By Larry Steckel, Extension Weed Specialist

As soon as the rains move through and soils dry, corn planting will take priority along with some starting to think about early soybean planting. 

With the exception of Select Max, there is no major problem with planting corn right around a burndown application, which makes it much easier than in soybean where many of the burndown herbicides have plant back restrictions. Moreover the main target weed, marestail (horseweed), may be larger than 4 inches by the time fields are fit to cross, which makes getting good control more difficult. 

So what would be some good options to burndown small to larger horseweed and then plant soybeans quickly behind? There are really only four herbicides to choose from for this and they are FirstRate, Gramoxone, Sharpen and Verdict. 

Sharpen and Verdict (premix of Sharpen and Outlook) are the most often used herbicides for burndown when you want to plant soybeans right away after application. In our research, Sharpen applied at a 1-ounce-per-acre rate or Verdict applied at a 5-ounce-per-acre rate has historically provided good control of marestail up to about 4 inches tall. Last year on burndown tests conducted near April 1, those herbicides applied at those rates performed very well (greater than 95% control). 

In one trial, we sprayed different herbicides at three different times of the day (sunrise, mid-day, or evening). Sharpen provided very good control of horseweed regardless of what time of day it was applied. Soybeans may be planted any time after Sharpen or Verdict are applied at the rates mentioned above. 

Our research has also shown that once horseweed gets in that 4-to-6-inch range or taller it is best to go with a rate of 1.5 ounces per acre of Sharpen or a 7.5-ounce-per-acre rate of Verdict to get more consistent control. The plant back to soybean after those rates is 14 days for most soil types in Tennessee.  However, for fields with low organic matter, the plant back is much longer (30 days). 

Another option to save time on larger horseweed is to tankmix 2,4-D at a pint per acre (ester formulation 4 lb a.i./ gallon) with either the 1-ounce-per-acre rate of Sharpen or the 5-ounce-per-acre rate of Verdict. In our research we have achieved good control of larger horseweed with those tankmixes. The plant back to soybean is 7 days with those mixtures. 

FirstRate can be applied as a burndown application at a rate of 0.3-0.75 ounces per acre. Soybeans can be planted at any time after a FirstRate application. Tank mix FirstRate with glyphosate for best results on horseweed. Use higher rates of FirstRate once horseweed is larger than 4 inches tall.  

Please keep in mind that we have documented ALS-resistant horseweed in some of the northern counties in West Tennessee. Moreover, it is likely that the ALS-resistant horseweed can be found in a few additional counties. FirstRate will not control ALS-resistant horseweed. Gramoxone applied alone typically provides 50-80% control of horseweed. Gramoxone works best on horseweed if tankmixed with metribuzin.  

Utilizing Gramoxone at 48 ounces per acre plus 4 ounces per acre of metribuzin has often provided 80% control or better. Recent research we have conducted has shown that Gramoxone has provided about 40% better horseweed control if it is applied early in the morning or late in the evening compared to the middle of the day. With so many folks planting LibertyLink soybeans this year, another option would be to plan to control the horseweed POST in your soybean with Liberty. In our research, Liberty can control very large horseweed if it is applied during a warm day (high in the 80s).