Controlling annual weeds postemergence in Roundup Ready soybeans is always easier when the weeds are small – less than 2 inches tall is preferable for good control. Once weeds get taller, they are often considerably more difficult to control. However, conditions are not always conducive to getting optimal postemergence weed control. The wet weather in many areas this spring may cause weeds in some fields to get larger than you intended. The following are some suggestions for controlling larger troublesome weeds in soybeans.
Marestail
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.
Figure 1. Growth stages of marestail from seedling, rosette, to bolting state. Photos by Dallas Peterson, K-State Research and Extension.
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. Unfortunately, some marestail may also be ALS resistant, in which case FirstRate, Classic, and Synchrony would also be fairly ineffective. This just further emphasizes the importance of early spring weed control. Liberty 280 herbicide has provided fairly good control of large marestail as a burndown treatment or postemergence in Liberty Link soybeans.
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, was effective for many years, but because of the heavy reliance on glyphosate for weed control, glyphosate-resistant waterhemp has become fairly common in eastern Kansas and glyphosate-resistant Palmer amaranth is now common throughout much of central and western Kansas.
Figure 2. Glyphosate-resistant Palmer amaranth escapes in soybeans. Photo by Dallas Peterson, K-State Research and Extension.
The best way to manage these pigweeds in soybeans is to use effective preemergence herbicides followed by postemergence treatment. However, if the preemergence herbicides weren’t applied or didn’t get activated in a timely manner, early-emerging pigweeds may not have been controlled and can grow rapidly. Flexstar, Cobra, Marvel, 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 glyphsosate should be applied within 3 to 4 weeks after planting to optimize performance. Producers often try to cut the rates of these herbicides to reduce soybean injury. However, lower rates of these burner herbicides still cause similar soybean burn symptoms and weed control is often reduced.
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.
Velvetleaf
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, Marvel, FirstRate, Harmony, and Synchrony.
Sunflower and Cocklebur
Fortunately, sunflowers and cocklebur are still 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 residual control of later-emerging plants.
Conclusion
If weeds have gotten large, 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
dpeterso@ksu.edu
Doug Shoup, Southeast Crops and Soils Specialist
dshoup@ksu.edu
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