Marestail – Many populations are resistant to both glyphosate and ALS inhibitors, and so the chlorimuron, cloransulam, or imazethapyr in many premixes fails to contribute residual marestail control. The nonALS active ingredients that have residual activity are flumioxazin, sulfentrazone, metribuzin, and saflufenacil (primarily at rates higher than the base rates that are used for burndown). While we rate the premixes containing some of these as an “8” on marestail, this can be optimistic, especially when applied a week or more ahead of planting. Our research increasingly shows that combinations of two nonALS actives provide more consistent residual control than just increasing the rate of a single active. Examples – using a mixture of Sonic + metribuzin rather than just Sonic; using a mixture of Canopy DF + metribuzin + Sharpen rather than just the Canopy DF + metribuzin. For the latter example, the higher rate of Sharpen would have to be applied 2 weeks ahead of planting to use 1.5 oz of Sharpen and maximize residual. Something else that’s important to keep in mind for marestail – where something has been applied the previous fall or early spring to control emerged marestail, it can be adequate to apply glyphosate+2,4-D closer to planting. Any residual herbicide can be included with glyphosate and 2,4-D. Where nothing was applied the previous fall or early spring, there can be a need for a more aggressive burndown such as glyphosate+2,4-D+Sharpen, and this can limit the ability to use flumioxazin and sulfentrazone or force earlier application. The bottom line here is that if it appears that there will be a need for an application fairly close to planting that includes Sharpen, stick with products that contain metribuzin – Canopy DF, Matador, Boundary, etc – and add metribuzin as appropriate to bring the rate up to 8 to 12 oz where soil type allows.
Giant ragweed – residual control of giant ragweed comes primarily from two ALS inhibitors that are components of many premixes – chlorimuron (Classic) and cloransulam (FirstRate). We rate these as “7” in the guide but they can be better or worse depending upon density of the population, rainfall etc. The imazethapyr (Pursuit) in some premixes also provides some control, which we rate as a “6”. None of these work on ALS-resistant giant ragweed, and nothing else provides much control either – maybe a little from fomesafen or the higher rates of Sharpen. Populations resistant to ALS inhibitors are most effectively controlled by two POST applications of glyphosate or Liberty, or Flexstar followed by Cobra in nonGMO soybeans.
Common ragweed – A similar story to giant ragweed but a few more choices. In addition to the ALS inhibitors mentioned above, the PPO inhibitors flumioxazin (Valor, etc) and fomesafen (Prefix, etc) have fair to good activity. Pryroxasulfone (Zidua, Anthem, etc) has at least some activity, as does metribuzin. In ALS-resistant common ragweed populations, premix products containing flumioxazin have the edge over those containing sulfentrazone, because the latter has no activity on ragweeds. There are some populations of common ragweed in Ohio with resistance to both ALS and PPO inhibitors (sites 2 and 14), and it’s difficult to obtain even fair residual control of these with pyroxasulfone and metribuzin. LibertyLink soybeans can be a good option for these multiple-resistant populations.
Waterhemp and Palmer amaranth – Both of these weeds are challenging because they can emerge well into the growing season. ALS inhibitors are largely ineffective due to a high frequency of ALS resistance, but the following active ingredients provide fair to good residual control for the early part of the growing season (combinations or premixes that contains two of these can be more effective then single actives, especially for Palmer): acetochlor, metolachlor, pyroxasulfone, dimethenamid, pendimethalin, flumioxazin, sulfentrazone, and metribuzin. Some of these can be applied POST in combination with herbicides that control emerged Palmer amaranth and waterhemp, in order to extend the residual control and minimize the need for additional POST treatments. This strategy, known as “overlapping residual”, does not replace the need for residual herbicides at or before planting. It should therefore be implemented primarily where these two weeds are already a problem, since it is added cost and does not work for marestail or giant ragweed. An exception might be if other fields in the area already have the problem, and the goal is to prevent the spread of waterhemp and Palmer amaranth.