http://www.ag.ohio-state.edu/~corn/

CORN
Crop Observation and Recommendation Network

April 2-9, 2001
C.O.R.N. 2001-7

In This Issue:

A) Newsletter Begins Weekly Schedule
B) Wheat Development Way Behind Last Year
C) Scout Emerging Soybeans For Bean Leaf Beetle
D) Bean Leaf Beetle And Soybean Viruses
E) Planter Inspection - A Key Step In Achieving "Picket Fence Stands"
F) Some Weedy Corrections
G) Is There A Marestail Problem In Your Future?
H) Strategies For Managing Marestail In Soybeans

A) Newsletter Begins Weekly Schedule - Greg La Barge CORN Questions

This issue of the CORN Newsletter begins our weekly update of current crop situations and recommendations. If you have questions, observations or general comments that you would like to see addressed in this newsletter you can reach us at the following e-mail address: corn-questions@postoffice.ag.ohio-state.edu

B) Wheat Development Way Behind Last Year - Pat Lipps CORN Questions

Cold weather during March has limited the spring regrowth of wheat throughout Ohio. According to our records the crops development is almost three weeks behind last year, but only about a week behind the 10-year average. Many fields look rough when viewed at a distance because of the cold injury to leaf tips. This injury is of no consequence to the wheat plant that has not begun regrowth. Some varieties look greener than other varieties at this time because certain varieties (less winter hardy varieties) broke winter dormancy earlier. Wheat producers should not be overly concerned with the lack of growth up to now, but to achieve high yields the crop must begin to grow quickly soon. Cold soil temperatures have limited tiller development this spring. Plants have the opportunity to produce additional tillers until the day length reaches about 14-15 hours of light per day and temperatures get much warmer. Unfortunately, continued cold weather will delay the heading date of the crop and may limit the grain filling period resulting in less yield potential. As usual, it all depends on the weather!

Wheat producers should begin to assess their fields as regrowth begins. Frost heaving has been a problem in some areas, especially shallow planted fields with high clay content and water drainage problems. Plants developing from seed planted too shallow are particularly prone to frost heaving. Frost heaved plants have their crowns pushed out of the ground exposing the base of the crown and roots. These plants will likely die as the soil dries and the plants attempt to regrow. There are also some fields with water-damaged areas that should be assessed for plant regrowth. Examine the plants by digging them up and separating the individual tillers. The crowns at the base of the tillers should be firm and white in color. Any dark yellow or brown coloration to the crowns indicate that the plants may stressed and unhealthy. Those that struggle to survive will likely produce only a couple of head bearing tillers.

The wheat plant can compensate for loss in plant stand to some extent at this time of year. A stand of 15 tillers per square foot is considered adequate for wheat (15 tillers/ foot of row on 7.5 inch row spacing), while 10 plants per square foot are minimal (9 tillers/foot of row on 7.5 inch row spacing). Probably more important is the uniformity of the stand. Large dead areas in the field lead to increased management problems (weeds) and reduced yield.

C) Scout Emerging Soybeans For Bean Leaf Beetle - Ron Hammond, Bruce Eisley CORN Questions

Growers should be prepared to monitor their soybean fields for the presence of abnormally high populations of bean leaf beetle. Although overwintered beetles usually do not reach economic levels in the spring, we saw many fields in 2000 with extremely high populations causing significant defoliation, stunting of the plants, and in some cases, stand loss. When defoliation reaches 50%, plants appear stunted, and stand loss appears imminent and insects are still active, insecticide treatment is recommended. Bean leaf beetles tend to be most numerous in early planted fields. Thus, growers who plant their soybean fields early, especially relative to other fields in their areas, should watch their fields closely.

D) Bean Leaf Beetle And Soybean Viruses - Ron Hammond, Anne Dorrance CORN Questions

There were many fields in Ohio last year that experienced symptoms associated with soybean viruses, especially bean pod mottle virus. Symptoms included "green stem syndrome" and crinkled leaves. The bean leaf beetle vectors this virus, especially in early season during feeding by the overwintering beetle. Bean pod mottle virus is one of several factors that can cause "green stem syndrome" in soybean. Whether bean pod mottle virus is the primary disease agent for green stem remains to be determined. To date, the evidence is circumstantial and based on correlations, but the trend is such that bean leaf beetle is thought to be the primary vector of this soybean virus. Researchers at the OARDC have established a research program to examine this situation in Ohio in the coming years and to explore potential management tactics.

E) Planter Inspection - A Key Step In Achieving "Picket Fence Stands" - Peter Thomison, Greg LaBarge CORN Questions

Uneven stands of corn are a common problem some years - often associated with fields where corn has been planted using reduced tillage. Tall and short corn, large within-row gaps, and groups of crowded plants characterize fields that exhibit stand uniformity problems. Lack of stand uniformity is a problem involving uneven seedling emergence and within-row spacing of plants.

Variable seedbed soil moisture associated with uneven crop residue distribution and weather conditions is a major factor causing uneven corn emergence, whereas high planting speeds (in excess of 6 mph) and poor planter maintenance/adjustment are primarily responsible for uneven within-row plant spacing.

Uneven plant stands characterized by large within-row gaps of 4 to 6 feet can reduce grain yields up to 5%. About 1/4 pound of potential yield can be lost for every ear in a tight grouping of two or more plants. Uneven emergence can also have a major impact on potential yield even if within-row spacing is relatively uniform. If 1/2 or more of a stand is delayed two weeks in emerging, yield losses of 5 to 8% can be expected. If 1/2 or more of a stand is delayed in emerging three weeks or later, yield losses of 20% or more can occur.

Corn sometimes emerges unevenly because of environmental conditions beyond the control of growers. However, timely planter servicing and adjustment, as well as appropriate management practices, can help prevent many stand uniformity problems. The following are some tips for improving the uniformity of seed placement during planting.

1. Avoid excessive tillage trips and tilling wet soils.
2. Check seed depth and seed soil contact periodically during planting.
3. Distribute residues over row areas evenly.
4. Plant at speeds between 4 1/2 to 5 1/2 mph.
5. With plate-type planters, match the seed grade with the correct planter plate.
6. Planters with finger pick-ups should be checked for wear on the back plate and brush, use a feeler gauge to check tension on the fingers, and then tighten them correctly.
7. Check for wear on double-disc openers and seed tubes.
8. Make sure the sprocket settings on the planter transmission are correct.
9. Check for worn chains, stiff chain links, and improper tire pressure.
10. Lubricate all chains and grease fittings.
11. Make sure seed drop tubes are clean and clear of any obstructions.
12. Clean seed tube sensors if you have a planter monitor.
13. Make sure coulters and disc openers are aligned properly.
14. With air planters, match the air pressure to the weight of the seed being planted.
15. Make sure press wheels are adjusted to close the seed slot.

A properly setup planter will help singulate and uniformly space seed to obtain a "picket fence" stand. Even stands reduce the plant-to-plant competition and take best advantage of sunlight to make corn grain. Doubles, triples or worse and large gaps can result in lost yield potential. Recent studies at Purdue University indicate that for each inch increase in standard deviation of plant spacing, yield lost is equal to 2.5 bushel per acre. A 1987-1996 study of stand observations from 354 corn fields in Ohio and Indiana, found that 84% of the fields had a standard deviation in plant to plant spacing of more than 4 inches or predicted yield loses of 5-12.5 bushels per acre.

Studies at University of Illinois concluded that skips contribute to the standard deviation or plant-to-plant variability slightly more than doubles but both increase plant spacing variability. They further concluded that skips reduce yield in fields where the intended population is at or below the optimum, while doubles increase yield when populations are less than optimum. One conclusion that can be drawn from this study is that population is the first variable that must be considered in planting time decision making. A second conclusion from this study is that standard deviation in plant spacing alone may not always predict yield loss. A third conclusion is that skips are much more limiting to yield than doubles.

F) Some Weedy Corrections - Mark Loux CORN Questions CORN Questions

In the 2001-6 newsletter article on winter annual weed control research, we listed the incorrect rate for Express. The rate used was 0.2 ounces/A, not 2 ounces/A. The labeled rate for fall or early spring applications is 0.17 to 0.33 ounces/A. Express must be applied at least 45 days before soybean planting, so it's utility at this time is limited.

There are also several corrections in the 2001 edition of the OSU weed control guide, with regard to effectiveness ratings for corn herbicides. We gave postemergence application of Hornet an effectiveness rating of 6 and 7 on ALS-resistant giant and common ragweeds, respectively. The clopyralid (Stinger) in the product actually provides effective control of ALS-resistant ragweeds, so this rating should be increased to 9 for both weeds. The rating for Harness Xtra on giant ragweed (ALS susceptible or resistant) should be 8, not 6, which would make it similar to Bicep II Magnum. Any products containing the equivalent of about 1.5 lbs active ingredient of atrazine per acre should have a rating of 8, and the rating decreases with the amount of atrazine.

G) Is There A Marestail Problem In Your Future? - Mark Loux and Jeff Stachler CORN Questions

Among the many weed problems that we try to manage, we consider marestail (horseweed) one of the most intriguing (some producers might use a "stronger" adjective than intriguing). Marestail has been a problem weed for several decades, although the severity of the problem seems to have been extremely variable from year to year. In 2000, marestail populations were high in many fields, and we seem to be detecting an increase in marestail populations in general over the past several years. This may reflect the recent overall increase in winter annual weeds, but we are concerned that marestail could be extremely problematic over the next decade. What is it about marestail that leads us to be concerned?

- Marestail can emerge over a much wider period of time compared to most other winter annuals, and is sometimes classified as a summer annual because of this. The 2000 growing season provided a good example of the plasticity in marestail emergence under certain environmental conditions. We observed marestail emergence from late summer of 1999 through early summer of 2000, and we attributed this long window of emergence to cooler and wetter than normal conditions in 2000. When marestail exhibits this type of emergence pattern, late fall or early spring herbicide applications will not necessarily provide adequate control. In fact, herbicides applied at planting may not control later-emerging plants, resulting in the need for a postemergence application.

- Marestail matures later in the summer compared to most other winter annuals. So, while most winter annuals have senesced around the time of planting to the point that they do not interfere with early soybean growth, marestail can interfere with soybeans for a month or more after planting especially if it emerges in late spring.

- We have confirmed a number of marestail populations to be resistant to ALS inhibitors, and suspect that the number of resistant populations will continue to increase. In the fields where we have confirmed resistance, selection seems to have been the result of preplant applications of Canopy products and postemergence applications of Synchrony, Classic, or FirstRate. These are among the most effective herbicides for control of marestail.

- Resistance to glyphosate has been confirmed in several marestail populations on the Eastern Shore. While we do not know of resistance to glyphosate in Ohio, the abundance of marestail and the intense use of glyphosate here could result in the development of resistant populations. Marestail exhibits some tolerance of glyphosate under cool conditions or when large, and our concern is that we will select over time for populations that can tolerate normal use rates.

- Soybean herbicide options for control of marestail populations that develop combined resistance to ALS inhibitors and glyphosate would be extremely limited. The most effective preplant herbicide program for control of these populations would consist of 2,4-D plus metribuzin plus paraquat. In addition, we would have no options for postemergence control of marestail in soybeans at this time (Liberty is effective on marestail, but Liberty Link soybeans are not a viable option at this time).

H) Strategies For Managing Marestail In Soybeans -Mark Loux and Jeff Stachler CORN Questions

In most years when marestail behaves as a true winter annual and emerges primarily in the fall, late-fall or early-spring application of 2,4-D ester is an effective strategy. A rate of 1 pint/A is sufficient when marestail are small, but this should be increased to 1 quart/A for control of large plants. Other effective treatments at this time include 2,4-D plus either Canopy, Python, metribuzin (Sencor, Domain, Boundary), or glyphosate.

Control of marestail within a few weeks before planting can be more difficult, partly because the 1 pint rate of 2,4-D ester that can be used at this time may be inadequate and glyphosate activity can be more variable. The most effective treatments at this time include Canopy, Python, or metribuzin plus 2,4-D - the addition of paraquat can improve control especially when using a metribuzin-containing product.

Where resistance to ALS inhibitors is suspected, we suggest you: 1) avoid use of Python- or Canopy-based treatments; and 2) strongly consider a mix of Sencor plus 2,4-D plus paraquat to ensure complete control. Glyphosate plus 2,4-D ester can also be effective for preplant control, but use the appropriate glyphosate rate based on marestail size and try to apply during periods of warm weather. This treatment will not control later-emerging plants, which can be a disadvantage compared to Canopy, Python, or Sencor in those years when marestail emerges after planting.

In a Roundup Ready soybean program, postemergence application of glyphosate should control later-emerging plants or plants that might survive preplant herbicide application. In non-Roundup Ready soybean fields, postemergence options include FirstRate, Classic, and Synchrony - no other postemergence soybean herbicides have adequate activity on marestail.

Two final thoughts on marestail management: where ALS-resistant marestail populations are suspected or have been confirmed, it is absolutely essential to: 1) plant Roundup Ready soybeans to have the option of a postemergence glyphosate treatment, or 2) make sure that complete control is achieved at planting with metribuzin plus 2,4-D plus paraquat - the metribuzin will also provide residual control.

Rotate herbicide site of action over and within years to reduce the risk of resistance to ALS inhibitors or glyphosate. We suggest use of ALS inhibitors or glyphosate as the primary weed control only once in three years. When soybean programs are dependent upon ALS inhibitors and glyphosate, consider use of atrazine, dicamba, Liberty, and other herbicide alternatives in corn instead of ALS inhibitors or glyphosate. A three-year crop rotation including wheat will also help reduce the rate of development of resistant populations.

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.

State Specialists: Pat Lipps & Anne Dorrance (Plant Pathology), Peter Thomison (Corn Production), Mark Loux (Weed Science), Jeff Stachler (Weed Science), Bruce Eisley (IPM) and Ron Hammond (Entomology) District Specialists: Ed Lentz (Agronomy) Extension Agents: Dave Jones (Allen), Clark Hutson (Seneca), Barry Ward (Champaign), Gary Wilson (Hancock), Greg La Barge (Fulton), Howard Siegrist (Licking), and Glen Arnold (Putnam)

Editor: Greg LaBarge Web Editor: Tom Rosati

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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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CORN Crop Observation and Recommendation Network