URBANA, Ill.-(UIUC)--Weeds! Until someone invents a purpose for them, we still have to fight them. If not, your beans will be starved for nutrients and moisture and have a dismal future. With plenty of rainfall this spring, the weed crop has taken over many fields and wet soils have delayed timely efforts to control them.
If the weather breaks, or if you are farming in the perfect world, Ohio State University weed specialist Mark Loux says once you have applied a post emergent herbicide, wait about 10 days, and scout your fields. In his latest newsletter Loux says at that time you will be able to determine what weeds were controlled, and which ones were either not controlled or will need a second herbicide application. He says do not always wait three weeks after the first application, because the weeds that escaped the first dose will be harder to control with a second dose.
Loux says the decision to make a second application is usually done with the help of a windshield and a speedometer reading 50 miles per hour, because that is when the weeds can be seen watching you above the canopy. Loux says the general belief is that many weeds which escaped an initial herbicide application can be controlled when a second is sprayed three weeks after the first. This allows enough time for recovery and limited regrowth. While some farmers think weeds will remain hidden below the canopy, Loux says that has not been a problem for researchers, whose spray droplets have found the weeds when small sizes make them easier to control. Consider also the timing, since new weeds are not likely to emerge after early July.
Your first application probably caught weeds that were reasonably small and had a fatal impact on them. Loux says the exception to the herbicide application three weeks later is when there was little or no impact the first time through the field because of resistance, and if that is the case, respray immediately. An early scouting after an initial application will indicate if the desired impact was achieved. He says research with glyphosate on giant ragweed has shown some to have resistance, and if that is your problem, an immediate respray with an alternative herbicide is warranted before they get too far out of control. If ragweed has been impacted by the glyphosate, but is still growing, a second application with glyphosate three weeks later will reach newly grown plant tissues.
Loux says a second application of the same herbicide will not always control weeds that escaped the first application. For ragweed with glyphosate resistance, he says an alternative may be required, such as FirstRate, Classic, Flexstar, or Cobra/Phoenix (although FirstRate or Classic can be used only if the population is known to still be sensitive to ALS inhibitors). He also says an oil-based adjuvant may be required to optimize the activity, since the surfactant in the glyphosate may be ineffective. Use the highest glyphosate rate possible or tank mix it with an alternative if in doubt about its effectiveness.
When glyphosate is tank mixed with an alternative, either in the first or second application, choose the alternative based on its potential impact on the weeds you have seen on your initial scouting of the field. Keep in mind that tank mixing glyphosate and a second herbicide may cause some minor leaf burn on soybeans, but there is little potential for yield loss. The greatest potential for yield loss will occur with late planted soybeans which could not grow out of the injury due to an extended dry period following the herbicide application.
Summary:
Following an initial post emergent herbicide application on soybeans, scout for its impact on weeds ten days later. That will guide your decisions on what to apply at a three week timetable following the first application. The second application may require the use of an alternative herbicide or a tank mix with glyposate and a crop oil adjuvant, if there are resistant weeds to an initial spray of just glyphosate.
(Source: Stu Ellis, http://www.farmgate.uiuc.edu)
