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Ohio State University Extension


C.O.R.N. Newsletter 2006-15

Dates Covered: 
May 30, 2006 - June 6, 2006
Tammy Dobbels

Corn Replanting Considerations

Authors: Peter Thomison

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. (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's 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 to 6 feet within rows and a 2% penalty for gaps of 1 to 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.

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 postemergence 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 planting dates generally increase the possibility of damage from European corn borer (ECB) and may 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 outyielded their non-Bt counterparts and usually produced yields comparable to commonly grown hybrid maturities (108 day or greater).

Understand that replanting itself does not guarantee the expected harvest population. Corn replant decisions early in the growing season will be based mainly on plant stand and plant distribution. Later in the season as yields begin to decline rapidly because of delayed planting, calendar date assumes increased importance.

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.

Nielsen, Bob. 2006 Corn Replant Decision-Making. Corny News Network. Online at [URL verified 5/30/06]

Nielsen, Bob and Peter Thomison. 2002. Delayed Planting & Hybrid Maturity Decisions. Purdue Univ. Cooperative Extension Service publication AY-312-W. Online at [URL verified 5/15/06].

Nafziger, E. 2005. University of Illinois Interactive Agronomy Handbook - See Corn Chapter, “Replant Decision Aid” On-line at [URL verified 5/30/06]
(select “Corn”, then “Decision Tools” and “Replant Decison Aid”)

Using Bt Corn Borer Protection in Late Planting

Authors: Ron Hammond, Bruce Eisley

Corn fields that have not been planted might benefit from the use of a Bt corn borer hybrid, either YieldGard Corn Borer, Herculex 1, or Agrisure CB. Our research over several years at the OARDC Western Station found that when we plant corn in late May, we often see a yield increase with Bt hybrids compared to closely related hybrids without the Bt trait. Some of this increase in yield may be because of control of second brood corn borer larvae because second brood is normally higher in late planted corn. So, if you need to plant or replant, consider a Bt corn borer hybrid.

Armyworm in Corn

Authors: Ron Hammond, Bruce Eisley

Last week after the CORN newsletter was sent out, we became aware of a serious outbreak of armyworm in corn planted into a rye cover crop in Seneca County, which is the perfect scenario for such problems. We added an update to the CORN newsletter on this situation. Please read last week’s newsletter for this information. As a quick reminder, corn fields along with wheat fields should be checked for possible problems. Also be aware of possible armyworm populations in rye or wheat fields that might make the move into adjacent corn fields. We do not believe armyworms are a widespread problem, but isolated fields might be at risk.

Leaf Rust on Wheat

Authors: Pierce Paul, Dennis Mills

We have received reports of the occurrence of wheat leaf rust in some parts of the state. The leaf rust-causing fungus may have overwinterd in some field (especially in southern counties), leading to the relatively early appearance of the disease in some locations. In most areas, spores of the fungus are usually blown in from southern states. Because of this, it normally takes a little while for the disease to become established and spore numbers to build up to a high enough level to be of concern in most years in Ohio. The earlier spores are blown in (or released from fields where the fungus may have overwintered), the greater are the chances of this disease spreading. Leaf rust develops best under slightly warmer conditions than those suitable for powdery mildew - temperatures between 66 and 77F and extended periods of humid, wet conditions. The development of this disease tends to be faster on susceptible varieties than on resistant varieties. We typically recommend fungicide application for leaf rust control on susceptible varieties when 5-10 pustules are detected on the flag leaf.


Efficacyof Fungicides for Wheat Disease Control in Ohio Based on Application atThreshhold
Fungicide Rate/A Powdery Mildew* Stagonospora
Leaf Blotch*
Leaf Rust**
Headline 9 fl oz Fair Good Excellent
PropiMax 4 lf oz Excellent Good Excellent
Quadris 6.2-10.8 fl oz Fair Good Excellent
Quilt 14 fl oz Excellent Good Excellent
Stratego 10. fl oz Fair Good Excellent
Tilt 4 fl oz Excellent Good Excellent
*PowderyMildew efficacy based on application at flag leaf stage (growth stage 8)
**Stagonospora and Leaf Rust control based on application at head emergence (growthstage 10.1)

Check product labels for application timing restrictions. Some fungicides (Tilt, PropiMax, Quadris, Stratigo, and Quilt) should ONLY BE APPLIED PRIOR TO FLOWERING. The wheat crop has already flowered or is flowering in most locations.


Potato Leafhopper on Alfalfa

Authors: Bruce Eisley, Ron Hammond

Potato leafhoppers have arrived in the Midwest, including Ohio. Now that many alfalfa fields have been or are being harvested, growers should plan on initiating scouting for the leafhopper as the re-growth reaches sufficient height for sweep-net sampling. Thus, growers in southern to central Ohio should probably begin sampling now, while those in the north should be thinking about this within a week or so. Sampling is done using a sweep net, and taking 15 samples throughout a field; each sample should consist of 10 sweeps with the net. Search through the net counting all potato leafhoppers, adults and nymphs (although mostly adults will be seen). When the average number of leafhoppers in a single sample (10 sweeps) equals or is greater than the height of the alfalfa, treatment should be considered. For example, if the alfalfa is 6 inches tall and the average number of leafhoppers per sample is 6 or higher, treatment is warranted. If the average is 5 or lower, the grower should come back within a few days to see if the population is higher or lower. Pictures of PLH adults and nymphs can be found on the WEB at:, This site lists those insecticides labeled for PLH on alfalfa:

Pythium on Corn and Soybeans

Authors: Anne Dorrance, Pierce Paul, Dennis Mills

We received several reports of stand issues in several areas of the state. Pythium and flooding appear to be the primary culprits. Pythium are soil borne water molds, that are most active when the soil temperatures are cool and soils are saturated. We did report a few years ago, where Ohio does have some Pythium spp. that were not sensitive to metalaxyl or mefenoxam. We do not know how widespread this is at this time. Extensive sampling will begin later this summer. Some of the other seed treatments do have efficacy against these Pythium spp. Some things to take field notes on this year as you are assessing stand include: Was a seed treatment used? What was the rate of seed treatments? What were the active ingredients in the seed treatment? This information taken now will help make a better decision for next year.

Raising Corn or Weeds?

Authors: Mark Loux

We don’t like to beat a subject into the ground, but as the soil dries out making traffic possible, it is essential to scout corn fields, and treat with herbicides where scouting reveals the need to do so. Priority should go to those fields planted in April or early May that either have not received any herbicide treatments, or where the preemergence herbicides were largely ineffective. Corn is subject to weed interference sooner after planting than soybean, so weedy corn fields should generally get priority over weedy soybean fields with regard to POST herbicide applications.

Soybean Aphid Update

Authors: Ron Hammond, Bruce Eisley

We are currently keeping a watch on the situation with the soybean aphid in Midwest state. So far, observations suggest little overwintering or presence of aphids on buckthorn. Based on these observations and those from last fall, we are still predicting that soybean aphids will not reach economic levels in our state. At this time, we recommend that growers NOT make plans on treating based on unsubstantiated rumors. As always, treatment should be based only on thresholds being reached after populations begin to increase. Treatments should NOT be done for preventive purposes. Because we are still not 100% sure what will happen this summer, we recommend growers continue to read this newsletter to keep informed.

Warrior on Soybean: Change in Preharvest Interval

Authors: Ron Hammond, Bruce Eisley

Syngenta recently informed us that Warrior has just received a new label with a reduced PHI, or preharvest interval, for soybeans. The new PHI for soybean is 30 days, down from the 45 day PHI that was on the previous label. This will allow Warrior to be used much later into the summer, at least 2 weeks longer. We will remind growers of this as we get into the later months of summer.

Soybean Rust

Authors: Anne Dorrance

Soybean rust has taken a step back in several areas of the south east this past week. Continued drought and hot temperatures have stalled any progress. Texas in particular remains negative. Louisiana is finally getting some light rains this week. Ohio’s sentinel plots are in, they range in size from VC to V2, 2 to 5 inches high. Plants were stalled due to the cool weather, and no diseases or insects were present. The warm weather should advance things and with the rains, we may begin to see brown spot in the next few weeks.

Tomorrow’s Field Day (May 31st) Focuses on Resistance Management

Authors: Bonnie Norris

As you are all aware, our heavy reliance on certain chemicals has generated a new opportunity to study resistance in weeds such as marestail (horseweed). Preble County has joined Montgomery County's designation as having with the distinct opportunity to see marestail plants which show evidence of both ALS and glyphosate resistance in the same plant. Jeff Stachler, Program Specialist, OSU Hort and Crop Sciences, has a weed plot here in Preble County, just south of Eaton on Consolidated Road, just West of US 127.

We have worked with the farmer and a neighboring farmer to set up a field day so we can see first-hand the research and the effects of various treatments. This particular field also has an interesting dandelion variable. The field day will be Wednesday, May 31st from 9 - 11 a.m. Attendees will receive the new "Biology and Management of Horseweed" booklet and a very good picture of this latest opportunity.

From I 70: Take I70 West to exit 10 US 127, Eaton. Take 127 South through Eaton. Consolidated Road is the first road South of Carter Lumber and past the bridge construction. The road will off to the right (West). Consolidated is NOT a cross road over 127, it goes under, so it looks almost like an exit ramp. Keep going West, you will see the field on the North (right) side of the road. Parking is exactly .1 miles beyond the field drive on the South (left) side. I will have it marked. Please just pull off on the side of the lane, NOT in the fields.

From the South: Take 127 North through Camden. Consolidated is the second road past the County Landfill. Consolidated is NOT a cross road over 127, it goes under, so it looks almost like an exit ramp. Keep going West, you will see the field on the North (right) side of the road. Parking is exactly 0.1 miles beyond the field drive on the South (left) side. I will have it marked. Please just pull off on the side of the lane, NOT in the fields.

I hope you can join us for this informative day. Call Bonnie Norris at 937-533-1446 if you need more

Modified Relay Intercropping

Authors: Steve Prochaska

Farmers in Crawford and Wyandot counties are planting soybeans not only in conventionally rotated fields but also into growing wheat fields. This system is called Modified Relay Intercropping (MRI) and it is the planting of soybeans into standing wheat. This system of intercropping does not use coated soybeans and wheat rows are typically 10 to 12 inches in width. Also, MRI planting occurs in late May or early June as opposed to planting in early to mid May for other systems of intercropping done in wider rows.

The MRI system offers producers many benefits. Because two crops with vastly different growing seasons and cultural requirements are produced in the same field in the same year, MRI offers producers both a production and marketing hedge. Thus farmers in the MRI system utilize their time, equipment, land and labor during a period (normally early June) to grow a second crop. The MRI system is also very favorable from an environmental perspective in that soil erosion is minimized with the small grain crop and often very little herbicide is needed to grow soybeans sown into the standing wheat.

Soybeans in the MRI system are generally sown into wheat around the pollination time period, with a grain drill or tool bar planter. The wheat has a tramline to facilitate soybean planting. Light or the lack of it, has a profound effect on the growth of intercropped soybeans. Soybeans planted too early into well tillered wheat often will become tall and spindly and result in weak plants that do not grow or yield well. This year farmers, have started to plant soybeans into wheat about week earlier than normal. MRI can be done to until wheat stems begin to break over. This system is highly dependent on timely rain in July and August.

The wheat plant, by virtue of its wide adaptability, is able to tolerate slightly wider row spacing and the stress of soybean planting with minimal yield loss. Six years of replicated field trials on the MRI system have resulted in favorable average soybean and wheat yields (73 bu/acre for wheat and 30 bu/acre for soybeans). Wheat yields in good years have exceeded 80 bushels per acre and soybean yields over 40 bushels per acre.

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), Gary Wilson (Hancock), Tammy Dobbels (Montgomery), Todd Mangen (Mercer), Ed Lense (Seneca), Mark Koenig (Sandusky), Jim Lopshire (Paulding), Jim Skeeles (Lorain) 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.