|
|
C.O.R.N
Newsletter
2004-11
April 27, 2004 -
May 4, 2004
Back
to main issue
|
 |
 |
|
Eight Years Later - What we’ve learned about managing weeds in Roundup Ready Soybeans
by
Mark Loux
|
|
Glyphosate may be the best herbicide ever developed, and the ability to use it in corn and soybeans has certainly changed our expectations of weed control. Especially in soybeans, Roundup Ready systems are among the most consistently effective when properly implemented. We hear much less about a number of weeds than we used to, including Canada thistle, hemp dogbane, common cocklebur, and several annual grasses (e.g. yellow foxtail) that were becoming more problematic in the mid-1990’s. On the flip side, a number of weeds can still be a problem in Roundup Ready systems, especially when producers decide that the flexibility of glyphosate allows them to throw basic weed management principles out the window. The result has been an increase in a number of weeds that can take advantage of poor management or a lack of diversity in herbicide use in Roundup Ready systems. Some examples are dandelion, marestail, lambsquarters, and winter annuals. So – what have we learned about glyphosate and weed management in Roundup Ready soybeans after eight years of use?
1) Weeds can and will develop resistance or reduced sensitivity to glyphosate. Use of glyphosate to the exclusion of other types of herbicides may be a primary cause of resistance, and rotation away from glyphosate for a year does not necessarily prevent resistance from occurring or adequately mitigate existing resistance problems. We have confirmed glyphosate resistance in marestail in Roundup Ready soybean fields following a year of corn. A common theme in these fields seems to be use of only glyphosate in the year the soybeans were grown.
2) When it comes to annual weed control, bigger is not better. Even given the flexibility of glyphosate and its ability to control large weeds at high rates, a number of issues arise when weeds are allowed to become too large before application. Problems with letting the weeds get too large include: A) possibly reduced control, especially if rates are not increased, which leads to a need for resprays and increases the risk of resistance; B) the stalks of several weed species (e.g. ragweeds, marestail, lambsquarters) can be infested with insects, possibly reducing herbicide effectiveness, and the potential for stalk damage increases the longer weeds remain in the field before herbicide application; C) reduced sensitivity of some lambsquarters populations to glyphosate appears to be expressed to a greater degree in larger plants (more than 4 to 6 inches tall); D) large weeds may prevent herbicide particles from reaching smaller weeds lower in the crop canopy; and last but not least, E) weeds that emerge with the crop can reduce yield when they are allowed to reach a size of more than 4 inches (corn) or 6 inches (soybeans).
3) Skipping the early-season burndown application can be a big mistake in no-till soybeans. Delaying the first herbicide application until mid to late May allows a number of weeds, including dandelion, chickweed, purple deadnettle, and annual bluegrass, to flower and produce seed. We have attributed the increase in these weeds across the state to the failure to apply early enough in spring to prevent seed production. This is based partly on our research in producers’ no-till fields in the early 1990’s, when an early-season application of 2,4-D ester plus glyphosate (or 2,4-D plus residual herbicides if applied early enough) was critical for the success of weed management programs. Most of the fields did not have a real problem with winter annuals or dandelions because seed production was prevented. In addition, glyphosate and 2,4-D are much less effective on dandelion after seed production, because movement of herbicide to the root decreases at this time.
4) There are still plenty of good reasons to use other herbicides in combination with glyphosate in Roundup Ready systems. A primary reason to integrate other herbicides is to reduce the overall dependence upon multiple glyphosate applications, thus delaying the development of herbicide resistance. In burndown treatments, the addition of 2,4-D to glyphosate can greatly improve control of dandelion, atriplex, marestail, and lambsquarters, among other weeds, and help mitigate the effects of cold weather or time of day on glyphosate activity. The addition of CanopyXL can improve control of dandelion, marestail, and other weeds, and FirstRate/Amplify can improve control of marestail and ragweeds. In postemergence glyphosate applications, the addition of Classic can improve marestail, morningglory, and dandelion control, and the addition of FirstRate/Amplify can improve marestail, morningglory, and giant ragweed control and provide some residual control of ragweeds.
5) There are still plenty of good reasons to use a residual herbicide, such as CanopyXL, Sencor, Valor, etc, in Roundup Ready soybeans, including the following: 1) seedling dandelion and marestail control; 2) early-season giant ragweed control/suppression to minimize the competitiveness of this extremely competitive weed during the first month of soybean growth and reduce the need for a second postemergence glyphosate application; 3) residual control of lambsquarters, which has developed reduced sensitivity to glyphosate in some fields; 4) minimizing weed/crop interference during the first month on soybean growth, so that unplanned delays in the postemergence glyphosate application do not result in crop yield loss; and 5) control of weeds during the first month or more to compensate for slow soybean development due to poor soil conditions or weather.
The bottom line for Roundup Ready soybeans: glyphosate is not a substitute for basic weed management skills, and there is no substitute for a multiple-pass, diverse herbicide program that creates flexibility in postemergence application windows, protects crop yield, and provides effective control and prevents seed production of winter annual, summer annual, and perennial weeds, reducing future weed populations.
|
|
Readers can subscribe electronically to this newsletter by signing up at http://agcrops.osu.edu/services/email.html. E-mail labarge.1@osu.edu
if you have problems subscribing or no longer wish to receive this newsletter.
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.
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.
All educational programs conducted by
Ohio State University Extension are available to clientele
on a nondiscriminatory basis without regard to race,
color, creed, religion, sexual orientation, national
origin, gender, age, disability or Vietnam-era veteran
status.
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.
TDD # 1 (800)
589-8292 (Ohio only) or (614) 292-1868
|
|
Email this story
