By Mark Loux, Ohio State University Extension
Dicamba can have a good fit in spring preplant burndown programs, especially for control of overwintered marestail in fields not treated the previous fall. We typically recommend a preplant burndown that includes at least two herbicides with substantial activity on marestail in this situation, such as Sharpen + 2,4-D or Gramoxone + 2,4-D + metribuzin. Dicamba is the most effective burndown herbicide on glyphosate-resistant marestail in the spring though, and in our research has usually killed or at least stopped emerged marestail in their tracks without help from other herbicides. We have occasionally observed larger marestail plants escape complete control, due partly to what appears to be antagonism from other herbicides in the mix. Low rates of dicamba added to other less than effective burndown mixtures can also improve control to adequate levels.
With regard to use of dicamba in burndown programs, there are distinctly different situations which define how it can be used and what can be mixed with it. Dicamba has been approved for preplant use prior to any type of soybean for a long time, but the restrictions on use can make it difficult to easily integrate into burndown treatments. For most of the 4 lb/gallon dicamba products that have this label, the wording is as follows (from Clarity label here): “Following application of Clarity and a minimum accumulation of one inch rainfall or overhead irrigation, a waiting period of 14 days is required for 8 fluid ounces per acre or less, and 28 days for 16 fluid ounces per acre. These intervals must be observed prior to soybean planting or crop injury may occur”. Restating – you can apply up to up to 8 fluid ounces per acre of Clarity prior to planting soybeans (any type), but following application, one inch of rain or irrigation must occur, and then you must wait another 14 days to plant. The Engenia, FeXapan, and XtendiMax labels carry these same restrictions and wording, with rates adjusted to account for different product loading. This restriction ensures that enough soil moisture exists and time elapses to allow for dissipation of dicamba to levels too low to injure soybeans. There is at least one product, Spitfire, a premix of 2,4-D and dicamba, with a more liberal restriction at the low end of the rate range. For Spitfire rates of 1 to 1.25 pints/A, the label allows a wait of 7 days to plant following application and the accumulation of ½ inch of rain or irrigation. The 7-day wait starts after the ½ inch of water occurs.
This use of the older dicamba products can allow for a lot of flexibility in tank mix partners. The Clarity label states” “Clarity herbicide may be tank-mixed with other herbicides registered for early preplant use in soybeans including burndown herbicides such as glyphosate and 2,4-D and residual herbicides….”. So the assumption for most of the dicamba products labeled for this use is that any product can be mixed with them as long as that product has a preplant soybean label, and there is no specific prohibition mentioned. There are exceptions – as far as we can tell the Spitfire label has a somewhat incomplete list of approved tank mix partners, and does not have a statement similar to that mentioned above. This non-Xtend soybean part of the XtendiMax/FeXapan labels does not mention tank mixing at all. And the Engenia label refers the user to the list of approved tank mix options at the Engenia website, the same list that pertains to use in Xtend soybeans.
We assume the preplant use discussed above pertains to any type of soybean, including Xtend soybeans, as long as the restrictions on rate, rain/irrigation, and waiting period are met. There is not a less restrictive way to use dicamba on non-Xtend soybeans. With regard to Xtend soybeans, it is possible to apply the approved dicamba products, Engenia, XtendiMax, or FeXapan, anytime prior to planting without a waiting period regardless of product rate (within the specified range), and then also at or after planting as well. These are the only three dicamba products that are approved for this less restrictive type of use. Since the use is specific to these products and Xtend soybeans, the user is required to consult the individual product labels and websites for approved tank mixes, nozzles, etc. Furthermore, all of the stewardship guidelines and restrictions on application listed on labels and covered in the dicamba training sessions apply here. Tank mix options are generally not as numerous, since every individual tank mix partner must be on the approved list. Just for the sake of example, the Engenia and XtendiMax list of approved partners does not currently include Canopy, Cloak, Fierce or Fierce XLT. The FeXapan list includes Canopy and Canopy Blend, but not Cloak or the Fierce/Fierce XLT. Many of the other commonly used preplant/preemergence soybean herbicides are approved though.
We have received a few phone calls on this situation, and the confusion tends to be mostly with regard to Xtend soybeans, and the ability to tank mix. The question that needs to be addressed in each situation is – which dicamba product is being used and what label are you working from? Here’s our take:
1) When using an older dicamba product such as Clarity on Xtend or any other type of soybean, and following the more restrictive label (inch of rain and 14 days), tank mix partners can include essentially any other preplant soybean herbicide if there is a statement on the label to this effect. If there is no such statement, then partners include only the specific herbicides listed on the label.
2) When using one of the new dicamba products, Engenia, FeXapan, or Xtendimax, it appears to us that tank mix partners have to be approved and listed on the respective websites, regardless of whether the more restrictive label for non-Xtend soybeans or the less restrictive label for Xtend soybeans is being followed. The Xtendimax and FeXapan labels are not completely clear on this though, with regard to the more restrictive use (14 day wait) on non-Xtend soybean.