Starting off with a good burn-down program in soybeans is a must these days. In many systems and areas it is the most important weed control decision that is made for the whole year. Weeds have evolved, especially in reduced tillage systems that are complicating many burn-down scenarios.
Glyphosate-resistant horseweed has now been identified in almost every Delta county in Arkansas. Horseweed and other difficult-to-control weeds such as cutleaf evening primrose, wild garlic, flowering winter annuals, eastern black nightshade, pokesalad, common ragweed, and giant ragweed are making for some hard decisions early in the growing season.
If horseweed is not controlled on the front end with a good burn-down program, then there are few good options to come back with post.
Most burn-down programs for horseweed will begin in late February or early March. University of Arkansas research and the work of others has shown that 8 ounces per acre of Clarity (dicamba) herbicide in a tank mix with glyphosate is the best choice for control of this weed.
From an economic standpoint, glyphosate is still in the mix to control other weeds at burn-down. This tank mix is also effective on cutleaf eveningprimrose and other tough broadleaf weeds.
My second option is to substitute 1 quart per acre of 2,4–D for the dicamba. It is typically less than $1 per acre cheaper, but can work if everything is right.
Gramoxone and Ignite are pretty good if the horseweed is very small. Neither product has a rotational issue with soybeans; however, both need dicamba in the tank in order to be very good.
Other products just do not perform as well as dicamba and 2,4–D on emerged horseweed and as mentioned before, you need to get them the first try. Adding 2,4–D will usually pick up the other winter annual junk present in the field. I prefer it to dicamba if flowering winter broadleaves like henbit and buttercups are the main target.
You can also do combinations of reduced rates of dicamba and 2,4–D to approach fields with mixed populations of weeds that include horseweed.
Burn-down treatments with dicamba or 2,4–D need to go out an absolute minimum of 14 days prior to planting soybeans. These dates assume at least 1 inch of rainfall. So, the way the plant-back interval works is you spray your burn-down, wait until you get a rain, then start counting. After 14 days with dicamba you can plant your soybeans.
These are University of Arkansas recommendations. These labels vary by product, so as always read the label.
Another area of concern with these treatments is application. It is essential to get good coverage for good control. I recommend a minimum of 10 gallons per acre by ground.
Also, speed, boom height and tip selection can play parts in getting spray on as much of the horseweed plant as possible. Last year common mistakes made at application were spraying in high winds and boom height being too high for effective coverage.
There is less concern about herbicide drift this time of year, but it is still important to keep it in the field. If the application is going out by air, be sure to talk to your aerial applicator about his best configuration for this application. Commercial applicators must understand the importance of this treatment and do everything they can to make it work.
In areas where horseweed is bad or where pigweed is a problem, many soybean growers are adding Valor, Synchrony XP, Canopy EX or another residual component to their dicamba plus glyphosate burn-down programs. These treatments looked good in our trials over the past few years.
Valor can be applied through planting, so your only restriction is on the dicamba plant-back. Plant-back intervals on the other residual treatments vary by product. Consult the label or the (Arkansas publication) MP-44 for more information.
Another burn-down consideration is Palmer amaranth (pigweed). Using Valor preplant is an excellent start to a program approach for pigweed, especially for later-planted soybeans. Valor has a different mode of action and will control pigweeds that are resistant to glyphosate, SUs and DNA (Prowl/Treflan) herbicides.
If you do not need the dicamba or 2,4–D, adding Valor (or one of the Valor pre-mixes like Gangster or Envive) to your burn-down applied 10 to 14 days ahead of planting can get you pigweed control a couple of weeks into the season. This helps avoid Valor injury to soybeans, and is a good option for conventional soybeans or where glyphosate-resistant pigweed is suspected.
Remember, if you miss horseweed early or it comes up and needs treatment in-season in soybeans, the best thing that I have looked at is a full rate (0.3 ounce per acre) of FirstRate. Unfortunately, this is really a payback, suppression-type treatment. It will not completely kill the horseweed.
You can make two applications of FirstRate per season and that can help, but I would rather have it controlled up front with the burn-down. The MP-44 and a podcast on soybean burn-down can be found at www.uaex.edu.