Several years ago a dealer commented to me that he didn’t know why he would ever need to recommend metribuzin to a grower. I remember thinking that there was a need somewhere for almost every residual herbicide available to us, and that some would likely become more important due to herbicide resistance issues. We seem to have arrived at that point with metribuzin. This is of course not news to those readers who never stopped using it, but for the rest of us, this article hopefully provides the reason to use or recommend metribuzin in some soybean fields.
Metribuzin is a triazine herbicide and photosynthetic inhibitor that has been used in soybeans since the mid 1970’s, and it was a primary component of soybean herbicide programs prior to the introduction of ALS-inhibiting herbicides in the late 1980’s. Sold by Bayer under the name Sencor for several decades (but no longer), generic metribuzin is currently available from several companies (e.g. Metri DF, Tricor, Dimetric). Metribuzin is a component of several premix products also, including Canopy/Cloak DF, Boundary, Matador, Intimidator, and Authority MTZ.
Metribuzin is one of the few residual herbicides that also have substantial burndown activity, primarily through non-systemic activity on small annual weeds. While this activity by itself is usually not adequate for control of emerged weeds, combining metribuzin with other burndown herbicides can improve the overall effectiveness of the burndown. Metribuzin’s contribution is maximized by mixing it with other burndown herbicides that work via contact activity (non-systemic), such as Liberty, Gramoxone, and Sharpen. This can be an advantage that metribuzin has over the two other herbicides that are also widely used for residual control of marestail, flumioxazin (Valor) and sulfentrazone (Authority/Spartan), which do not provide any control of emerged weeds and have some potential to antagonize the activity of systemic herbicides.
The spectrum of residual control for metribuzin includes most small-seeded annual broadleaf weeds. It is most effective on lambsquarters, pigweeds, Pennsylvania smartweed, ladysthumb, marestail, and waterhemp, but also has some activity on common ragweed, velvetleaf, and annual grasses. Metribuzin does not control weeds that are resistant to triazine herbicides of course. We consider the rate of 0.28 lb ai/A (6 oz of 75DF) to be about the minimum for effective control of any annual weeds, and we have observed more effective control with increasing rate between 0.28 and 0.56 lbs/A. Where the goal is to attain a certain metribuzin rate by combining it with another premix that also contains metribuzin, it’s obviously important to know how much metribuzin the other product contains. This information can be found in Table 18 of the “2013 Weed Control Guide for Ohio and Indiana”. For example, the 4 oz rate of Canopy DF contains 0.16 lb ai of metribuzin, so reaching a metribuzin rate of 0.47 lbs ai would require the addition of another 0.31 lbs, or 6.6 oz of a metribuzin 75DF product.
Metribuzin is only moderately persistent in soil, so where it is applied a few weeks or more ahead of planting, increasing the rate or making two applications can improve the longevity of control. The rate used also depends upon whether metribuzin is being applied with other herbicides that have substantial activity on the weeds of interest. So while we suggest that rates of 0.47 to 0.56 lbs ai/A could be the most effective for residual marestail control in the absence of other residual herbicides, a rate of 0.28 lb ai/A could be adequate to improve the control when applied with Valor XLT, Authority First, Sonic, etc.
The table in the metribuzin label that shows application rates based on soil texture and organic matter content has more detail than similar tables on many other herbicide labels. Metribuzin can occasionally injure soybeans and it is important to follow the rate guidelines on the label to minimize the risk of injury. Injury is more likely where soil pH is 7.5 or higher, and some soybean varieties can be sensitive to metribuzin. Check with the seed company for more information, although many seed companies no longer screen their soybean varieties for metribuzin sensitivity. The risk of injury may be lower than it was 30 years ago when higher rates of atrazine were being used in the preceding year’s corn crop. In addition, applying several weeks or more before planting in no-till probably reduces the risk of injury compared with application at or after planting.
The two major components of a marestail management program in no-till soybeans are 1) ensuring that the existing emerged marestail are controlled prior to soybean emergence, and 2) using residual herbicides to control later-emerging marestail for another 6 to 8 weeks after planting. There are several strategies that can be implemented to achieve this, and more information can be found in the OSU/Purdue fact sheet, “Control of Marestail in No-till Soybeans” (https://agcrops.osu.edu/specialists/weeds). The purpose of this article is to provide more in-depth information and rationale to help in the selection of residual herbicides. The variable emergence pattern of marestail, and it’s tendency to emerge late in the season, seem to mean that there are no completely “bulletproof” marestail management programs. Not all residual herbicides are effective for marestail control, so it’s possible to inadvertently use or recommend residual herbicides that ultimately make the program even less “bulletproof”. The bottom line in marestail management is that we are trying to avoid having to control it with postemergence herbicides, since they largely do not work (exception – Liberty in LL soybeans).
We have a number of residual herbicides that are relatively effective for control of marestail, but the presence of ALS resistance in many marestail populations reduces the number of viable options. When selecting residual herbicides, most effective control will generally result from: 1) assuming that the population is ALS-resistant, and 2) making sure that the residual herbicide treatment contains herbicide(s) that are effective on ALS-resistant populations, and are used at high enough rates. Table 10 in the “2013 Weed Control Guide for Ohio and Indiana”, which shows residual herbicide effectiveness, has ratings for both ALS-sensitive and ALS-resistant marestail.
To go into more detail - the ALS inhibitors cloransulam (FirstRate) and chlorimuron (Classic) are among the most effective herbicides for control of marestail, and their activity contributes greatly to the effectiveness of various premix products of which they are a component. For chlorimuron, these are Canopy/Cloak, Valor XLT, Envive/Enlite, and Authority XL, and for cloransulam they include Gangster, Authority First, and Sonic. However, the chlorimuron or cloransulam is not effective on ALS-resistant marestail populations, so in that case the second component of these premixes carries the load for control - the sulfentrazone (Authority/Spartan, flumioxazin (Valor), or metribuzin. The increasing frequency of ALS resistance in marestail, combined with the lower rates that are commonly used for these premix products, is partly responsible for the variability in residual control that we hear about. Another reason for this is that chlorimuron and cloransulam have a longer period of activity in soil than the other three herbicides. We have observed more variable control of marestail in our research when relying on just the single non-ALS component of these premix products.
The application rate and timing of application becomes more of a factor in ALS-resistant populations, along with the overall complexity of the residual herbicide treatment. Applying earlier in spring, or using the lower “Roundup Ready rates” of products can reduce the residual activity in late May and June when marestail is still emerging. Metribuzin, sulfentrazone, and flumioxazin seem to be relatively similar in their residual activity on marestail. We conducted a study in 2012 where metribuzin greatly outperformed the other two herbicides, and that has led us to recommend that growers consider adding at least some metribuzin to treatments that contain sulfentrazone or flumioxazin (see accompanying article on metribuzin in this C.O.R.N. issue). The addition of metribuzin can improve burndown and the use of two herbicides with activity on marestail may also result in more consistent residual control. In an attempt to boil all of this down to a few key points, we offer the following suggestions:
- use a residual herbicide treatment that includes metribuzin, sulfentrazone, or flumioxazin. When using a premix that contains one of these, increase rates above the base “Roundup Ready” rates in order to provide more of these components. Consider adding some metribuzin to products that contain sulfentrazone or flumioxazin.
- products that contain flumioxazin: Valor, Valor XLT, Envive, Enlite, and Gangster. It is possible to use lower rates of the premix products, but add more Valor in order to improve marestail control.
- products that contain sulfentrazone: Authority XL, Sonic, Authority First, Authority Broadleaf, and Spartan. It is possible to use a low rate of one of these and add more Spartan to increase the sulfentrazone, but this may not be cost-effective compared with just increasing the premix rate.
- when using metribuzin as the primary residual for marestail, increasing rates can improve burndown and the longevity of control. We suggest aiming for rates of 10 to 12 oz of 75DF where soil type allows. Consult the labels for information on rate based on soil texture and organic matter.
- products that contain at least some metribuzin but need more added to be consistently effective: Canopy/Cloak DF, Matador, Intimidator, Boundary.
- it is possible to split the spring residual herbicide treatment into two applications – one in early spring where about half of the residual herbicide is applied with burndown, followed by the rest at the time of soybean planting with additional burndown as needed. This may help maintain control later in the season, compared with applying everything in early spring.
- products that are ALS-inhibitors and may be effective in ALS-sensitive populations: Python, Canopy/Cloak DF and EX, and FirstRate. We don’t recommend this approach. Don’t guess wrong about whether a population is ALS-resistant.
- products that we do not consider effective for residual control of marestail: Scepter, Prefix, Pursuit, Optill, Optill Pro, Sharpen, Outlook, Verdict, metolachlor, pendimethalin, and Warrant. Sharpen can contribute some residual but is generally not adequate unless mixed with another effective herbicide. It is possible to add a full rate of metribuzin to any of these to create a mix that works.
- Labels no longer allow mixtures of Sharpen with products that contain flumioxazin or fomesafen (Prefix, Intimidator), and we do not expect this to change between now and the planting season. Sharpen can be mixed with Authority products (see label for rates based on application timing), along with metribuzin and the other products mentioned in this article.
The annual Northwest Corn/Soybean Day program is scheduled for January 24th at Sauder Farm and Craft Village’s Founders Hall from 8:30 a.m. to 3:30 p.m. (Optional CORE & Category 6 sessions for pesticide applicator credits are available from 3:45 to 5:15 pm). The program has a variety of speakers and 28 exhibitors sharing quality information on management practices for the 2013 crop production season.
The lead off speaker will be Dr. Emerson Nafziger (University of Illinois Extension) who will be discussing Managing Corn Following a Drought. Harold Watters, OSU Extension Field Agronomist, will be discussing Weed Disasters: The Top 10 Ways to Avoid Being a Statistic. The Soybean Health – Insect/Disease Update will be discussed by Dr. Anne Dorrance, Soybean Specialist from OARDC. The last keynote speaker will be Greg LaBarge, OSU Extension Field Agronomist, discussing Soil Factors for High Yields.
In addition, exhibitors from seed and input suppliers, banking, crop insurance, grain marketing and the machinery industry will be on site to share information about products and programs. A light breakfast and full lunch will be provided with registration. Additionally, continuing education credits for pesticide applicators are offered throughout the day:
*Private: 1 hour CORE, 2 hours Category 1, ½ hour Category 6
*Commercial: 1 hour 2A, 1 hour 2C
*Michigan: 4 hours total credit
*Certified Crop Advisors: 4 hours total credit
Pre-registration is $25 and is requested by January 16th. At the door registrations are $45 and available on a limited basis. A more detailed agenda and registration information can be found at http://fulton.osu.edu. Contact Eric Richer, Extension Educator, Agriculture & Natural Resources 419-337-9210 or firstname.lastname@example.org for more information.
February 14, 2013 is this year’s date for the Northern Ohio Crops Day to be held at Ole Zim’s Wagon Shed, 1375 N. State Route 590, Gibsonburg, Ohio. Featured on this program will be: “Pesticide Safety, Reminders on Common Situations” “Wood Lots, Now what after the Emerald Ash Borer” “Weed Control Strategy” “Tools in Soybean Production” and “ Ponds and Water Issues”. These are topics that have been suggested by area producers. This year’s presenters are new to Northern Ohio Crops Day. We are very fortunate to have our new soybean and small grain specialist, Laura Lindsey, presenting at this year’s program.
Program has been approved for Private and Commercial pesticide recertification credit. Participants can obtain all private recertification credits, and commercial credits are 1 hour each in Core, 2D plus ½ hour in 2C, 6A, and 3A. We have also received 4 CEU’s for Certified Crops Advisers. Due to support from our Ag sponsors pesticide recertification credits for private applicators is $30. Commercial credit will again be $15 per hour.
The meeting starts at 9:00 a.m. and continues until 3:00 p.m. A $10.00 donation will be accepted at the door to help with expenses that includes a copy of the 2013 Ohio & Indiana Weed Control Guide publication which has been updated this year.
Lunch will be provided courtesy of the Northern Ohio Crops Day Exhibitors. The program is a joint effort of Erie Basin EERA Ohio State University Extension. Please call Sandusky County office (419) 334-6340 or e-mail email@example.com with any questions or special needs.
Fischer Auditorium, OARDC Campus, Wooster, OH
Topics that will be covered include: The 9 Major Points of a 2013 Weed Control Program, Real World Application of the 4 R’s, Modified Relay Intercropping, and The ABC’s of Cover Crop Use.
Conesville United Methodist Church, 195 State Street, Conesville, OH 43811
Weed Control for soybeans and nutrient management will be featured topics.
Fairfield County Ag Center, Lancaster, OH
Cover crops, nutrient management are highlighted topics
OSU Extension Address, 503 Fairground Dr, Paulding, OH 45879
Soil Quality, Soybean production, weed control and growing corn after a drought are higlighted topics.
Fayette County Agriculture Service Center, Washington Court House, Ohio 43160
Variable Rate Planting in Corn, Soybean Genetics vs. Management, Cover Crop Update, Weed Resistance Update, and Nutrient Management.
Kalida K of C Hall. Kalida, OH
The annual Putnam County OSU Extension Agronomy night is scheduled for Thursday, January 26 at the Kalida K of C Hall starting at 6:30 p.m. Once again Agronomy Night will have a wide variety of topics presented by Extension specialists and local industry people.
Robert Fulton Agricultural Center, 8770 State Route 108, Wauseon OH 43567
Controlled drainage management, tiling system layout and design, economics and nutrient management.
Dayton Convention Center, Dayton, Ohio
Complete requirements for Ohio commercial pesticide recertification in one day. Speakers include Ohio State University Extension state specialists, field specialists, and researchers. Ohio Department of Agriculture personnel and key industry leaders are also on program.
- Glen Arnold (Nutrient Management Field Specialist),
- Debbie Brown (Shelby),
- Ron Hammond (Entomology),
- Rory Lewandowski (Wayne),
- Nathan Douridas (FSR Farm Manager),
- Mike Gastier (Huron),
- Andy Michel (Entomology),
- Steve Prochaska (Agronomy Field Specialist),
- Greg LaBarge (Agronomy Field Specialist),
- Pierce Paul (Plant Pathology),
- Ed Lentz (Hancock)