Controlling weeds more than 6 inches tall in Roundup Ready soybean fields
It’s been a crazy spring with the wet weather in much of Kansas, which has complicated herbicide application and weed control. Consequently, a number of weeds have gotten away from us and are growing rapidly in soybean fields. Controlling emerged weeds more than 6 inches tall is often considerably more difficult than controlling weeds less than 3 inches tall. 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. 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 also provided fairly good control of large marestail as a burndown treatment or postemergence 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, Synchrony, 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 was effective for many years, but because of the heavy reliance on glyphosate for weed control, glyphosate-resistant waterhemp and Palmer amaranth have become fairly common in many areas of Kansas.
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 are now growing wild. Flexstar, Cobra, Ultra Blazer, Marvel, and Prefix can be fairly effective for controlling small pigweed (< 3 inches tall). 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. These products are less effective as the pigweed, especially Palmer amaranth, gets larger. Fomesafen products (Flexstar, Marvel, Prefix, and others) should not be used in counties west of U.S. Highway 281, and fields treated with fomesafen products cannot be planted to wheat for 4.5 months, corn for 10 months, or alfalfa, canola, grain sorghum or sunflowers for 18 months after application. Because these burner herbicides are contact in nature, it is key to achieve as good of coverage on the leaves as possible. Using a minimum spray volume of 15 to 20 gallons per acre will be critical to get good control especially on larger pigweeds.
Additional residual pigweed control can be achieved by adding Outlook, Zidua, Dual Magnum, or Warrant to the postemergence treatment, but these herbicides will not control emerged pigweeds. Pursuit, Synchrony, and Harmony were once fairly effective for pigweed control and can still provide good control of susceptible populations, but many fields already 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, but be mindful that rotation restrictions with these two herbicides can be fairly lengthy as well.
If weeds have gotten larger than 6 inches tall, 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
Doug Shoup, Southeast Area Crops and Soils Specialist