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Ohio State University Extension


C.O.R.N. Newsletter 2004-02

Dates Covered: 
January 22, 2004 - February 4, 2004
Greg La Barge

Managing Marestail I: Biology

Authors: Mark Loux

Marestail has become increasingly problematic in Ohio soybean fields over the past five years and starting to become more prevalent in corn fields. The ability of this weed to successfully infest soybeans appears to be due to a combination of factors, including its biology, its tendency to readily develop herbicide resistance, and current tillage and herbicide use practices. In this issue of C.O.R.N., we discuss these factors in more detail, and provide guidelines to effectively manage marestail and reduce the rate of development of herbicide resistance.

Marestail is more difficult to manage than many other winter weeds because of its emergence pattern and the duration of its life cycle. Marestail is classified as a winter or summer annual, because it can emerge throughout much of the year. Periods of peak emergence appear to be late summer/early fall and April/early May. In OSU research, we have observed considerable new emergence of marestail following early April herbicide applications, but very little new emergence following early May applications. Plants that emerge in late summer/early fall overwinter in the rosette stage – larger plants with elongated stems do not survive the winter. Plants that overwinter typically begin stem elongation in early May. Plants that emerge in the spring lag behind the overwintering plants in their development, but the warm weather in May allows the spring emergers to begin stem elongation sooner after emergence than overwintering plants (i.e spring emergers pass through the rosette stage more rapidly). Herbicides are less effective on plants with elongated stems, and marestail becomes increasingly difficult to control as it gets taller.

Unlike many winter weeds that flower and produce seed in May, marestail continues to grow vegetatively through mid-summer, so it directly interferes with corn and soybeans. In general marestail is a weak competitor in soybeans and corn compared to most broadleaf weed species, especially giant ragweed. However, marestail populations, especially those where herbicide resistance has developed, can become numerous enough to decrease soybean yield. In one Michigan State University field study, soybean yield was reduced by 83% due to the presence of marestail. Marestail can reach heights of 6 feet before growth ceases. Marestail plants flower and produce seed in late August through October, so plants that emerge in late summer/early fall can have a life cycle of one full year or more. A marestail plant can produce up to 230,000 seeds, which are wind-blown due to the presence of a pappus. Marestail seeds are smaller than dandelion seeds, and can easily move with wind at least 0.75 miles, but may travel farther.

Similar to many winter weeds, the seed of marestail are viable as they leave the plant, and do not need further conditioning in order to germinate. Seed produced in the early fall typically germinate within the next year or so. Marestail seed does not have much longevity in soil or on the surface, on the order of only 3 to 10 years or so. Marestail is primarily self-pollinated, but some out-crossing can occur. Considerable variation in a number of characteristics has been observed among marestail populations. This variation may be partly responsible for its tendency to readily develop herbicide resistance.

Managing Marestail II: Herbicide Resistance

Authors: Mark Loux

Over the past 23 years, resistance to many different herbicide sites of action has been observed worldwide in marestail, including triazines, paraquat, glyphosate, ALS-inhibitors, ureas, and amides. OSU research indicates that ALS resistance is widespread in Ohio marestail populations, especially in the western half of Ohio (where most of our samples have come from). Resistance to glyphosate appeared to be initially confined to four counties in southwest Ohio in 2002, but has now been confirmed in at least 13 new counties in the southwestern quarter of the state, as far north as Miami, Clark, Union, and Franklin Counties. We collected marestail seeds from 50 fields (primarily non-GMO soybean fields) in the fall of 2001. Over 80% of these populations were ALS-resistant in greenhouse research, but none were resistant to glyphosate. Another 15 populations collected from Clermont, Clinton, Highland, and Brown Counties in the fall of 2002 were glyphosate-resistant, but not ALS-resistant. We screened 35 populations from as far north as Marion County and as far east as Muskingum County in the fall of 2003, and 21 of these were glyphosate-resistant. In addition, at least one of the populations from 2003 has multiple resistance, to both glyphosate and ALS inhibitors. This population is from Montgomery County. Glyphosate-resistant populations from 2002 were nearly all from fields planted continuously with no-tillage Roundup Ready soybeans, whereas those from 2003 were also from no-tillage Roundup Ready soybeans grown in rotation with corn. A consistent factor for the glyphosate-resistant populations in both growing seasons is that glyphosate was the only herbicide used the year soybeans were grown.

What is the overall significance of herbicide resistance in the management of marestail? We view 2,4-D ester, glyphosate, chlorimuron (CanopyXL, Classic Synchrony), and cloransulam (FirstRate/Amplify) as the most important tools for control of emerged marestail, especially after stem elongation has started. Plants that are more than a few inches tall generally require a combination of two or more of these herbicides. CanopyXL and FirstRate/Amplify are two of the most effective herbicides for residual control of marestail that is not ALS-resistant. Glyphosate, Classic, Synchrony, and FirstRate/Amplify are the only herbicides for postemergence control of marestail in emerged soybeans. So, when marestail develops resistance to glyphosate or ALS inhibitors (such as CanopyXL and FirstRate/Amplify), options for control become limited. Populations with resistance to both types of herbicide could be extremely difficult to control, especially after stem elongation begins.

Managing Marestail III: Crop Rotation, Tillage, And Herbicides

Authors: Mark Loux

Since marestail is generally much less of a problem in crops other than soybeans and the seed is not long-lived in soils, crop rotation helps reduce marestail populations. Occasional tillage also reduces marestail populations. Marestail seed germinate from shallow soil depths, so burial of seed with tillage can reduce marestail populations. Preplant tillage will remove marestail that emerge the previous fall or earlier that spring. However, tall plants with well-established roots may be difficult to completely control with minimum tillage.

Herbicides are the most commonly used tools for marestail control, and the only option this spring in a no-tillage soybean field. The following sections contain information on spring marestail control in soybeans for six possible scenarios based on the approximate date and marestail size, and whether herbicides were applied last fall. These suggestions are based on the premise that ALS-resistant marestail are likely to be present in many fields, and that selection pressure for glyphosate resistance should be reduced (i.e. glyphosate should not be the only herbicide that is being used to control marestail). Keep in mind the following basic principles: 1) applications prior to early May should include a residual herbicide to control later-emerging plants; 2) marestail should always be controlled before it exceeds 4 to 6 inches tall; and 3) 2,4-D ester should be included in spring herbicide treatments. Where residual herbicides are specified, best choices in fields where marestail may be ALS-resistant (or if you are not sure): Canopy XL (3.5 oz or more), Valor, Gangster, or Sencor. FirstRate/Amplify or Python may be used in fields with marestail that are not ALS-resistant. The first two of the following six sections assume the field received an herbicide treatment last fall, while the last four assume that herbicides were not applied last fall.

Senario 1. Fields treated with only non-residual herbicide (glyphosate, 2,4-D) last fall. Fields should be free of overwintering marestail, but will be subject to spring emergence. Fields planted by early May should be treated just prior to or at planting with a residual herbicide plus either 2,4-D or glyphosate, or a combination of Sencor plus Gramoxone. Fields planted after early May should not require a residual herbicide for marestail control, but should be treated prior to or at planting with one of the following to control small, spring-emerging marestail: 2,4-D plus glyphosate; CanopyXL plus glyphosate; or FirstRate/Amplify plus glyphosate.

Senario 2. Fields treated with residual herbicides (CanopyXL, Sencor, Valor, Backdraft, etc) last fall. The residual herbicide should control marestail into May, and may control marestail into June. Burndown herbicides should not be needed in fields planted by mid-May, but small marestail may be present at the time of postemergence applications. Notable exceptions to this would be where a fall application of CanopyXL or Backdraft failed to provide residual control of ALS-resistant marestail into the spring, in which case the recommendations are the same as in 1 (above).

Scenario 3. Early-April application – marestail in the seedling or small rosette stage. Emerged plants should be adequately controlled by 2,4-D ester (0.5 lb ai/A should be adequate, but 1 lb ai/A can provide more consistent control of larger plants and improve dandelion control). Another option is Sencor plus Gramoxone. Include a residual herbicide for control of later-emerging marestail. Best choices for residual in fields where marestail may be ALS-resistant (or if you are not sure): Canopy XL (at least 3.5 oz/A), Valor, Gangster, or Sencor. FirstRate/Amplify or Python may be used in fields with marestail that are not ALS-resistant. This has been an extremely effective program for marestail control in OSU research.

Scenario 4. Mid to late April – marestail in seedling through large rosette stage. Similar to number 3, but emerged plants become more difficult to control. Emerged plants can be controlled with 2,4-D ester, but only at 1 lb ai/A (and only if the stem has not elongated). For lower rates of 2,4-D, combine with glyphosate. Sencor plus Gramoxone plus 2,4-D is also an option. Still need to include a residual herbicide for control of later-emerging marestail.

Scenario 5. Early to mid May – marestail stem elongated but not more than 4 to 6 inches tall. Most effective treatment is glyphosate plus 2,4-D ester plus either CanopyXL or FirstRate/Amplify. Glyphosate plus 2,4-D ester is effective where glyphosate resistance is not an issue, and glyphosate plus either CanopyXL or FirstRate/Amplify is effective where ALS resistance is not an issue. Use a glyphosate rate of at least 0.75 lb acid equivalent/A. Somewhat more variable but generally effective on this size plant is the combination of Sencor plus Gramoxone plus 2,4-D ester.

Scenario 6. Late May – large marestail with elongated stems (more than 8 inches tall). Pull out all the stops – the marestail are likely to be tall and difficult to control. Tillage is an option, but must be aggressive enough to kill large plants with well-established root systems. Anything less than a 3-way mixture of glyphosate plus 2,4-D ester plus CanopyXL or FirstRate/Amplify is not recommended. Use a glyphosate rate of at least 1.5 lbs acid equivalent/A. Resistance to glyphosate and/or ALS inhibitors can result in situations where effective control is not possible.

Applying Nitrogen On Frozen Ground For Wheat

Authors: Edwin Lentz

Many of us would like to apply N to wheat in January and February to save time and labor. In addition, it is easier to drive across a frozen field in February than waiting for proper soil conditions in March. However, there has been insufficient data to address the strengths and weaknesses of a January/February N program.

To address this issue, a three year research study at the OARDC Northwestern Research Station was completed in 2003. A single application of N (70 lb/A) was applied approximately three to four weeks before greenup and at greenup as urea, urea-ammonium nitrate solution (28%), and ammonium sulfate. Prior to the spring application, all treatments had received 30 lb/A of starter N applied in the fall. Actual pregreenup N application dates were 2/21/01, 2/14/02, and 3/11/03 and N applications for greenup treatments were 3/26/01, 3/11/02, and 4/02/03.

Across all N sources, yields from pregreenup applications were similar or slightly larger than N applied at greenup in 2001 and 2002. However, in 2003, pregreenup treatments yielded 19% less than greenup treatments, regardless of N source. Plant analysis confirmed that the 2003 pregreenup treatments had much less N in the upper leaves than the greenup treatments. These results suggest that the potential for N loss significantly increases with surface applications prior to greenup and may cause significant yield reductions. Also, this loss may still occur even with increased N rates or alternative N sources. Since we are unable to predict conditions from year to year, we would not recommend a single N application prior to greenup. The following website has more detail information about this research

Soybean Cyst Nematode Resistant Varieties

Authors: Nancy Taylor, Dennis Mills, Anne Dorrance

Soil samples for SCN testing submitted by Ohio growers during 2003 indicate that for the 464 fields sampled representing approximately 8,000 acres:

5% have very high levels of SCN (more than 5,000 eggs/200 cc),
5% have moderate levels (2000-5000 eggs),
30% have low levels (200 - 2000 eggs) and
60% have none to trace levels (0 to 200 eggs).

There are two things in this last series of results that raise concern. One is the low number of fields total sampled and secondly, the proportion that have very high levels of soybean cyst nematode present. We also know that there are at least 5 fields in the state where the resistance from PI88788 is no longer effective. We are amazed at the size of some of the SCN infestations that are beginning to really develop in this state. Circles the size of football fields, where the beans are knee high, mature earlier than the rest of the field and yields are hitting 20 to 50% of the long term average for a given field. Soybean cyst is really a wimp of a pathogen, it robs the plant of yield, doesn’t necessarily kill it or produce dramatic above ground symptoms – in fact – it is the rare field that has the knee high beans we described above. So it is an easy pathogen to forget. However, to keep fields profitable for the LONG TERM— it is economical to manage this pest and keep tabs on what these populations are doing.

Soybean cyst nematode is best managed with crop rotation. When a non-host crop is planted such as wheat or corn – then the cyst are actually starving. The eggs hatch and the juveniles cannot find food or a place to reproduce – their total populations decline. To ensure that this strategy works – the fields must be free of host plants. Unfortunately, SCN can reproduce on some weeds – including purple dead nettle – so these weeds must be managed as well.

When the SCN populations in a field range from 200 to 2000 eggs per cup of soil a soybean cyst resistant variety can be planted. A listing of SCN resistant varieties available for 2004 can be found at

DO NOT PLANT SCN RESISTANT varieties in fields with SCN populations greater than 2,000. In most varieties only one source of resistance is present. If this source is planted into fields with high populations, you will favor those nematodes that can reproduce on that source and the nematodes will build up in a very short period of time. The end result will be a field with a high SCN population with no resistant varieties available. Additional information is available

Answers To Questions From The Crop Profit Game

Authors: Jim Beuerlein

The Crop Profit Game was an interactive satellite program held at 38 Extension Office across the state on January 15th. The program featured Extension Specialist discussing current Agronomic concerns and issues. Some questions raised during the program were not answered and we will be sharing those in the next few issues.

Question: What soybean seeding rates are recommended by OSU and how much can this impact my cost of production?

The proper soybean seeding rate is a function of planting date, row spacing, soil type and variety maturity. The number of seed per acre increased with later planting, decreases for wider rows, decreases for later maturing varieties, and is higher for light colored soil than black soil. Typical seeding rates range from 130,000 seeds per acre for early planting of a full season variety in 30" rows on good black soil to 270,000 seeds per acre for a mid-season variety planted in 7" rows for double cropping. Typical seeding rates for a Group III variety planted the first three weeks of May in 7.5" rows would range from 175,000 to 225,000 depending on soil type and anticipated growing conditions. When growing conditions are good and plants grow fast and become large, maximum yields are produced with the lower seeding rates. When poor growing conditions exist, then the higher seeding rates are needed for maximum yield. The most profitable plant population is about15,000 fewer plants than where maximum yield occurs. It is usually more profitable to under-seed than over-seed.

A table with various in row spacing suggestions by planting date and maturity can be found at

More information on seeding rates and other production tips can be found at

Crop Management Workshops

Authors: Greg LaBarge

The 2004 Crop Management Workshops begin next week with the Corn Production and Management Workshops and followed by Soybean Production and Technology Workshops.


For more details

Tuesday, January 27, 2004
Delaware County, Ostrander
Scioto Twp. Hall
3737 Ostrander Rd.
Host Agent: Rob Leeds

Thursday, January 29, 2004
Miami County, Piqua
Upper Valley Applied Technology Center
Looney Rd.
Host Agent: Harold Watters

Friday, January 30, 2004
Pickaway County, Circleville
Pickaway Co. Public Library
1160 N. Court Street
Host Agent: Mike Estadt


For more details

Tuesday, February 3, 2004
Lorain County, Pittsfield
Pittsfield Twp. Hall
SR 303 & SR 58
Host Agent: Jim Skeeles

Thursday, February 5, 2004
Mercer County, Celina
Central Services Bldg.
220 W. Livingston St.
Host Agent: Todd Mangen

Advanced Crop Management Workshop

Authors: Edwin Lentz

Registration will be opened till the end of January for Advanced Crop Management Workshop for CCA’s and other Ag Professionals. Details can be found at or request a copy from the West District Extension Office (419/422-6106,

Workshop sessions will be taught by recognized Ohio specialists, and will allow the CCA to obtain 3 hours of soil and water management, 2 hours of nutrient management, 3 hours of crop management and 3 hours of integrated pest management. This workshop will be on the Ohio Agricultural and Research Development Center main campus in Wooster. Dates and times are February 11 (10:00-6:00 pm) through February 12 (8:00-noon).Cost of workshop will be $150 which includes educational materials for participants, food and refreshments.

Conservation Tillage & Technology Conference February 26 & 27

Plan now to attend the 15th Annual Conservation Tillage and Technology Conference scheduled February 26 & 27, 2004 at Ohio Northern University at Ada, Ohio.

This year's conference will cover a wide range of topics including soil fertility, planting and seed placement, tillage systems comparisons, soil density, root development, and weed, disease, and pest management. Over 65 sessions will be offered over the two day conference with nearly 70 different speakers including University Specialist, Ag Industry Representatives, and producer panels. Featured speakers include Francis Childs, record-setting corn grower; Dr. Bob Nielson, Prudue University Corn Specialist; Jim Kinsella, nationally known no-till/strip till producer; Bill Richards, former NRCS Director.

Nearly 30 credits wil be made available for CCA's. Early registration is $25.00 per day or $40.00 for both days. Registration after February 18th is $35.00 a day or $50.00 for both days. For a copy of the conference agenda and registration information contact the Hancock County Extension Office at 419-422-3851 or visit the web site at:

Archive Issue Contributors: 

State Specialist: Pat Lipps (Plant Pathology), Peter Thomison (Corn Production), Jim Beuerlein (Soybeans & Small Grain), Mark Loux (Weed Science), Jeff Stachler (Weed Science), Ron Hammond (Entomology), Ed Lentz (District Agronomist); Extension Agents: Barry Ward (Champaign), Todd Mangen (Mercer), Greg La Barge (Fulton), Mark Keonig (Sandusky), Harold Watters (Miami)& Dusty Sonnenberg (Henry).

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.