Several Ontario agronomists recently shared their most successful strategies for managing glyphosate-resistant weeds. Four general strategies were identified:
1. Inclusion of winter wheat in the crop rotation
By early spring, winter wheat’s dense ground cover does not provide a good environment for annual weeds, like waterhemp, to germinate. There are also many herbicides that are effective at controlling glyphosate-resistant weeds in winter wheat. A large post-harvest window also gives farmers an opportunity to reduce weed seed production and attack persistent perennial weeds.
2. Work towards having soils with adequate fertility
Inadequate fertility levels can result in a less vigorous crop that is slower to close its canopy. This allows light and heat to reach the soil and stimulate weed seed germination. Attaining a base fertility level for phosphorus and potassium at 20 and 120 ppm, respectively, will put your crop at a competitive advantage.
3. Regular scouting
Scouting is a critical task to ensure optimum timing of herbicide applications. Waterhemp seedlings can grow as fast as 2.5 cm per day and control with post-emergent herbicides is reduced dramatically as seedlings get bigger.
4. Budget for and plan to use a “two-pass” herbicide program
With long windows of seedling emergence, most “one-pass” herbicide programs are inadequate at providing season-long control of many common weed problems.
Now let’s look at the effectiveness of herbicides. Several herbicide programs that provided consistent control of the following glyphosate-resistant weeds during the 2020 season were identified.
Canada fleabane (see photo at top): Easily controlled in winter wheat with Infinity FX, Pixxaro or Lontrel XC.
In corn, both Acuron and Integrity + Aatrex have provided very good control of Canada fleabane. Post-emergent applications of Lontrel XC or Shieldex + Aatrex have done a good job of controlling late-emerging seedlings.
In soybeans, the pre-plant application of Eragon LQ + Sencor has worked very well. Some agronomists have also had good results mixing Eragon LQ with 2,4-D ester or Elevore. Late-emerging seedlings are best controlled with herbicides that can only be used if planting E3 Enlist or Roundup Ready 2 Xtend soybean cultivars.
Giant ragweed (see photo above): Easily controlled in winter wheat with 2,4-D ester, Pixxaro, Trophy or Lontrel XC. In both corn and soybeans, most residual herbicides don’t provide season-long control, so followup post-emergent applications are needed. For example, in corn the use of a soil-applied herbicide like Acuron, followed up with a post-emergent application of Lontrel XC provides very good season-long control of giant ragweed. In soybeans, a pre-plant application of 2,4-D ester will control any emerged giant ragweed. However, late-emerging giant ragweed seedlings are best controlled with herbicides that can only be used if planting E3 Enlist or Roundup Ready 2 Xtend soybean cultivars. There are practically no effective options to control glyphosate-resistant giant ragweed when growing non-GMO and Roundup Ready soybeans.
Waterhemp (see photo below): There is generally less waterhemp found in winter wheat because it provides significant ground cover before peak germination of waterhemp. However, both Infinity FX and Pixxaro have provided very good control of waterhemp in winter wheat.
In corn both Acuron and Integrity + Aatrex have provided very good control of waterhemp.
Soil-applied herbicides such as Fierce EZ, Authority Supreme or Zidua SC have provided very good early-season control in soybeans. Late-emerging waterhemp seedlings are best controlled with herbicides that can only be used if planting E3 Enlist or Roundup Ready 2 Xtend soybean cultivars. Post-emergent control in non-GMO soybean and Roundup Ready soybean is less consistent. This is because Reflex and Ultra Blazer are the only herbicides that provide control of waterhemp, and plants resistant to both of those herbicides have been identified in many fields.