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Ohio State University Extension


C.O.R.N. Newsletter 2005-01

Dates Covered: 
January 11, 2005 - January 25, 2005
Harold Watters

Preserving the Value of Glyphosate

Authors: Mark Loux

The following article was co-authored by the following, all of whom conduct extension education in weed management in the Midwest: Bob Hartzler and Mike Owen, Iowa State University; Chris Boerboom, University of Wisconsin; Kevin Bradley, University of Missouri; Jeff Gunsolus, University of Minnesota; Bill Johnson, Purdue University; Jim Kells and Christy Sprague, Michigan State University; Mark Loux and Jeff Stachler, Ohio State University; Aaron Hager and Dawn Nordby, University of Illinois; Bryan Young, Southern Illinois University

It is well known that glyphosate-resistant horseweed (also known as marestail) populations have been selected in Roundup Ready soybean and cotton cropping systems. Resistance was first reported in Delaware in 2000, a mere 5 years after the introduction of Roundup Ready soybean. Since that initial report, glyphosate-resistant horseweed is now reported in 12 states and is estimated to affect 1.5 million acres in Tennessee alone.

A person could ask if this is any indication of what might lie ahead. On one hand, it has been proposed in a popular advertisement that glyphosate-resistant weeds are unlikely to occur if glyphosate is frequently used, as long as glyphosate is applied at full rates. The recommendations in this advertisement, in part, are based on several long-term university studies of Roundup Ready cropping systems. We feel a person should ask whether or not these studies can prove that resistance will or won’t happen. It is our belief that these studies are not large enough to test if resistance will develop. For example, the University of Wisconsin has a 7-year Roundup Ready cropping system trial. This trial has horseweed in the no-till plots. Despite burndown and in-crop treatments with glyphosate, glyphosate-resistant horseweed has not developed in this study. Since this trial did not find glyphosate-resistant horseweed, does this mean that glyphosate-resistant horseweed cannot develop? Does it mean that the resistant horseweed in Ohio or Tennessee or Delaware is not truly resistant? Obviously not. Small scale trials cannot prove that some event will not occur when a larger scale is considered. In total, these Roundup Ready cropping system trials may only be testing continuous glyphosate use on perhaps 50 acres, which is an extremely small area compared to the total acres relying on glyphosate as a primary management tool. The true trial to determine if a rare event like glyphosate-resistant weeds will develop is actually being tested on the tens of millions of acres of Roundup Ready corn, soybean, and cotton that are sprayed each year.

Many weed scientists across the Midwest have warned of the potential for additional glyphosate-resistant weeds if a “high selection pressure” is maintained. In this case, “high selection pressure” refers to the repeated use of glyphosate without interruption by herbicides with other modes of action or other weed management practices. This potential was confirmed at the recent North Central Weed Science Society Meeting where glyphosate-resistant common ragweed weed was reported. This is the first report of glyphosate-resistance in common ragweed. It was identified in a Missouri no-till soybean field that has been in continuous soybean production (with some double crop wheat) for many years and in Roundup Ready soybean since 1996. This field had a high selection pressure for glyphosate resistance with one or more glyphosate applications per year. This is the second example of a glyphosate-resistant weed developing in a Roundup Ready cropping system. We do not know which weed species will be the next to develop glyphosate resistance or when it will occur, but high selection pressure will likely result in additional cases of resistance.

Midwest weed scientists appreciate the value that glyphosate and Roundup Ready crops offer to growers. We hope that growers and crop advisors will evaluate how they use glyphosate and the Roundup Ready technologies to gain the value of these technologies without increasing the risk of resistance. Ideally, we recommend:

1) tank mixing glyphosate with another mode of action like 2,4-D in burndown treatments when glyphosate will be applied in the subsequent crop,
2) alternating glyphosate use with other herbicide modes of action between years, and
3) incorporating appropriate integrated weed management practices such as cultivation.

Lambsquarters Control Issues in Roundup Ready Soybeans

Authors: Mark Loux

Producers in Ohio have for several years reported problems controlling lambsquarters with postemergence glyphosate applications. We know that there is substantial variability among populations of lambsquarters with regard to its appearance and aspects of its biology, as well as its response to glyphosate. Environmental conditions and lambsquarters size appear to also influence its response to glyphosate in the field. As early as 2003, OSU weed scientists had characterized several lambsquarters populations as having “reduced sensitivity” to glyphosate based on the results of preliminary greenhouse research. We have collected additional biotypes since then, and we now characterize several of these as “glyphosate-resistant”.

The basis for labeling these as resistant is our ability to repeatedly demonstrate in the greenhouse the need for a higher rate of glyphosate to achieve the same level of response from a resistant biotype, compared to a sensitive biotype. Similar to other species with resistance to glyphosate, lambsquarters demonstrates a relatively low level of resistance, especially when compared to the high level of resistance exhibited by most ALS-resistant weed species. For the glyphosate-resistant lambsquarters, a glyphosate rate of two to four times the labeled rate (0.75 lbs glyphosate acid/A) must be applied to obtain the same response in resistant versus the sensitive biotypes. In the greenhouse, resistance tends to be expressed to a greater degree as lambsquarters increase in size. Smaller resistant plants are more easily controlled, although small plants may not be completely killed at four times the labeled rate. We conducted a field study in 2003 on the farm where one of the resistant biotypes was collected. Over a range of glyphosate rates, from 0.38 to 7.5 lbs of glyphosate acid/A, lambsquarters up to 6 inches tall were completely controlled, while plants 10 to 12 inches tall were not completely controlled in some areas of the field even at the highest rate.

Based on this research, it appears that some lambsquarters populations in Ohio are developing resistance to glyphosate. We expect no abatement of this trend, due to the selection pressure resulting from over-reliance on glyphosate in weed management programs. Several other factors probably influence the activity of glyphosate on lambsquarters in the field, including glyphosate rate, adjuvant use, lambsquarters size and age, and environmental conditions. Monsanto claims that they have occasionally had areas of poor glyphosate performance on lambsquarters, and they attribute this primarily to environmental conditions around the time of postemergence glyphosate applications.

We conducted a field study in 2004 to investigate the effect of glyphosate rate and formulation on control of a glyphosate-sensitive lambsquarters biotype. Treatments were applied to 2 to 4 inch lambsquarters immediately following the passage of a cold front, in an attempt to induce weather-related stress on the plants. The result was generally reduced glyphosate activity on lambsquarters, which increased our ability to differentiate between treatments. In one study, we observed increasing control of lambsquarters with increasing rate of Roundup WeatherMax, with 33 oz/A necessary to obtain 90% control. However, adding 6 oz/A of nonionic surfactant to 16 oz/A of Roundup OriginalMax achieved the same level of control. Otherwise, control with 16 to 22 oz/A of WeatherMax or OriginalMax did not exceed 78%. In a second study, we compared a number of glyphosate formulations at a glyphosate rate of 0.56 lb glyphosate acid/A. Touchdown Total controlled 90% of the lambsquarters, and control with other formulations ranged from 47 to 78%. No differences in giant ragweed control were observed among treatments in either experiment, and university weed scientists have typically observed few differences in weed control when comparing glyphosate formulation. Lambsquarters appears to be more sensitive to glyphosate formulation and surfactant rate, and possibly surfactant type, than other weed species. We used a spray volume of 20 gpa in both experiments, which is higher than the volume many applicators use for postemergence glyphosate applications. The higher spray volume has the effect of reducing the effective surfactant concentration in the spray solution. Our research in 2005 will be designed to determine whether the differences in control observed here would occur at lower spray volumes.

So, what’s the bottom line on all of this? It appears that, for a variety of reasons, we will continue to have problems controlling lambsquarters with glyphosate in some fields in Ohio. There appear to be issues with control arising from environmental conditions and application parameters, including the glyphosate formulation and the effective surfactant rate. We know that lambsquarters becomes more difficult to control with increasing size. We have also confirmed variability in response to glyphosate among lambsquarters populations, and have characterized several as glyphosate-resistant. We expect that the expression of resistance in the field would vary with all of the previously mentioned variables, but it is probably safe to assume that the effect of these other variables could be magnified when trying to control a resistant biotype.

The simplest and most effective method for avoiding problems with postemergence lambsquarters control may be to include an herbicide that provides residual control in preplant or preemergence burndown treatments. Many preplant soybean herbicides can provide adequate season-long lambsquarters control, including Valor, Gangster, Canopy XL/EX, Prowl, Scepter, Sencor, Python, and FirstRate. Suggestions for maximizing glyphosate activity on lambsquarters include the following (Note: increasing the rate of “loaded”glyphosate products results in an increased rate of glyphosate and surfactant):

- apply when lambsquarters are less than 6 inches tall;
- increase glyphosate rate to at least 1.1 lbs of glyphosate acid/A if plants are more than 6 inches tall;
- when lambsquarters are subject to stress from unfavorable environmental conditions, increase the glyphosate rate to at least 1.1 lbs/A and consider adding surfactant;
- consider use of additional surfactant when using a spray volume of more than 15 gpa;
- include 2,4-D ester with preplant glyphosate applications.

For information on weed management and product choices, the 2005 Weed Control Guide for Ohio and Indiana is now available at University Extension offices.

Cruiser Labeled on Soybean

Authors: Ron Hammond, Bruce Eisley

Syngenta recently announced that EPA granted Cruiser a label on soybean. The company is currently obtaining registration in Ohio. Cruiser will be labeled for some secondary soil pests, early season bean leaf beetle, and soybean aphid. Our experience with Cruiser has been mostly against seedcorn maggot, which Cruiser controls very well. However, growers should remember seedcorn maggot is only a potential problem when a green, living cover crop is incorporated into the soil in early spring. We have not observed any problems with this insect in other situations, including no-till systems.

Our experience with Cruiser for bean leaf beetle and soybean aphid control has been very limited. Over the past two years, we established numerous research plots with Cruiser for these two insects, but have not had sufficient insect populations to determine its ability to control either one. This helps to point out that most soybean fields probably do not need preventive protection with a seed treatment. Additionally, in the three plots established in 2004 where there was no insect pressure, no significant yield differences were obtained between Cruiser-treated plots and check or control plots. Although we are predicting problems with soybean aphid this year, IPM philosophy still calls for scouting for the insect, and taking curative action when thresholds are reached.

Now that Cruiser will have a label this spring, we intend to establish numerous plots throughout Ohio to examine its ability to manage bean leaf beetle and thus, possibly bean pod mottle virus, and soybean aphid. We need to explore its effectiveness in different situations, such as early plantings vs. late plantings, and in areas of the state where bean pod mottle virus is thought to occur. Perhaps in future years when more data are obtained, we might begin recommending preventive seed treatments in general. However, at this time, we would urge growers to continue with their regular pest management strategies of scouting and spraying when necessary. As more information is gathered, we will make it available through this C.O.R.N. newsletter.

Using Skip-Rows for Soybean Pest Management

Authors: Jim Beuerlein

There are several soybean pest problems that Ohio growers may have to deal with in 2005. The soybean aphid was a very serious pest problem in some Ohio soybean fields in 2001 and 2003, and is predicted to be serious again in 2005. Other problems include the bean leaf beetle, Japanese beetle, Mexican bean beetle, and possibly soybean rust. Most fields are planted in 7.5-inch rows and will need to be sprayed in late July or in August, well after the soybean canopy has closed which will cause a yield loss due to soybean plants being run down. Fortunately, most commercial sprayers have narrow tires so only two rows are run down as the sprayer crosses a field.

When forming skip rows it is important that they be the correct distance apart to accommodate the sprayer that will be used. Many sprayers are adjustable and can accommodate wheel spacings of eight to twelve feet. The ideal combination of drill and sprayer width is when the sprayer is three times as wide as the drill. With that combination the sprayer will use the 2nd, 5th, 8th, etc. pass of the drill when making applications. If the sprayer is either two or four times as wide as the drill, the first sprayer pass should be positioned to spray from the edge of the field to the center of a drill pass by disabling nozzles on the end of the sprayer. It will then be possible to use the skip rows for spraying the remainder of the field. Sprayers that are not full multiple widths of the drill will not be able to use skip row systems unless the sprayer size is either increased or decreased to meet that requirement. More information about the economics of skip rows can be found on the Internet at:

Annual Ohio Wheat Growers Meeting January 26

Authors: Harold Watters

The 2005 Ohio Wheat Growers Annual Meeting and Trade Show will be held at The Centre, 601 North Main St. Bluffton, Ohio on January 26, 2005 from 9:00 - 3:00. See the website for more information and directions. Doors open at 8:00 for Registration ($20). Topics to be covered include information on breeding for quality traits and identity preserved programs.

2004 Quadris/Warrior Strip Trials

Authors: Dennis Mills, Anne Dorrance

Numerous acres have been treated in Ohio with Quadris and Warrior over the past few years. This was based on reports that increases in yield were achieved in some fields when applications were made at the R3 growth stage. Historically, foliar diseases and for that matter insect feeding damage have been isolated problems in Ohio. Unlike more southern states, we rarely deal with stem anthracnose, Cercospora and only recently have we seen substantial amounts of brown spot. However, with the arrival of soybean aphid every other year, insecticide applications are becoming more routine. During 2004, we put this pesticide treatment to the test with a statewide study witch consisted of on-farm strip trials in 13 locations as well as 2 small plot studies.

Quadris/Warrior strip trials were coordinated by county extension agents where growers applied treatments in strips on their own fields with their own sprayers (40-90 ft wide), with each treatment replicated 3 to 5 times in each field. Treatments were Quadris alone, Warrior alone, Quadris plus Warrior and a non treated control. Measurements were taken on foliar disease, which was negligible at all locations and insect feeding, which was only measurable in 3 locations. Yields recorded ranged from 37.6 to 65.7 bu/A with two locations being significantly better for Quadris plus Warrior vs the control at 5.5 and 2.5 bu/A respectively. One of the reported effects of Quadris plus Warrior has been a delay in defoliation or “greening” of the variety ie: delayed maturity. This effect was only noted in the Wood County site.

Data from other states is also quite variable. Approximately 50% of the fields do see yield increases greater than 4 bu/A while the remaining were less than 4 bu/A or had a decrease in yield. What is going on? There are some indications that managing foliar and stem diseases may be playing a role in this response. In Ohio, this past year, the field with the largest increase (5.2 bu/A) also had the highest levels of insect feeding. More data is needed to verify these results but also to determine if we will see more differences under aphid pressure. The question we will try to answer this next year is does plant stress (from aphid feeding) play a role in this yield response.

Do other strobilurin fungicides have the same effect. In Ohio’s studies at two locations we had very similar results for both Quadris/Warrior and Headline/Warrior. So at this point there appear to be little differences, although another year may shed some different light on this.

How will soybean rust affect the timing of applications? Quadris is one of the fungicide choices to be used as a protectant to manage soybean rust. Timing for best management of soybean rust will readily become more of a focus than applying at a specific growth stage to reap a 50-50 shot at a yield response. If rust arrives in Ohio at late R5 – then that will be the time to put these treatments on. Well timed fungicide applications closest to the time of fungal spores reaching fields will see the greatest response for these protectants.

Table 1. Yields and difference in bu/A for nontreated control plots and Quadris/Warrior treatments in on-farm field trials in 13 locations in Ohio during 2004.






6.4 fl oz and

3.2 fl oz/A


Yield (bu/A)

LSD (P<0.05)























































































Table 2. Comparison of strobilurin materials with and without Warrior insecticide in small plot studies at OARDC research branches in Wooster and Hoytville, Ohio, 2004.


Treatment and rate/A

Yield bu/A


Yield bu/A





Warrior3.2 floz



Quadris6.4 floz



Quadris 6.4 fl oz + Warrior 3.2 floz



Headline12.0 floz



Headline12.0 fl oz + Warrior 3.2 fl oz.









Acknowledgement: Many thanks go to the soybean producers and county extension agents that assisted in this trial. These experiments can be tedious but we had 13 locations with excellent data to pull from. We look forward to working with them all again in 2005!

Archive Issue Contributors: 

State Specialists: Peter Thomison (Corn Production), Pat Lipps, Anne Dorrance and Dennis Mills (Plant Pathology), Ron Hammond and Bruce Eisley (Entomology), Jim Beuerlein (Soybean Production) and Mark Loux and Jeff Stachler (Weed Science). Extension Agents and Associates: Roger Bender (Shelby), Steve Foster (Darke), Gary Wilson (Hancock), Howard Siegrist (Licking) and Harold Watters (Miami).

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.