roundup Ready or otherwise.

Information from product labels that pertains to preplant use in corn:

- Poast and Poast Plus labels specifically state to not apply these products as a preplant or preemergence treatment before planting grass crops, such as corn.

- The Select label does not allow replanting sensitive rotational crops for 30 days, although corn is not mentioned specifically. The Select Max label indicates that it is only labeled for soybean and should not come into contact with sensitive crops such as corn.

- The Assure II label states that only specific crops (which do not include corn) can be planted within 120 days after application.

- The Fusion label states that it may only be applied before, during or after emergence of soybean.

So, although these products are effective at controlling volunteer corn, they are labeled only for the control of volunteer corn in a soybean crop, not in a corn replant situation.

Our assessment is that there are really only three effective and legal options to kill an existing stand of Roundup Ready corn in a replant situation – tillage, Gramoxone, or Liberty.

Our experience has been that tillage will be the most reliable method, but not desirable for those in a long-term no-till situation.

The second best options of using Gramoxone or Liberty may not always be 100% effective, but the labels for these products do allow this type of use. In University research trials, Gramoxone (2-3 pt/A) + Sencor (4-6 oz/A) or 32 to 34 oz/A of Liberty has been effective for control of small corn (V1 to V3). Application of Gramoxone alone, without the addition of Sencor, is likely to be less effective. Corn that has advanced past the V3 growth stage will generally be more difficult to control. A combination of Gramoxone plus Sencor is likely to be more effective than Liberty on this size corn, unless Liberty Link corn is planted and a followup treatment of Liberty can be used after emergence of the new stand to control plants that survived the first application.

A Diversity of Insects Being Found

Authors: Bruce Eisley, Ron Hammond

Over the past few weeks, we have received reports of numerous insects causing concern from all field crops. This spring has been one of the highest in the past few years for the diversity of problems we have had to handle. For the most part, these problems are also the same ones being reported by neighboring states. This update is a reminder to check all your field crops for possible problems.

Corn – European corn borer adults are being caught in traps in Ohio and the rest of the Midwest states. This population will be producing the first brood of larvae. Black cutworms continue to cause problems in fields, as do slugs and armyworms. We have received numerous reports of insecticide seed treatments not providing the level of black cutworm control that was expected; all fields should be checked. The armyworm situation is mostly in corn fields having had a rye cover crop, or in fields adjacent to wheat. Reports of armyworm populations in timothy are also being made. We have also seen some corn fields with lesser cornstalk borer problems.

Soybeans – Reports on higher than normal bean leaf beetle populations continue to be received. Although their numbers should be going down within the next few weeks, make sure they are not causing undue defoliation. Slugs are also causing problems to newly emerged soybeans, as well as those just emerging. Although we do not have any confirmed findings of soybean aphid yet in Ohio, neighboring states have located small populations, which is not unexpected. Whether these aphids build up to damaging levels on a widespread basis across Ohio like last year remains to be seen; vigilance and continued reading of the CORN newsletter is suggested.

Alfalfa – although weevil numbers have dropped, potato leafhopper are now being reported in new alfalfa growth. Sampling for leafhoppers should have already started, or should begin this week.

Wheat – Although not receiving many calls on armyworms on wheat, some neighboring states are now reporting problems. As noted under corn, we have also been called about armyworms in timothy. We also had a call or two on cereal leaf beetles in wheat in populations higher than normally seen.

Oats – We had one report on cereal leaf beetles causing problems on oats, which is highly unusually. Only Lannate, endosulfan, and some malathion formulations are labeled for this insect on oats.

Needless to say, there are numerous pests out there on almost every crop being grown. As IPM principles suggest, sampling should be occurring on a weekly basis to make sure that nothing gets out of hand. See Bulletin 827 for more information on many of these insects ( and Bulletin 545 for labeled insecticides and rates (

What Soybean Maturity Should I Plant in June?

Authors: Alan Sundermeier, Jim Beuerlein

Late planting reduces our cultural practice options in soybeans for row spacing, seeding rate and variety maturity. The row spacing for June planting should be no greater than 7.5 inches. Appropriate seeding rates for the first half of June are about 200,000 to 225,000 seeds per acre.

Soybeans are photo period sensitive. The date of physiological maturity is due to both day length and the stage of seed development in the uppermost pods on the plants. Relative maturity (RM) has little effect on yield for plantings made during the first three weeks of May but the effect can be large for late plantings. In May each 3-day delay in planting delays the physiological maturity date by about one day. During the first half of June, a 4-day delay in planting delays physiological maturity about one day. In the last half of June it takes a 5-day planting delay to delay physiological maturity a day. As planting is delayed, yield potential goes down and there is concern about whether late maturing varieties will mature before frost.

When planting late, the rule-of-thumb is to plant the latest possible maturing variety that will reach physiological maturity before the first killing frost. The reason for using late maturing varieties for late planting is to allow vegetative growth for as long as possible to produce nodes where pods can form before flowering and pod formation. More nodes equals more pods and more yield. So we need late maturing varieties that will mature before getting frosted.

Assuming normal weather and frost dates, varieties in Northern Ohio planted between June 1 to June 15 with a relative maturity of 3.2 should mature before frost and produce maximum possible yields. Varieties with an earlier relative maturity will mature earlier but will produce reduced yields.

When evaluating existing soybean plant stands for replant decisions, farmers need to take random plant counts to determine the average number of existing plants per foot of row. In a 7.5 inch row spacing, soybeans that emerged around May 15 will produce acceptable yields with 1.2 plants per foot of row. An emergence date of June 1 in 7.5 inch row spacing needs 1.4 plants per foot of row. Many times growers have better yield potential with a thinner soybean population planted in early May compared to replanting in June. More on soybean stand evaluation can be found in a previous article at:


Extension Educators Providing Weekly Crop/Pest Updates

Authors: Greg LaBarge

Regional Crop and Pest Reports are being developed in several regions of the state. These reports will be updaed on a weekly basis with weather conditions plus reports on corn, soybeans, wheat and forages from observation during the week. The reports will updated on Thursday or Friday highlighting development and the presence of insect, disease and other cultural problems being reported. The is a great way to keep up on what is happening in your area and other areas that can you in your scouting efforts. The page can be found at:

Corn Growth Stage and Postemergence Herbicides

Authors: Mark Loux

Postemergence corn herbicide labels specify the maximum size or most advanced growth stage of corn to which herbicide can be applied, and following these guidelines reduces the risk of herbicide injury. We also summarize height and growth stage restrictions in the herbicide descriptions and in Table 7 (pages 73-74) of the current Weed Control Guide for Ohio and Indiana located at

Where herbicide use is based on corn growth stage, this is usually indicated on the label as the “V stage” or number of leaf collars that have developed. When determining the growth stage, remember to include the very first corn leaf, which is usually no longer evident by this time of the season. So, a corn plant that appears to have 4 leaves with fully developed collars is actually in the 5-collar or V5 stage. Some labels indicate both corn size and growth stage, such as “can be applied broadcast up to 6 collars or 20-inch corn”. Use the more restrictive guideline in this case, so that if corn reaches 6 collars before it is actually 20 inches tall, the herbicide should no longer be applied broadcast.

A few other relevant comments:

- corn becomes more sensitive to dicamba after about the 5-leaf stage, or when it exceeds 8 to 10 inches in height. Here, 5-leaf is a more vague guideline, since it does not refer to leaf collars, but just number of leaves. The most conservative label language for dicamba indicates that full rates can be applied broadcast up to 5-leaf or 8-inch corn, and a directed spray should be used on larger corn. The Distinct label is much less conservative, allowing broadcast applications on corn up to 24 inches tall. The Distinct rate should not exceed 4 oz/A where corn is more than 10 inches tall, however.

- Low rates of dicamba are a standard component of many postemergence corn treatments, and can be applied broadcast to corn larger than 8 inches tall. Where dicamba is applied in combinations with other herbicides (such as ALS inhibitors), spray additive recommendations can vary with dicamba rate and corn size. See labels for more information on mixtures containing dicamba.

- Herbicides and herbicide treatments that include ALS inhibitors (NorthStar, Accent, Hornet, Equip, etc) can usually be applied broadcast up to the 4- to 6-collar stage of corn, depending upon the herbicide. Labels for most ALS inhibitors also indicate a maximum corn height for broadcast applications, which is often 20 inches. A number of these herbicides can be applied to somewhat larger corn, as long as the spray is directed toward the lower part of the corn plant using drop nozzles. Any product containing atrazine will also indicate that it should not be applied to corn more than 12 inches tall, which is not due to injury concerns, but is just a standard guideline for postemergence use of atrazine.

- Some herbicides for which the label specifies a relatively wide window for broadcast application:

Product Stage
glyphosateon RR corn  2 – 8 collars or 30inches
Libertyon Liberty Link corn 7 collars or 24 inches
Aim 8 collars
Basagran large but not specifiedon label
bromoxynil before tasselemergence
Callisto 8 leaves or 30 inches
Permit up to layby stage (noteto manufacturer - a more definitive guideline would be useful here)
Priority 8 collars
Resource 10 leaves
Yukon 36 inches
Some of these can be applied to even larger corn if using a directed spray.


"Patching In" Poor Corn Stands

Authors: Peter Thomison

Ponding and poor drainage conditions have reduced corn stands in fields across Ohio, especially areas that received considerable rain since about May 11. The remaining plants in affected fields are often unevenly spaced within rows and not developing uniformly. Questions often arise as to whether to patch-in these poor stands, replant stands with poor emergence, or to protect late emerging plants during row cultivation. The following are some guidelines to consider in these situations based on findings of Illinois and Wisconsin research. A good source for more information on this subject is the National Corn Handbook Chapter 36, "Effects of Uneven Seedling Emergence in Corn" which is available on-line at

Growers will sometimes attempt to plant over or "patch in" a poor stand rather than kill the existing plants and replant at a full population. However, "patching in" is generally of limited benefit unless the surviving plant population is less than one half that of the original. The success of such an approach is even less likely late in the planting season (i.e. after June 1). Later planted corn cannot compete effectively with the remnants of the original plant population for sunlight, water, and nutrients. In these late planting situations, late emerging plants often function more like weeds, and contribute little to grain yield.

• If you replant within 2 weeks of planting the original, patching-in may be a viable option. Yields will be similar to those from a uniform-emerging replanted stand, if you can get relatively uniform plant spacing within the row between the old and new plants. However, within 2 weeks of planting, it probably will be too early to determine what the final stand will be (and whether patching will be needed).

• If you replant within 3 weeks after the initial planting, yield potential is about 10% greater if you tear up the field and start over with an even emerging stand rather than just patch-in the original stand. Balance this possible yield increase against the additional cost of tillage, seed, and dryer fuel.

• If the delay in emergence is less than 2 weeks, replanting will have a minimal effect on yields, regardless of the pattern of unevenness.
• If one half or more of the plants in the stand emerge 3 weeks late or later, then replanting may increase yields by up to 10%. To decide whether to replant in this situation, estimate both the expected economic return of the increased yield compared to your replanting costs and the risk of emergence problems with the replanted stand.
For more information on replanting corn, check last week’s CORN newsletter (May 30-June 6, 2006 C.O.R.N. 2006-15 ("Corn Replanting Considerations") available on-line at

• If the delayed plants emerge only 1 " to 2 weeks late, use shields and avoid burying the late-emergers during cultivation.
• Protect plants emerging 3 weeks late if one half or more of the plants in the stand are late-emergers.
• If less than 1/4 of the stand emerges 3 weeks late or later, it probably will not pay to encourage their survival. Yields will be about the same whether or not these delayed plants are buried during cultivation.

Archive Issue Contributors: 

State Specialists: Ann Dorrance, Pierce Paul and Dennis Mills (Plant Pathology), Mark Loux and Jeff Stachler (Weed Science), Peter Thomison, (Corn Production), Ron Hammond and Bruce Eisley (Entomology). Extension Educators: Howard Siegrist (Licking), Harold Watters (Champaign), Glen Arnold (Putnam), Roger Bender (Shelby), Steve Foster (Darke), Steve Bartels (Butler), Steve Prochaska (Crawford), Bruce Clevenger (Defiance), Greg LaBarge (Fulton), Todd Mangen (Mercer), Ed Lentz (Seneca), Mark Koenig (Sandusky), Steve Ruhl (Morrow), and Keith Diedrick (Wayne)

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.