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Agronomic Crops Network

Ohio State University Extension


C.O.R.N. Newsletter 2007-17

Dates Covered: 
June 13, 2007 - June 18, 2007
Greg LaBarge

Control of Volunteer Roundup Ready Corn

Authors: Mark Loux

As the acreage of Roundup Ready corn increases, so will the prevalence of volunteer Roundup Ready corn in the following year’s soybeans. In nonGMO soybeans, control of volunteer Roundup Ready corn is really no different than control of nonGMO volunteer corn. Most postemergence grass herbicides, including quizalifop (Assure II, Targa), Fusion, Fusilade DX, and clethodim (Select, Arrow, Select Max, etc) effectively control volunteer corn when applied at the appropriate rate and with the recommended adjuvants. All of these herbicides are generally most effective when applied with crop oil concentrate, with the exception of Select Max, for which nonionic surfactant plus AMS is the standard adjuvant recommendation.

Where volunteer Roundup Ready corn is present in Roundup Ready soybeans, the postemergence grass herbicide is typically applied in combination with glyphosate. Control of the volunteer corn can be reduced in this mixture, because the adjuvants usually include just AMS and the nonionic surfactant that is packaged with the glyphosate.

An example from a field study we conducted in 2005 – Select applied at 6 oz/A with crop oil concentrate controlled 100% of the volunteer Roundup Ready corn, but this decreased to 43% control when applied in a mixture with Roundup WeatherMax and AMS (no crop oil concentrate). As a result, selection of the appropriate postemergence grass herbicide becomes more critical in mixtures with glyphosate, along with the rate applied. The primary recommendation here is to avoid use of Select, Arrow, and other generic clethodim products in mixtures with glyphosate, as well as any Poast products.

A summary of the research from studies we conducted on control of volunteer Roundup Ready corn over the last three years follows (not all treatments were repeated all years). AMS was included in all treatments, but no additional surfactant was added. The control shown reflects evaluation at 28 to 42 days after application.

2004 - % control when mixed with glyphosate: Assure II at 4 or 6 oz/A – 100%; Select Max at 6 and 8 oz/A – 80 and 90%; Fusion at 6 oz/A – 100%; Select at 4 and 6 oz/A – 65 and 85%.

2005 - % control when mixed with glyphosate: Assure II at 4 and 5 oz/A – 98 and 92%; Select Max at 6 oz/A – 99%; Select at 4 and 6 oz/A – 37 and 43%.

2006 - % control when mixed with glyphosate: Assure II/Targa at 4 oz/A – 86 to 95%; Select Max at 6 oz/A – 88%; Fusilade DX at 4 and 6 oz/A – 95 and 97%; Arrow at 4 oz/A – 65%.

Variable Corn Size and Postemergence Herbicides

Authors: Mark Loux

An article in last week’s C.O.R.N. covered the issue of weed control and the possible need for residual in postemergence corn treatments where corn is still relatively small due to variable corn size within fields. One of the related issues is the problem with timing postemergence applications in field with variable corn size and growth stage, in order to ensure that the risk of corn injury and yield loss is minimized.

For example, when applied broadcast, ALS-inhibiting herbicides should generally be applied before the V4 to V6 stage, depending upon the product, to avoid problems with ear development and lower the risk of yield loss. It could be possible to have within the same field a range in corn growth stage from V3 to V6, and the presence of V6 corn could preclude use of an ALS inhibitor.

A caution here is to make sure that both corn height and growth stage are noted in field scouting, since herbicide labels may refer to one or both of these with regard to postemergence timing. In addition, this is one of those years where corn may be relatively small and still be advanced enough in growth stage that certain herbicides should no longer be applied, and measuring just height would not provide the needed information.

The bottom line – be sure to make herbicide choices and time herbicide applications based on the corn plants that are most advanced in growth stage and/or size, since these are likely to determine which herbicides can still be applied.

Grass herbicides tend to be of the greatest concern here, since most of them are ALS inhibitors. Several of these can be applied to corn that is past the V4 to V6 stage, but only as a directed spray using drop nozzles. Herbicides with this type of label include: Accent – directed spray up to 36-inch or 10-collar corn; Option – directed spray up to 36-inch corn; and Equip – directed cpray up to 36-inch or V8 corn.

Several broadleaf herbicides can be applied broadcast to corn up to 30 inches or taller with little risk of corn injury, including Status, Impact, bromoxynil, and Callisto. However, the leaf canopy of corn this large can prevent spray particles from reaching weeds growing under the crop canopy. Drop nozzles may provide more effective delivery of herbicide to the intended target while minimizing risk of corn injury.

Please refer to the information corn growth stage and postemergence herbicides can be found in Table 7 on page 73-74 of the 2007 Weed Control Guide found at

OSU Weed Science Field Day

Authors: Mark Loux

The OSU Weed Science field research tour will be held at the OARDC Western Agricultural Research Station on Wednesday, July 11. The tour, which is self-directed in nature, starts at 9 am and runs until noon. OARDC Western Agricultural Station is north of South Charleston on State Route 41, approximately 5 miles south of Interstate 70.

Cereal Leaf Beetle, Leafhopper and Soybean Aphid Update

Authors: Bruce Eisley, Ron Hammond

Growers should be aware that insect activity is about what we would expect for this time of year and with the conditions being experienced in Ohio. The most important thing for growers is to keep on sampling and monitoring their fields. There are a number of insects that should be given some special attention.

Cereal Leaf Beetle on Wheat – Numerous fields have experienced problems from this insect, but growers should realize their injury should be diminishing. Most larvae should or will be pupating, and in those fields with extensive injury, the damage is probably already done to the flag leaf and treatment would be more for revenge. Thus, growers should be cautious on making insecticide application at this time unless the situation calls for it. This is an insect that will demand further watching in coming years.

Potato Leafhopper on Alfalfa – Populations are rising in new growth and fields are being sprayed in Ohio. Growers should now be sampling their fields for leafhoppers and paying attention to the threshold. When the average number of leafhoppers in a single sample (10 pendulum sweeps) is equal or greater than the average height of the alfalfa stand, insecticide treatment is warranted for varieties not resistant to the potato leafhopper.

For potato leafhopper-resistant varieties of alfalfa, the economic threshold established from our research is three leafhoppers per inch of growth (30 leafhoppers for 10” tall alfalfa, for example). In areas having little moisture and the alfalfa is beginning to show stress, we recommend lowering the threshold. If your soils are turning quite dry and you find from 0.5 to 1 or more leafhoppers in 10 sweeps per inch of alfalfa seedling height, it would be prudent to apply an insecticide treatment. See the CORN newsletter from May 30 for more information on this insect.

Soybean Aphid – We are seeing aphids in most fields we are sampling in northern Ohio. Currently, they are in extremely low numbers. However, Michigan and Ontario, two areas to our north, are both seeing some fields with relatively high densities, well over 100 per plant. They are having to determine the need for spraying in those fields. Thus, Ohio growers in more northern areas are advised to check their fields so as not to be caught off-guard by high aphid populations.

Alfalfa Field Day – Defiance County, June 21st

Authors: Bruce Clevenger

OSU Extension is sponsoring an Alfalfa Field Day on Thursday, June 21, 2007 from 9:00 a.m. to 2:00 p.m. at the Andy Helmke Farm, 07730 Egler Road, Defiance, OH. Ohio and the U.S. hay stocks are reported down as of May 1, 2007 as compared to one year ago. Local demand for alfalfa by the livestock industry and mills has increased the interest in growing alfalfa as a cash crop.

The Alfalfa Field Day will feature presentations by industry and educational representatives.

Greg LaBarge, OSU Extension Educator, Fulton County will discuss the fertility and pH requirements for establishing alfalfa. Gary Wilson, OSU Extension Educator, Hancock County will discuss creating the best stand with variety selection and cultural practices.

Dr. Mark Sulc, OSU Extension Forage Specialist, will share his research and experiences on “why the stand declines?” It will include the cumulative stress load, insect and diseases, and evaluating winter injury risk.

Harvesting Quality Alfalfa will also be discussed by Dr. Sulc. Taking forage quality samples and field and storage factors that will influence quality will be discussed.

A panel discussion of a farmer, industry reps and an extension educator will discuss marketing alfalfa as a cash crop to area forage markets – dehydration mills, livestock producers, horses, etc.

17 companies and organizations are sponsoring the event with equipment demo and displays on-site.

The cost for the Alfalfa Field Day is $10.00 which includes lunch. Pesticide applicator credits will also be available to private and commercial license holders. The program offers CCA continuing education credits in crop management, nutrient management and pest management. Please RSVP by June 18, 2007.

Agenda and registration material can be found at: Questions and RSVP may be directed to the Defiance County OSU Extension office (419) 782-4771. The event is sponsored by the OSU Extension offices in Defiance, Fulton, Paulding, and Williams Counties.

Corn Ear Formation: Effects of Early Season Dry Weather

Authors: Peter Thomison

Many corn fields across Ohio are experiencing exceptionally dry conditions. Moisture stress during the early vegetative stages of corn development is atypical in Ohio. Some areas of the state have received less than a 1/2 inch of rain since mid-April.

Based on observations made at the end of last week, most corn across the state ranged from about V5 (the five leaf collar stage) to V7 or slightly beyond. Ear formation is probably well underway in fields at more the advanced stages of development. However, as early as the V4/V5 stage, ear shoot initiation is completed and the tassel is initiated on the top of the growing point. During the rapid phase of corn vegetative growth (which generally starts by V7), ear yield components are being determined. Kernel row numbers per ear are generally established by about V12.

Will the recent moisture stress impact ear formation and yield potential? It takes fairly severe stress conditions during the early vegetative growth stages to impact kernel row numbers per ear. Kernel row numbers are usually less affected by environmental conditions than by genetic background.

Therefore, in most cornfields, it’s unlikely that kernel row numbers have been impacted significantly by recent dry conditions. However, unlike kernel rows per ear, kernels per row can be strongly influenced by environmental conditions. Determination of kernels per row (ear length) is usually complete about one week before silking (R1) or about the V17 stage. Severe drought stress during the two weeks prior to pollination can reduce kernels per row and lead to a reduction in grain yield.

The lack in uniformity in crop emergence and development resulting from dry soil conditions (and soil crusting in some areas) is more like to affect yield potential adversely than ear formation. It’s not unusual to see fields that were planted in late April and early May with corn ranging in growth stage from V2 to V7 and plant height from 5 to 20 in.

Uneven Plant Height in Corn: Effects on Yield

Uneven height in corn stands is receiving considerable attention across the state. In many corn fields, it’s not unusual to see differences in plant growth stage and height within and between corn rows. Although much of the variability in plant height can be related to uneven emergence, it’s important to recognize that plant height is not a reliable indicator of plant growth stage in corn. In some fields that show variability in plant height, tall and short plants may actually be at fairly similar stages of growth based on leaf collars.

In 2007, the principle causes of delayed emergence and plant heights are probably dry soil moisture, and in some areas due to soil crusting from heavy rains shortly after planting, Other factors contributing to the problem include poor seed to soil contact due to cloddy soils, seeding depth, residue distribution, etc.

What impact will variability in plant height have on crop yields? It’s been well documented that uneven emergence affects crop performance because competition from larger, early emerging plants decreases the yield from smaller, later emerging plants. According to a popular rule of thumb, if two neighboring plants differ by two or more leaves, the younger plant will almost always be barren or produce a nubbin ear at maturity. The impact is less pronounced at lower population (below 24,000 plants/A).

In a 2006 article in the Iowa State University Integrated Crop Management Newsletter (on-line at, Roger Elmore and Lori Abendorf reviewed past research to determine how later emerging plants performed within a field of normal emerging corn. Figure 1 in this article includes findings from a number of research studies and shows percent yield loss when a certain percentage of the stand is late compared to the rest of the field.

The studies described in this article usually involved delaying the planting of a certain percentage of corn plants with a field to simulate variable emergence.

Research in Ontario indicated that when one of six (17%) plants was delayed in emergence by two leaves overall yield was reduced 4 percent; when delayed by four leaves, 8 percent yield losses were observed. Plants neighboring late emerging plants only partially offset yield losses.

Illinois and Wisconsin research considered the response of corn when 25, 50, or 75 percent of the plants were planted either 10 or 21 days after the original planting date. Overall, grain yields were reduced 6 to 7 percent by a delayed planting of 10 days regardless of the percentage of plants delayed. However, when planting was delayed 21 days, yields were reduced 10 percent when 25 percent of the plants were delayed, 20 percent when 50 percent were delayed, and 23 percent when 75 percent of the plants were delayed.

Regarding the prospects for late emerging corn, good weed control will be important to limit competition for moisture and nutrients to that from neighboring older plants. If we receive timely rains throughout the growing season, late emerging corn plants may still yield well if they are growing within rows in sufficient numbers or in “patches” that limit competition with neighboring older, taller plants.

Also, remember that the later emerging corn will be at a higher risk of silk clipping damage from Japanese beetles and corn rootworm beetles at pollination than the taller, earlier emerging corn. Fields with significant patches of late emerging corn should be monitored for these silk clipping insects at pollination.

Archive Issue Contributors: 

Anne Dorrance, Pierce Paul,and Dennis Mills (Plant Pathology), Ron Hammond and Bruce Eisley (Entomology), Peter Thomison (Corn Production), Mark Loux and Jeff Stachler (Weed Science), Robert Mullen (Soil Fertility). Extension Educators: Roger Bender (Shelby), Howard Siegrist (Licking), Glen Arnold (Putnam), Keith Diedrick (Wayne), Steve Prochaska (Crawford), Todd Mangen (Mercer), Alan Sundermeier (Wood), Gary Wilson (Hancock), Mark Koenig (Sandusky), Harold Watters (Champaign), Greg LaBarge (Fulton), Steve Foster (Darke), Steve Ruhl (Morrow), Wesley Haun (Logan), Ed Lentz (Seneca), Jonah Johnson (Clark) 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.