Controlling Broadleaf Weeds in Small Grains Stubble (08/05/21)
Small grain harvest is underway in eastern North Dakota and western Minnesota. Some small grain fields have broadleaf and grass weed escapes; weeds coexisting with small grains due to dry conditions affecting small grain growth and development (Image 1). Small grains did not out compete broadleaf weeds in 2021.
We suggest you WAIT to till fields following wheat harvest. Dry soils will make tillage much more difficult as compared to tillage after a rainfall event. Moreover, insects and rodents forage on crop and weed seeds, which are a source for their energy. University research reports greater than 5% per day loss of weed seeds, and total losses ranging between 20% to 90%, when weed seeds remain on the soil surface. Tillage after harvest greatly reduces predation of weed seeds since predators prefer not to dig for seeds.
We suggest waiting ten to fourteen days to till after harvest, or until waterhemp or kochia is approximately 4-inch tall (Image 2). We know that glyphosate alone does not provide control of glyphosate resistant waterhemp or kochia. Field research to evaluate waterhemp control from mixtures with glyphosate was conducted in August and September 2020 in wheat stubble (Peters TJ and Lystad AL. 2021. Waterhemp control in small grains stubble. Sugarbeet Res. and Ext. Rep.51:38-39). (https://www.sbreb.org/wp-content/uploads/2021/04/2020-full-book.pdf).
We observed 90% waterhemp control from Roundup PowerMax at 32 fl oz/A plus Sharpen (saflufenacil) at 1 fl oz/A with Prefer 90 NIS at 0.25 % v/v plus N-Pak AMS at 2.5% v/v, 6 days after treatment (DAT). At 22 DAT, there was no statistical difference between Roundup mixtures with Sharpen or Roundup and 2,4-D at 64 fl oz/A with Prefer 90 NIS at 0.25 % v/v plus N-Pak AMS at 2.5% v/v. Please be aware of crop rotation restrictions! Sharpen has a 4-month crop rotation restriction between application and sugarbeet planting; total months when soils are unfrozen. Sharpen is degraded in soil by microbes.
Soil moisture, especially in the surface layer, is lacking in 2021 due to our drought conditions. Thus, it is not clear how much microbial activity might be occurring in soils. We believe Sharpen for fall broadleaf control might be a risky proposition, especially in fields to be planted to sunflower, drybean, sugarbeet, and potato in the spring of 2022. Therefore, Roundup PowerMax plus 2,4-D probably is a better option for waterhemp and kochia control following wheat harvest.
Gramoxone (paraquat) was not an entry in the 2020 experiment but may be an alternative to programs with crop rotation restrictions. Apply Gramoxone SL at 1.3 pt/A (3 lb gallon formulation) or 2 pt/A (2 lb gallon formulation) with non-ionic surfactant at 2 pt per acre in at least 15 gpa water carrier when broadleaf weeds are less than 4- to 6-inches tall. We observed improved control of broadleaves greater than 6-inches tall when 2,4-D is mixed with paraquat. We have also observed improved control from early evening application as there is some paraquat translocation in plants from late afternoon applications. Finally, please be respectful of neighbors’ sensitive crops especially when conditions are breezy.
Extension Sugarbeet Agronomist
NDSU & U of MN