One of the major weed problems in soybean production is marestail. Unfortunately marestail is able to germinate over a prolonged time period, beginning in the early fall and then new plants germinate again from early spring through early summer. The issue is that once the marestail plant gets more than 3 to 4 inches of growth there is not any effective post emergent herbicide control option. Therefore the key to good marestail control and good soybean production is a systems approach that utilizes several steps including fall herbicide application and split spring herbicide applications.
If we get some favorable weather, the end of March into early April offers a window of opportunity to manage marestail. This application can be very effective when combined with a previous fall treatment. However, if a fall burndown herbicide application was not possible then it is essential that this early spring herbicide application happen. This early spring treatment will provide control of the fall germinated and over-wintered marestail plants plus early spring germinating marestail plants. This herbicide application needs to include a burndown plus some residual herbicide and this application must be made before mid-April for best results. Some effective herbicide options include:
- glyphosate + 2,4-D plus half-rate of a sulfentrazone product such as Sonic or Authority or a half rate of a flumioxazin product such as Valor, Envive etc.
- glyphosate + 2,4-D plus metribuzin at 4-6 ounces/acre
Control is improved by using the highest rate of a 2,4-D ester product that is allowed and this rate will be determined by how early the application is made relative to a possible planting date. For all of these products, rates up to 0.5 lbs./acre of active ingredient carry a label restriction of 7 days between application and planting. Generally rates between 0.5 and 1.0 lbs./acre of active ingredient have a 30 day planting restriction with the exception of several products such as E-99, Salvo, and Weedone 650 that allow 1.0 lb./acre of active ingredient applied 15 days before planting.
The addition of a saflufenacil product such as Sharpen can help with the burndown control of emerged weeds plus give some additional residual control especially when combined with other residual products that contain flumioxazin or sulfentrazone. The Sharpen label permits the use of 1.5 ounces combined with these other products provided there is at least 2% soil organic matter and the application is made at least 14 days before planting.
Another option if the weather and field conditions permit, is the addition of dicamba to this early spring herbicide application. Research has shown that dicamba can be more effective than 2,4-D on marestail in the spring, but there is also more potential for soybean injury if the label restrictions are not heeded. In general there needs to be 14 to 28 days between the application of dicamba and soybean planting PLUS there must have been a minimum of one inch of rain cumulative total within this time frame.
More detailed information about marestail control along with more herbicide control options is available in the Extension fact sheet entitled “Control of Marestail in No-till Soybeans” by Weed Specialists Mark Loux from Ohio State University and Bill Johnson from Purdue University. This fact sheet is available on-line at: http://agcrops.osu.edu/specialists/weeds , or contact the Wayne County Extension office at 330-264-8722 for a copy.