CORN
Crop Observation and Recommendation Network

May 26 - June 2, 2003
C.O.R.N. 2003-15

In This Issue:

A) Weed Control in Slow Growing Corn
B) Postemergence Glyphosate Application in Roundup Ready Soybeans
C) Staging Vegetative Growth in Corn: Early Season Considerations
D) Wheat Head Scab Risk Low
E) Barley Yellow Dwarf Symptoms in Wheat
F) East Central Ohio Crop Diagnostic Clinic

A) Weed Control in Slow Growing Corn - Loux CORN Questions

Slow crop growth creates problems in weed management programs. We expect the crop to be advanced enough in its development to be able to suppress late-emerging weeds within about 6 to 8 weeks after planting. Weed suppression by the crop is needed at this point, because: a) the residual activity of preemergence herbicides has declined, and b) postemergence herbicides applied about 4 weeks after planting generally do not provide much residual weed control. In many corn fields in Ohio, preemergence herbicides were applied 3 to 6 weeks ago and the corn is not anywhere near tall enough to shade out newly emerging weeds. Consequently, we are likely to begin seeing weed escapes in some of these fields, which could require postemergence herbicide treatments (or cultivation for anyone with the time and equipment to do so). Some suggestions for managing weeds under these conditions:

• The marketing of preemergence corn herbicides is extremely competitive, and one of the big selling points is often a warranty of performance. Per label language for many products, warranties can exclude environmental conditions that do not allow for normal corn development. However, companies are using herbicide warranties to buy your business, so be sure to hold the manufacturer to any promises, etc made at the time of purchase. These promises may have included supplying product or financial support for postemergence treatments where weeds escape preemergence treatments.
  
• Scout fields frequently to see if new weeds are emerging. Applying postemergence herbicides when these weeds are small will allow use of less expensive treatments and lower rates. For example, Basis is a relatively low-cost option for control of emerged grasses, but must be applied when grasses are no more than 2 inches tall (HOWEVER - see next bullet also!). Taller grasses will require Option, Steadfast, or another product that is more costly than Basis. Low rates of dicamba or another broadleaf herbicide can be used when broadleaf weeds are small to reduce costs.
  
• In spite of the suggestion in the previous bullet, we have some reservations about postemergence application of ALS inhibitors to corn that is small, yellow, and not actively growing. Corn that fits this description, combined with relatively cool weather, increases the risk of crop injury. Where possible, we suggest delaying postemergence application until after several days of warm weather, and after green color returns to the corn and it appears to be growing.
  
• Keep in mind that weeds emerging at the same time as the corn are more competitive than later-emerging weeds in corn fields treated with preemergence herbicides 3 to 6 weeks ago (this assumes "normal" corn development). With regard to weed interference, therefore, timing of postemergence herbicides is more critical in a total postemergence weed management program than in a preemergence plus postemergence program. Where the field was tilled prior to corn planting and no herbicide has been applied, postemergence herbicides should be applied before weeds (and especially grasses) are more than 3 to 4 inches tall, depending upon the product applied.
  

B) Postemergence Glyphosate Application in Roundup Ready Soybeans - Loux CORN Questions

Many of the comments about corn in the previous article could apply to soybeans, except that many Roundup Ready soybean fields have received no herbicide application or just a burndown treatment without residual activity. Soybean development is extremely slow, which can stretch herbicide programs to the point that they will have to provide weed control for up to 10 weeks after planting - difficult to accomplish with even two glyphosate applications. The general rule that we use for weed interference in soybeans is the following: assuming soybeans were planted into a weed-free field, weeds should be treated with herbicide before they exceed 6 to 8 inches in height to avoid yield loss from weed interference. There are some exceptions to this rule, most notably giant ragweed, which will be 8 to 12 inches tall when other weeds are 4 to 6 inches tall. In non-Roundup Ready fields, herbicides should be applied before most weeds exceed 4 to 6 inches in height to ensure herbicide effectiveness. In no-till fields not treated with burndown herbicides at the time of planting, established weeds can be competitive soon after planting, and can prevent establishment of a uniform soybean stand. In those Roundup Ready soybean fields planted a number of weeks ago, where soybeans are struggling but weeds are still growing, consider the following suggestions:

• Were herbicide(s) wth residual activity were applied earlier in spring or last fall, the residual activity will limit weed emergence and growth during this period of slow soybean growth. This can allow for a delay in postemergence glyphosate application, so that soybeans are larger and later-emerging weeds are controlled. In these fields, apply when weeds are small enough to be effectively controlled by the lowest glyphosate use rate (22 oz of WeatherMax or 32 oz of most other glyphosate products).
  
• In Roundup Ready fields where a burndown treatment without residual was used at or just prior to planting, apply when most weeds are less than 6 to 8 inches tall. This allows use of the rate specified in the previous bullet and prevents yield loss due to weed interference. Scout fields several weeks after the first postemergence glyphosate application to see if later-emerging weeds and slow soybean development require a second postemergence application. Alternatively, include herbicides with residual in the first postemergence application, which will reduce the need for a second postemergence application. Some examples: Extreme - premix of glyphosate plus Pursuit; Backdraft - premix of glyphosate plus Scepter; tank-mix of glyphosate plus FirstRate.
  
• In no-till Roundup Ready fields planted several weeks ago where no herbicides have been applied, we suggest application of glyphosate as soon as field conditions allow where dandelions and other established competitive weeds are present. Otherwise, make the first application when weeds are 6 to 8 inches tall and follow with another as needed. Herbicides with residual can be used per the previous bullet.
  
• Be sure to increase glyphosate rates when required by weed size. Controlling emerged weeds effectively with the first application of the appropriate glyphosate rate is a more effective strategy than depending upon a second application to finish killing weeds not completely controlled by an initial application of too low a rate.
  
• The statements about herbicide warranties in the previous article also apply here. If you purchased a more expensive glyphosate product due to the promises and/or warranty of performance, be sure to hold the manufacturer to their warranty. This applies to the possible need for a third glyphosate application in Roundup Ready fields where soybeans fail to develop rapidly enough.

C) Staging Vegetative Growth in Corn: Early Season Considerations - Thomison CORN Questions

In making an assessment of yield losses in corn due to defoliation and other types of plant injury, it is necessary to establish the stage of plant growth at the time when damage occurred. It's also important to know the stage of development in order to use postemergence herbicides effectively with minimum crop damage.

Several systems are currently used to stage vegetative growth in corn. The "leaf collar" system is probably the method most widely used by Extension and seed company agronomists in the Corn Belt. With this method, each leaf stage is defined according to the uppermost leaf whose leaf collar is visible. The first part of the collar that is visible is the back, which appears as a discolored line between the leaf blade and the leaf sheath. The oval shaped first leaf is a reference point for counting upward to the top visible leaf collar. This oval shaped leaf is counted as the number 1 leaf when staging. If a plant has 4 visible leaf collars, then it is defined as being at V4. Normally a plant at the V4 stage will have parts of the 5th and 6th leaves visible, but only four leaves with distinct collars. A field is defined as being at a given growth stage when at least 50% of the plants show collars.

Another widely used staging method is the "hail adjustor's horizontal leaf method" developed by the crop insurance industry. Rather than using the uppermost leaf collar, hail adjustors identify the uppermost leaf that is 40 to 50% exposed and whose tip points below the horizontal. Typically a given "horizontal leaf" growth stage based on the hail adjustor's method will be 1 to 2 leaf stages greater than the collar method. From growth stage V1 through about V5 there is typically one additional leaf (above that leaf with the last visible collar) whose leaf tip is pointing below the horizontal. Beyond growth stage V5, two or more additional leaves with 'droopy' leaf tips will be evident above the leaf with the last visible collar (so a V6 plant according to the leaf collar method will typically be a 8-leaf plant according to the hail adjustor's horizontal leaf method). One problem with the horizontal leaf method is that it is often difficult to identify the uppermost horizontal leaves in fields that have recently experienced severe leaf damage. Hail adjustors get around this problem because they usually assess hail damage 5 to 10 days after the storm, by which time 1 or more leaves have emerged from the whorl.

Corn leaf stage is a more reliable indicator of corn development than plant height. This is especially true in a cool, wet spring when corn is growing more slowly from a height standpoint. Differences in tillage and soil type may have a pronounced effect on plant height but relatively little effect on the stage of vegetative development. For example, corn may be taller in a conventionally-tilled field than in a neighboring no-till field planted on the same date, yet plants in both fields may be at nearly the same stage when counting leaf collars.

At about the V6 stage or 8-leaf stage of the hail adjustor's method, increasing stalk and nodal growth combine to tear the smallest lower leaves from the plant. Degeneration and eventual loss of the leaves results. Hail damage, insect feeding, and fertilizer/herbicide burning promote this process.

There may also be occasions when the lower leaves are hard to identify prior to the V6 stage. When extensive early season leaf damage has occurred, identification of the first rounded leaf and subsequent leaf collars may be difficult. This year, a number of factors, including strong winds, persistent rains, and periodic flooding/ponding, have contributed to loss of leaves used in staging corn growth.

Dr. Bob Nielsen at Purdue has described a method for predicting leaf stage development using accumulated heat unit or growing degree day (GDD) information. Given an understanding of corn leaf stage development and heat unit calculation, a grower can predict what leaf stage of development a particular field is at given its planting date and temperatures since planting. It is useful if you know when the crop emerged, but if not you can estimate that event also. Corn emergence typically requires 100 to 150 GDDs.

The Purdue research indicates that corn leaf developmental rates can be characterized by two phases. From emergence to V10 (ten visible leaf collars), leaf emergence occurs approximately every 85 GDDs. From V10 to tasseling, leaf emergence occurs more rapidly at approximately one leaf every 50 GDDs. Previously, about 60-65 GDDs were generally associated with the appearance of new leaf collars during vegetative growth.

Example: A field was planted on April 28, but you do not know exactly when it emerged. Since planting, approximately 785 GDDs have accumulated. If you assume that the crop emerged in about 125 GDDs, then the estimated leaf stage for the crop would be between V7 and V8. We arrived at this estimate by first subtracting 125 from 785 to account for emergence, then dividing the result (660) by 85 to equal 7.8.

Dr. Nielsen warns that these predictions of leaf stage development are only estimates. One of the factors that most influences the accuracy of these estimates is the existence of other growth-limiting stresses and conditions (nutrient deficiencies, compaction, etc.). Despite these potential drawbacks, this heat unit method may be useful in timing when plants will reach an approximate stage of growth. For more information on using heat units to predict vegetative development, check a recent article by Dr. Nielsen available on-line at http://www.agry.purdue.edu/ext/corn/news/articles.03/LeafStagePredict-0520.html

D) Wheat Head Scab Risk Low - P. Lipps and D. Mills CORN Questions

Most of the wheat in central Ohio has been in flower over the past week and cool temperatures are extending the flowering period through this week. Wheat in north west Ohio is still in the head emergence growth stage and will likely be flowering the end of this week and into next week. In spite of the rain, the cooler temperatures (mid 40 to mid 60 degree F) are lowering the risk of head scab. Optimum temperatures for infection of the wheat heads during flowering are in the mid 70 degree F range. Our scab forecasting models are predicting low to moderately low risk of scab for most of the state at this time. Hopefully the wheat crop will escape infection as we progress through this period. We will be updating the scab risk forecasts as the week progresses. Please visit our web site (http://www.oardc.ohio-state.edu/ohiofieldcropdisease) if you wish to see forecasts for locations in your area.

E) Barley Yellow Dwarf Symptoms in Wheat - P. Lipps CORN Questions

As the heading growth stage has progressed from south to north in Ohio we have seen the flag leaves of plants in some fields become yellow. The affected plants are scattered in the fields and generally only a very low percentage of the tillers are showing this symptom. This flag leaf yellowing is due to barely yellow dwarf virus that is spread by aphids. Due to the very late development of these symptoms we believe that the aphids present in early spring probably transmitted the virus during the warmer weather in early April. Additionally, we expect very little yield loss, if any, from these late infections. If you see yellow flag leaves in your wheat fields do not become alarmed and do not assume you need to spray the field for aphids. There is no reason to spray for aphids in wheat at this time unless you have very high numbers (100 or so aphids per foot of row). Once the wheat is past the flowering stage (anthers shed from the head) most aphids common in Ohio probably have very little impact on yield. Any further spread of the virus will not impact yield because there is too little time for the virus to replicate in the plants over the next couple of weeks.

Economic levels of barley yellow dwarf result when aphids inoculate the plants in the fall of the year. We usually see barley yellow dwarf on the earliest planted wheat, usually in fields planted before the Hessian Fly Safe Date. Infections in the fall permit sufficient time for the virus to replicate in the plant causing all leaves of the tillers to turn yellow. Frequently these diseased tillers are stunted and appear dwarfed. Infections that occur in the spring rarely cause significant problems unless a very high percentage of the tillers in the field are showing symptoms. Having symptom expressed in the flag leaf only is an indicator that the virus was introduced into the plant not long ago and there was not sufficient time for the plant to translocate the virus to other leaves.

F) East Central Ohio Crop Diagnostic Clinic - H. Siegrist CORN Questions

The focus for this year's East Central Ohio Crop Diagnostic Clinic is "Evaluating Different Strategies and Tools Critical to High Success in Crop Development." The clinic is scheduled for Wednesday, June 4 and will be held at the David Miller Farm in Millersport, Ohio. The cost for the event is $7.00 per person, payable that evening, to cover a light dinner and materials. Registration will be from 5:30 6:00 p.m. From 6 p.m. to 8:30 p.m., there will be concurrent sessions dealing with (1) Twin Row Corn Versus 30-Inch Rows, (2) Strip Till Corn Compared to No-Till and Conservation Tillage Corn, and (3) Assessing Four Different Seed Treatments in Side by Side Plots-Strengths and Limitations of Each.

Instructors for the in-field program are Peter Thomison, OSU Agronomist; Troy Putnam, Agronomist with Dupont-Pioneer; Ed Lentz, OSU Agronomist; Howard Siegrist, OSU Extension; Pat Lipps, OSU Plant Pathologist; and Bruce Eisely, OSU IPM Program.
At 8:30 p.m. a light dinner will be served followed by a presentation on Weed Management by Jeff Stachler, OSU Weed Extension Associate. The clinic will adjourn at 9:30 p.m.

CCA continuing education credits of C.M. 1.5, S. & W. .5 and P.M. .5 as well as pesticide applicator re-certification for commercial and private have been approved.
Reservations are not essential but encouraged by phoning any of the sponsoring OSU Extension offices: Licking County (740) 349-6900, Fairfield County (740) 653-5419, Perry County (740) 743-1602 or e-mailing Howard Siegrist at siegrist.1@osu.edu.

Readers can subscribe electronically to this newsletter by sending an e-mail message to: corn-out-on@postoffice.ag.ohio-state.edu. A successful subscription message will receive by an automatic reply from the listserv. Contact your local Ohio State University Extension Office or e-mail labarge.1@osu.edu if you have problems subscribing.

Past versions of C.O.R.N. can be found on the World Wide Web at: http:/www.ag.ohio-state.edu/~corn/archive/

C.O.R.N. is a summary of crop observations, related information, and appropriate recommendations for Ohio Crop Producers and Industry. C.O.R.N. is produced by the Ohio State University Extension Agronomy Team, State Specialists at The Ohio State University and Ohio Agricultural Research and Development Center. C.O.R.N. Questions are directed to State Specialists, Extension Associates, and Agents associated with Ohio State University Extension and the Ohio Agricultural Research and Development Center at The Ohio State University.

Editor: Steve Prochaska        Web Editor: Nathan Watermeier

Information presented above and where trade names are used, they are supplied with the understanding that no discrimination is intended and no endorsement by Ohio State University Extension is implied. Although every attempt is made to produce information that is complete, timely, and accurate, the pesticide user bears responsibility of consulting the pesticide label and adhering to those directions.

All educational programs conducted by Ohio State University Extension are available to clientele on a nondiscriminatory basis without regard to race, color, creed, religion, sexual orientation, national origin, gender, age, disability or Vietnam-era veteran status.

Issued in furtherance of Cooperative Extension work, Acts of May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture, Keith L. Smith, Director, Ohio State University Extension.

TDD # 1 (800) 589-8292 (Ohio only) or (614) 292-1868

  | C.O.R.N. | Newsletter | Archive | Search | Questions? | Ohioline | Publications |