Big Weeds in Soybeans
  07/13/10 7:54:37 AM

Controlling large weeds in Roundup Ready soybean fields
Wet weather during much of May and June in eastern and central Kansas may have interfered with normal plans to apply burndown and residual herbicides in many soybean fields. Consequently, weeds in many fields may have gone uncontrolled earlier and are now growing rapidly with the warm, wet conditions. Controlling large weeds is often considerably more difficult than controlling smaller weeds. The following are some suggestions for controlling larger troublesome weeds in soybeans.
Marestail has become one of our most troublesome weeds in no-till crop production, especially in soybeans. Marestail tend to be difficult to control even when the plants are small and in the rosette stage, but become even tougher when plants get more than 6 inches tall. That is why fall and early burndown treatments are critical to the long-term management of marestail. Unfortunately, that doesn’t always happen. In addition, there are populations of marestail that have developed glyphosate resistance in many areas. However, some marestail populations are still susceptible to glyphosate, and even resistant plants are not completely immune to glyphosate.   
The most effective herbicide treatment for controlling marestail in Roundup Ready soybeans is probably a tank-mix of glyphosate plus FirstRate. The combination of the two herbicides seems to work better than either herbicide alone, even on resistant plants. It is important to use the full labeled rates of glyphosate and recommended adjuvants, including ammonium sulfate, to optimize control and help minimize the risk of developing more resistance. Other tank-mixes to consider with glyphosate for controlling marestail would include Classic and Synchrony herbicides. Ignite herbicide has also provided fairly good control of large marestail as a burndown treatment or postemergence application in Liberty Link soybeans. 
Velvetleaf has sometimes been difficult to control with glyphosate. There are no confirmed cases of glyphosate-resistant velvetleaf, but it is not extremely susceptible to glyphosate. Several application factors can affect control, including time of day, hard water, ammonium sulfate, and environmental conditions. Velvetleaf control with glyphosate can be optimized by using full rates of glyphosate and ammonium sulfate (17 lb/100 gal of spray), spraying during the daylight hours, and spraying when the plants are under minimal drought stress. Herbicide tank-mix partners with glyphosate that may enhance velvetleaf control would include Resource, Cadet, FirstRate, and Harmony. 
Waterhemp and Palmer amaranth
These pigweed species used to be some of the most common weeds in soybean fields prior to Roundup Ready soybeans. Glyphosate applied early, and possibly again as a follow-up treatment, has largely kept these weeds under control in recent years. However, we now have some glyphosate-resistant waterhemp in Kansas, and glyphosate-resistant Palmer amaranth has become a major problem in the southeast U.S. 
The best way to manage the pigweeds in soybeans is to use a preemergence herbicide followed by glyphosate. However, under very wet conditions like we’ve had this spring, the pigweeds may not have gotten controlled earlier and are now growing wild. Flexstar, Cobra, and Ultra Blazer can be fairly effective for controlling small pigweed, but are less effective as the pigweed gets larger, especially Palmer amaranth. These herbicides also provide some residual weed control, so tank-mixes of these herbicides with glyphosate should be applied within 3 to 4 weeks after planting to optimize performance. Pursuit and Harmony were once fairly effective for pigweed control and can still provide good control of susceptible populations, but many fields now have ALS-resistant waterhemp and Palmer amaranth.   
Sunflower and Cocklebur
Fortunately, sunflowers and cocklebur are quite susceptible to glyphosate. However, these weeds are fast growing and often have multiple flushes of germination. It is important to use the full rate of glyphosate and get good spray coverage when trying to control larger sunflower and cocklebur. Tank-mixing Scepter or Classic herbicide with glyphosate may improve control and help provide the residual control of later-emerging plants.
If weeds have gotten large because of wet conditions, it’s always best to start with the highest labeled rate of glyphosate, with the proper adjuvants, and add other herbicides as needed, depending on the weed species present. In most fields, there will be a combination of one of more of the weeds listed above, so producers will have to see how the herbicide options match up and select the best combination.
-- Dallas Peterson, Weed Management Specialist
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