Soybean herbicide systems have evolved back to a fairly high level of complexity to deal with the herbicide resistance we have in various broadleaf weeds. By the time we use a comprehensive mix of burndown and residual herbicides, we tend to be coming back with post-emergence herbicides primarily for marestail, ragweeds, and waterhemp (and grasses), says weed researcher Mark Loux of Ohio State University.
Post-emergence tools available for control of these broadleaf weeds vary with the type of soybean trait being used, but can include glyphosate, PPO inhibitors (fomesafen, Cobra), glufosinate, dicamba, and soon 2,4-D choline. ALS inhibitors have become somewhat irrelevant on these weeds due to widespread ALS resistance, although they may have activity on some ragweed populations still sensitive to ALS inhibitors. Resistance to various sites of action can further limit the number of options.
The following generalizations about resistance seem appropriate at this time:
Marestail – almost all populations resistant to glyphosate and ALS inhibitors
Common ragweed – populations is some areas/fields are resistant to glyphosate and ALS inhibitors, and in some cases also PPO inhibitors. Other populations still largely respond to all of these herbicides
Giant ragweed – most populations have lost sensitivity to glyphosate and some are resistant. Many populations also resistant to ALS inhibitors, especially if they have glyphosate resistance. No confirmed PPO resistance yet.
Waterhemp – all populations resistant to ALS inhibitors, most resistant to glyphosate, some resistant to PPO inhibitors also. Resistance is more widespread in areas/fields with longest history of waterhemp problem
In soybeans, we assume that regardless of the residual herbicides used at planting, giant ragweed and waterhemp will require post-emergence herbicides. The same can be said for common ragweed that has resistance to ALS inhibitors. Flumioxazin can provide some residual control of these populations, but not to the point that post-emergence herbicides are unneeded.
Marestail is the one weed in this group that can often be adequately controlled with residual herbicides (assuming an effective burndown). Residual control of marestail is essential in RoundupReady and nonGMO soybeans, since there are no post-emergence options. And also in Xtend soybeans, if the goal is to use dicamba only in burndown programs, avoiding post-emergence use.
Off-target movement of dicamba, which was widespread in 2017, has much greater potential to cause problems from post-emergence application. University weed scientists are fairly united in their opinion that dicamba use would be better restricted to early season, in burndown programs.
The other three weeds mentioned here do not necessarily require post-emergence dicamba use, since glyphosate/PPO inhibitor combinations can still be effective. However, failure to use an appropriate residual program in Xtend soybeans can result in mid-season marestail escapes, driving a need for post-emergence dicamba.
We have been making the same residual herbicide herbicide recommendations on marestail for several years, and they are based on the following:
► The ALS resistance means that the chlorimuron, cloransulam, or imazethapyr that is in many residual premixes provides no residual control
► The only other active ingredients with residual activity are metribuzin, flumioxazin, and sulfentrazone. Higher rates of Sharpen provide some residual. Dicamba and the 1-ounce rate of Sharpen provide negligible residual which is short-lived.
► The residual activity from any one of these actives varies from year to year and field to field in our research, resulting in inconsistent control. Control with metribuzin has been rate dependent — increasing with rate from 6 to 12 ounces of 75 DF.
Mixtures of two active ingredients provide more consistently effective control, which is the basis of our recommendation to add 6 to 8 ounces of metribuzin 75 DF to products that contain flumioxazin or sulfentrazone. It’s also possible to improve control by using 1.5 to 2 ounces of Sharpen with metribuzin, but this requires a 15 to 30 day wait to plant.
Based on this we can put premixes roughly into one of three categories with regard to residual marestail control: most consistently effective (two actives), variable (one active); and no control. This information can also be gleaned from the marestail ratings in the herbicide efficacy tables in the “2018 Weed Control Guide for Ohio, Indiana, and Illinois.” We can’t keep track of all generic equivalents so there are more trade names available for certain premixes than are listed here. Keep in mind that just because a premix contains the right components for maximum marestail residual does not mean it has the right ones for ragweeds.
Most consistent residual (two actives), higher labeled rates required in some cases:
Panther Pro, Trivence, Authority MTZ, Ransom
Variable residual (one active):
Afforia, Authority First/Sonic, Authority MAXX/XL, Authority Assist, Zone, Broadaxe XC, Envive/Enlite, Fierce, Fierce XLT, Latir/Militia, Surveil, Valor XLT
Variable residual sub-category - more metribuzin needed depending upon product rate:
Cloak DF, Canopy Blend, Matador, Intimidator, Boundary/Ledger/Tailwind
No or little residual:
Anthem MAXX, Prefix/Vise/Statement, Pummel, Torment, Warrant Ultra, Zidua PRO
Deficiences in residual on marestail for individual premixes can be compensated for by the strategies outlined above. For example, Zidua PRO has almost no residual activity on marestail, but is often applied with metribuzin. In this and any other case where metribuzin is carrying the full load, rates of 10 to 12 ounces 75DF will be more effective than 6 to 8 ounces.
Some products contain metribuzin, but require additional amounts to reach a total metribuzin rate that has a chance to provide enough residual. For example, 4 ounces of Canopy Blend contains the equivalent of 2.7 ounces of metribuzin 75 DF, so our recommendation would to add about another 4 to 8 ounces of metribuzin 75DF, depending upon soil type.