Most effective management of marestail occurs with either: 1) a program of fall followed by spring herbicides, with most of the residual herbicide applied in spring; or 2) a split-application approach in spring, where burndown and some residual herbicide is applied in late March or early April, and additional burndown and residual applied when soybeans are planted. Skipping the fall or early spring applications and applying all of burndown and residual herbicide together in spring introduces more variability in marestail control, and delaying application until late April or May can result in inconsistent control of emerged plants with burndown herbicides.
The burndown treatment for no-till soybeans should obviously include herbicides other than glyphosate that have effective activity on emerged marestail. Burndown options are outlined below. Keep in mind that: a) it’s possible to add 2,4-D to any of these as long as the waiting period between application and planting is followed; b) any of these should be combined with residual herbicides to control marestail that emerge after planting.
1. Glyphosate + 2,4-D ester. This was the go-to treatment for marestail for years. While still effective in the fall, and in many fields on small marestail in the spring, it appears to have become more variable on marestail over the past decade, especially when applied from mid-April on in fields not treated the previous fall. Increasing the glyphosate rate to 1.5 lbs ae/A and the 2,4-D ester rate to 1.0 lb ai/A can improve control. Allow 7 days between application and planting for 0.5 lb 2,4-D ester/A. Labels for some products specify a 15-day waiting period between application and planting for rates up to 1.0 lb/A, while the waiting period is 30 days for others.
2. Sharpen + glyphosate. Effective on marestail, but can be weak on large giant ragweed and dandelion unless 2,4-D ester is included. Must be applied with MSO (methylated seed oil) in a spray volume of at least 15 gpa. Sharpen rate of 1.0 oz/A can be applied anytime prior to soybean emergence. Higher Sharpen rates are allowed, but must be applied 14 to 44 days before planting, depending upon rate and soil type. Works well with products that contain metribuzin. Cannot be mixed with products that contain flumioxazin or fomesafen – Valor, Valor XLT, Gangster, Envive, Prefix, Intimidator. The label recommends flat fan nozzles for burndown applications. The active ingredient in Sharpen, saflufenacil, is also a component of Optill, Optill Pro, and Verdict, and mixtures of any of these with glyphosate should provide similar control.
3. Liberty. Burndown rates are 29 to 36 oz/A. Using the higher rates and adding metribuzin generally results in more consistently effective control, especially under cool, cloudy weather conditions. Apply in a spray volume of at least 15 gpa (20 gpa is preferable in most situations). Avoid use of nozzles that produce large spray droplets. Liberty will be most effective when used in spring following a fall herbicide application that removes many of the other “winter weeds” typically found in a no-till field.
4. Gramoxone + 2,4-D + metribuzin. Use Gramoxone rates of 3 to 4 pts/A and metribuzin rate of at least 4 oz 75DF/A. Activity is maximized when applied with COC (crop oil concentrate). Apply in a spray volume of at least 15 to 20 gpa and avoid nozzles that produce large droplets. Some dealers have commented that the mix of Gramoxone + 2,4-D has been effective in early spring without metribuzin, but we have observed the three-way mixture to be most effective on plants that are past the rosette stage.
5. Clarity and some other dicamba products. We have not previously suggested the use of this in the spring prior to soybeans, but this is an approved use on some dicamba product labels. Labels allow use of 8 oz product/A, but specify that it must be applied at least 14 days before planting and an inch of rain must occur within that 14-day period. Rates up to 16 oz/A can be applied, but the waiting period increases to 28 days. Dicamba can be more effective on marestail than 2,4-D, and marestail is one of the few weeds that dicamba has much residual activity on. So, if the burndown treatment is applied early enough in spring, the addition of a low rate of dicamba to glyphosate/2,4-D mixtures or other burndown treatments can improve burndown of marestail and add at least some residual.
6. Residual herbicides that contain chlorimuron (Classic) or cloransulam (FirstRate) can add some burndown activity, but only where the marestail populations are not resistant to ALS inhibitors. Products that contain chlorimuron – Canopy/Cloak, Valor XLT, Envive, Authority XL. Products that contain cloransulam – Sonic, Gangster, Authority First. In populations that are ALS-resistant, the Valor/Authority component of these products has the potential to cause antagonism and a reduction in the activity of glyphosate/2,4-D.