In today's crop production systems, starting off with a good burn-down program is a must. In many systems and areas it is the most important decision of the whole year, at least from a weed control standpoint.
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 eastern black nightshade, pokesalad, common ragweed, giant ragweed and ryegrass — are making for some hard decisions early in the growing season.
If horseweed is not controlled on the front end with a good burndown program, then there are few good options to come back with postemergence in soybeans and cotton.
Most burn-down programs for horseweed begin in late February or early March. University of Arkansas research and the work of others have shown that a tank-mix of 8 ounces per acre of dicamba herbicide with glyphosate is best for control of this weed. From an economic standpoint, glyphosate is 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 it can work if everything is right.
Gramoxone and Liberty are pretty good if the horseweed is very small. Both need dicamba in the tank to be very good.
Other products just do not perform as well as dicamba and 2,4-D on emerged horseweed; you need to get them the first try.
Burn-down treatments with dicamba need to go out an absolute minimum of 14 days prior to planting soybeans and and 21 days prior to planting cotton. These dates assume at least 1 inch of rainfall.
So, the way the plant-back interval works is you spray your burn-down, then wait until you get a rain, then start counting. After 14 days with dicamba you can plant your soybeans.
For 2,4-D, the intervals are 14 days for soybeans and and 28 days for cotton. The same rainfall rule applies. These are University of Arkansas recommendations.
Labels can vary by product, so as always, read them.
Have I mentioned that you may only get one shot to control horseweed and that is at burn-down?
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 the spray on as much of the horseweed plant as possible. Common mistakes made at application last year 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 what his best configuration is for this application. Make sure that commercial applicators understand the importance of this treatment.
There has been a tremendous interest, often by the herbicide companies, about starting in the fall with burn-down plus residual treatments for control of horseweed and other weeds.
A typical program is glyphosate plus 2 ounces per acre of Valor. You may see some of these fields as you drive around the Delta or other agricultural areas this winter. These fields for the most part look great.
Valor is an excellent product for this, but my concern is what happens later this spring as we get ready to plant. Will these fields hold until we can get some seed in the ground?
If you have to re-apply the burndown in the spring or the residual plays out at planting, you could be spending money to look at bare fields in the winter when they are not making you any money.
In areas where horseweed is bad, many soybean growers are adding Valor or Synchrony XP to their dicamba plus glyphosate burndown programs. Those treatments looked pretty good in our trials last year.
Valor and Synchrony can be applied up through planting, so your only restriction is on the dicamba plant-back.
I also like glyphosate plus dicamba as the burn-down and then go back with a pre-emergence treatment of Valor.
If you miss horseweed early or it comes up and needs treatment in-season in soybeans, the best thing I have looked at is a full rate of FirstRate. In cotton it is Envoke. Both programs provide suppression at best.