CORN
Crop Observation and Recommendation Network

April 24-30, 2000
C.O.R.N. 2000-10

In This Issue:

A) Planting Rates for Corn: How Much Over-Planting is Necessary?
B) Seed Treatments - When Will They Do The Most Good?
C) Delayed Planting Considerations for Weed Control- A Few Points to Consider
D) Will the Best Glyphosate Please Stand Up?
E) Is Powdery Mildew in Your Wheat: Should You Spray?
F) Rescue Options for Alfalfa Severely Damaged by Weevil
G) Products Labeled for Treatment of Alfalfa Weevil
H) Is Bermudagrass a Viable Forage Option in Ohio?

A) Planting Rates for Corn: How Much Over-Planting is Necessary? - Peter Thomison CORN Questions

When corn is produced for grain in Ohio, recommended plant populations at harvest (or final stand) can range from 20,000 to 30,000+ plants/A, depending on the hybrid and production environment. Populations for corn silage typically  exceed those for grain by 2,000 to 4,000 plants/A. Seed companies specify a range in final stands for the various corn hybrids they market, and these seeding rate guidelines should be followed closely to achieve optimal crop performance.

Yield potential of the production environment (i.e. soil productivity, etc.) is the primary factor determining hybrid yield response to increasing plant population. Seeding rate adjustments should be made on a field by field basis using the average yield potential of a site over a 3 to 5 year period as the major criterion for determining the appropriate plant population.

Higher seeding rates are recommended for sites with high yield potential which have high soil fertility levels and water holding capacity. On very productive soils which typically average yields of 160 bu/A or more (such as a drained Kokomo), final stands of 30,000 plants/A or more may be required to maximize yields. High plant populations are also warranted when irrigation is used to maximize crop performance. On soils averaging 120-150 bu/A, final stands of 26,000 plants/A may be needed to optimize yield. Lower seeding rates are preferable when droughty soils or late planting (after June 1) limit yield potential. On soils that average 120 bu/A or less, final stands of 20,000 - 22,000 plants/A may be adequate for optimal yields.

Final stands are always less than the number of seeds planted per acre. Cold, wet soil conditions, insects, diseases, cultivation and other adversities will reduce germination and emergence. Generally, you can expect from 10-20 percent fewer plants at harvest than seeds planted. To compensate for these losses, you need to plant more seed than the desired population at harvest. Many seed companies recommend over-planting by 10-15%.

To calculate your own planting rate, consider the following formula:

Planting rate = Desired population per Acre / (Germination  x  Expected Survival)

Germination is the percent germination shown on the seed tag (converted to decimal form). Expected survival is the percent of seedlings and plants that you expect to reach harvest maturity under normal conditions (converted to decimal form). Ninety percent survival (or 10% plant mortality) is about average. If you are planting very early when the soil will likely remain cool for several days following planting, you may want to reduce expected survival by 5 percent. A similar approach should be followed when planting no-till especially in heavy residues.

Example:

target final stand at harvest - 26,000 plants per acre
seed tag indicates 95 % seed germination
assume 90% survival (10% plant mortality)

Planting rate = 26000 / (.95  x  .90)  =  30,409 seeds per acre

The following table, based on the above formula, provides planting rates that compensate for reduced emergence and plant mortality ( 95% seed germination and 90% survival) during the growing season for a range of target final stands

* Rounded to nearest hundred

B) Seed Treatments - When Will They Do The Most Good? - Anne Dorrance CORN Questions

Soybean fungicide seed treatments may benefit producers the most for fields that have been in continuous soybean production, in no-till or reduced tillage systems, and poorly drained. Seed to be planted into fields  with a history of Phytophthora damping off or root rot should also be treated with the highest rates on the label of either Allegiance or Apron XL. In Ohio, P. sojae populations have developed that can cause disease on many of the resistance genes currently used in soybean varieties.

C) Delayed Planting Considerations for Weed Control - A Few Points To Consider - Mark Loux CORN Questions

As wet weather moves planting later, there will be a need to include glyphosate or Gramoxone in burndown programs at appropriate rates for the weed species and sizes. Be cautious about trying to control large weeds with products such as metribuzin, Canopy, Python, etc. These products are typically best-suited for control of relatively small annual weeds. Mixing them with 2,4-D does often result in broader-spectrum control, but as weeds become larger the use of Gramoxone or glyphosate will be necessary.

Some cool-season perennials such as quackgrass and Canada thistle are getting large enough that treatment with glyphosate can control the entire plant, rather than just the top growth. This is more likely for quackgrass, where a height of 6 to 8 inches may be sufficient. Canada thistle size is variable across the state, and preplant glyphosate is generally less effective at reducing populations compared to quackgrass. Another option than can be effective for reducing thistle popualations is preplant application of Hornet, which may be more effective for entire-plant control compared to glyphosate when plants are small.

D) Will the Best Glyphosate Please Stand Up? - Mark Loux CORN Questions

We have received questions about differences in control and crop safety with the various glyphosate products, partly because of possibly misleading advertisements. We have not conducted extensive testing of all of the glyphosate products available, but we can offer the following guidance: exclusive of Touchdown, the difference among glyphosate formulations is the surfactant package (and the other ingredients that allow it to made into a mixable and sprayable form). Roundup Ultra has what Monsanto considers to be their premium surfactant package. Likewise, Glyphomax Plus has a surfactant package not present in Glyphomax. We assume that manufacturers have a vested interest in formulating glyphosate with surfactants that optimize its activity. However, our limited field testing has not shown differences in the effectiveness of glyphosate formulations, especially with regard to postemergence applications under (typically) warm conditions with reasonably small weeds.

We have also not found significant differences in the yield of Roundup Ready soybeans among glyphosate products, and we have no idea where the data for herbicide advertisements claiming yield differences between Touchdown and Roundup comes from. Our Zeneca contact claimed ignorance of this also. The minor symptoms that Touchdown causes on soybean leaves do not reduce yield.

Most weed scientists in the region would probably agree that Touchdown activity appears to occur slightly more rapidly than other glyphosate products, but most would probably disagree that this results in differences in the final control observed.

The bottom line: when mixed or formulated with quality surfactants and properly used, there appears to be little difference between the glyphosate products available. However, we have not tested all products available, and it is possible that a product with a less than ideal formulation could be sold, resulting in reduced effectiveness.

E) Is Powdery Mildew in Your Wheat: Should You Spray? - Pat Lipps CORN Questions

Cool weather over the past week has slowed the development of the wheat crop somewhat. Wheat in southern Ohio will be in flag leaf emergence this week and some fields in northern Ohio will be in this growth stage by the end of this week. Flag leaf emergence is the best time to check for powdery mildew in the field. Flag leaf emergence is also the time when we see the greatest yield response from spraying a fungicide to control the disease. There are several factors that need to be considered before spraying a fungicide: 1) the susceptibility of the variety planted in the field, 2) the amount of disease in the field, 3) the predicted weather conditions for the next week and 4) the price of wheat.

The weather conditions over the past two weeks has been favorable for powdery mildew development and predictions for the next week or so indicates further mildew spread. Yield responses to fungicide control of powdery mildew are always greatest on susceptible varieties. Our current information lists the following varieties as susceptible:  Pontiac, Cardinal, Dynasty, Patterson, 529W, 539W,  558W, Classic RW151, Classic RW1517, Pioneer  2510, Pioneer 25R26, RS901, RS927, SG1545, SG1550, Benjamin, Podach, Reino SR204, TS5020, TS8040, W9830 and possibly others. Scouting the field is the only way to determine if disease is present and if the threshold has been reached. Scout each field by choosing 30-40 individual tillers throughout the field. Examine the leaves on each tiller to determine how far up the plant the disease is present. The threshold for powdery mildew is when the leaf below the flag leaf first becomes infected with 2 to 3 powdery mildew pustules. This threshold is good from flag leaf emergence to boot stage.

The major limitation to spraying some of these fields now is the price of wheat. With the market price below $2.00/ bu it is not likely that spraying a fungicide will be economic except on seed production fields. A yield loss prediction system that was developed for powdery mildew indicates that if the second leaf becomes infected before head emergence a 16% yield loss can be expected on a susceptible variety. The loss increases to 25% on susceptible varieties if the flag leaf becomes infected. On resistant varieties only a 5% to 8% loss is predicted, respectively.  In a field with a 70 bu/A yield potential, the loss on a susceptible variety will likely range from 11 to 18 bu/A. At $2.00 price for wheat the loss would be $22 to $36/A. Tilt (4.0 oz/A) is labeled for powdery mildew control. This material will cost from $9.50 to $12/A with application costs added to this price tag (total cost $15-$17/A). Economics of using a fungicide are much better on a susceptible variety than a resistant or moderately resistant variety.

F) Rescue Options for Alfalfa Severely Damaged by Weevil - Marc Sulc and Hal Willson CORN Questions

Reports of alfalfa weevil damage are variable across the state, and growers should be scouting all alfalfa fields on a regular basis now (see next article for more details). As temperatures warm, weevil damage can occur very rapidly and unpredictably. We have received reports of several fields that have already sustained considerable defoliation. The question is being asked "Should these fields be harvested now, or should a rescue insecticide treatment be applied"? Normally, weevil damage occurs much later when early harvest is an excellent control option. But at this early stage, a rescue insecticide treatment is probably the better option, even for heavily damaged fields in most cases (assuming weevil larvae numbers are still high). With the favorable moisture and temperature conditions, the alfalfa should be able to recover and make considerable new growth at this early stage. On heavily damaged fields that are "rescued", delay the normal first harvest date to allow more recovery time. A harvest now when the alfalfa is so young will result in extremely low yields, may weaken the stand, insecticide treatment on the re-growth may be needed anyway, and drying conditions are poor this time of year. Whether damage from weevil is evident now or not, continue to closely monitor alfalfa fields. Additional infestations may develop in the days and weeks to come, and damage can occur very quickly.

G) Products Labeled for Treatment of Alfalfa Weevil - Hal Willson CORN Questions

Alfalfa weevil is currently causing economic injury to alfalfa in most areas of the state. In most fields, weevil activity is above normal and occurring earlier than normal. As a result, the weevil has the jump on alfalfa growth and rescue treatments have been warranted in many locations.

A number of insecticide products are currently registered for use on alfalfa and most products labeled for alfalfa include a listing for alfalfa weevil. However, it should be noted that dimethoate products, often used for leafhopper control, may only provide suppression of weevil. In Ohio, we rarely conduct efficacy trials on alfalfa due to the fact that we rarely have much weevil activity at the Western Branch station. However, a review of reports from other states indicate that the new third generation synthetic pyrethroid products of Baythroid and Warrior will provide optimal control of weevil, and other products used over time like Ambush, Pounce, Furadan and Lorsban continue to provide effective control. In addition, a number of products like malathion, methoxychlor, Sevin and Lorsban continue to be available for use by the non-certified applicator. During the past year of reviews under FQPA, the use of Penncap-M on alfalfa was withdrawn.

Most insecticide products registered for use on alfalfa provide a choice in the rate of application that may be used in a treatment. It should be noted that the pre-harvest limitation (PHL) for products is often dependent on the rate of application.

To minimize adverse impact on beneficial parasites of alfalfa weevil, treatments should only be applied when justified by prior sampling of weevil abundance. Economic thresholds for weevil on alfalfa were given in the April 17th issue of this newsletter.

The following table provides a current listing of products currently available for use on alfalfa. The rates cited in the table apply to an entire complex of alfalfa pests. In regard to weevil control, the applicator should refer to the product label for the rate recommended for weevil control.

Other formations of dimethoate include Dimate 2.67, Dimate 4E, Dimethoate 267, and Dimethoate 400.

Rates and pre-harvest limitations of insecticides labeled for use on alfalfa. (Note: Read label for specific rates recommended for alfalfa weevil.)

H) Is Bermudagrass a Viable Forage Option in Ohio? - Marc Sulc CORN Questions

In the past few months, there has been renewed interest in bermudagrass for forage here in Ohio. I say "renewed interest", because the interest in bermudagrass has come and gone in Ohio over the past 50 years. The fact that the interest faded each time should be reason enough to raise a "red flag" for producers today. The recent interest has been fueled by advertising and the drought of 1999. Certain advertisements make bermudagrass sound like the wonder of all wonders, and a very new development. I encourage you to consider the following truths based on research and experience.

Bermudagrass is a warm-season perennial grass (where adapted) that is most common to the humid south, generally south of a line connecting the southern boundaries of Virginia and Kansas. Historically, bermudagrass has been found persisting (but not highly competitive) as far north as southern Ohio, Illinois, Missouri, and Kansas. Winter sensitivity has been its limitation. The turf specialists here at Ohio State University have evaluated it over the years, and have never been too excited about it since it winterkills almost completely in one out of every 5 years. Steve Barnhart, forage specialist at Iowa State University said, "I have inquired around the country this past Spring and have found that 'turf-type' bermudagrasses can be found growing in the St. Louis area, but to my knowledge, there are no long-term stands in Iowa."

Being a warm-season grass, bermudagrass will be productive only in warm months. Temperatures of 27 to 28 F kill stems and leaves back to the ground. It will grow on moderately well-drained soils provided it has adequate supply of moisture and plant nutrients. If very dry conditions prevail during May (as in 1999), bermudagrass likely will not be very productive because that is precisely when it begins growth and new root development for the season. It can tolerate flooding for several days, but it makes little if any growth on waterlogged soils.

The varieties being advertised must be established by sprigging. Sprigging cost is very high! Are you willing to pay over $250 per acre for sprigging a grass that may last five years at best?

Nutritive value of bermudagrass is highly dependent on the stage of growth when harvested and the environmental conditions. Late cut bermudagrass with low N fertility can be as low as 6-7% crude protein, but heavily N fertilized and frequently cut (every 2-3 weeks) bermudagrass can reach CP levels of 19 to 20%. To get high CP levels requires up to 400 lbs N/acre!

Keith Johnson, forage specialist at Purdue University recently wrote the following: "I evaluated bermudagrass 8 years ago. It didn't look too special at West Lafayette. I'd rather have Indiangrass or big bluestem be my warm-season grass in pasture rotation in Indiana. There are many ranchers in the southeast USA that use bermudagrass as their base forage and wish they could more effectively use alfalfa, red clover, ladino clover, orchardgrass, low-endophyte tall fescue, etc. Imagine that, we didn't know we had it so good!"

My advice to growers is to exercise great caution in considering bermudagrass for long-term forage production in Ohio. In my opinion, the summer annual grasses (pearl millet, sudangrass, sorghum x sudangrass, and corn) and other perennial warm-season grasses (indiangrass, big bluestem) are much more viable options over the long term for supplemental late summer forage production in Ohio.

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.

Contributors to C.O.R.N. this week include: State Specialists: Pat Lipps & Anne Dorrance (Plant Pathology), Mark Loux (Weed Science), Hal Willson (Entomology), Ron Hammond (Entomology), Mark Sulc (Forage Specialist); Extension Agents: Roger Bender (Shelby), Steve Bartels (Butler), Clark Hutson (Seneca), Barry Ward (Champaign), Dennis Baker (Darke), Gary Wilson (Hancock), Glenn Arnold (Putnam), Howard Siegrist (Licking), Bruce Clevenger (Defiance), Ray Wells (Ross) and Steve Prochaska (Crawford).

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.

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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.

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