Finally sunshine two days in a row and everyone is chomping at the bit to get something done. Judging by the phone calls, burndown applications are first on the priority list. The rain has us running a little late on the calendar for burndown applications, but if you look at what is growing in the fields, winter annuals have also been delayed due to the extended cold weather. Before making burndown applications, decide which crops will be planted to make sure the re-plant intervals following herbicide applications will match up well with the intended crop. For all burndown applications refer to the Row Crop Plant Back Intervals (MP-519) publication for replant guidelines. Many growers will focus on corn and rice acres first, because they will be the first planted. I know everyone is in a hurry, but the key to success with any burndown program is first to know what weeds you have in the field and second, wait at least 14 days after the application to be certain that the weeds are dead prior to planting. This will save some headaches later in the season, with weed and insect pests. Crop specific burndown recommendations are listed below by crop.
Corn: Several herbicide options are available with short plant-back windows into corn. These include Glyphosate (Roundup), Paraquat (Gramoxone), dicamba (Clarity), 2,4-D, Leadoff, Sharpen, and Verdict. Glyphosate-resistant horseweed is still a pretty big issue on all production acres. Burndown herbicides that provide good control of horseweed are dicamba (Clarity), Verdict, Sharpen and 2,4-D. Generally a three-way mix of Roundup, 2,4-D and Dicamba will provide control of most all emerged winter annuals. Roundup + Verdict (10oz/A) will also provide control of most winter annuals including horseweed as well as provide good residual control up through corn planting. Adding Leadoff to Roundup + dicamba or 2,4-D applications will also provide excellent residual control through planting. If Leadoff is used, make sure to plant a corn variety that is not sensitive to ALS herbicides. Atrazine and or Dual can be added to all burndown applications to provide residual control through corn planting and buy time for a POST application. Keep in mind, if atrazine is used it will lock that field into either corn or grain sorghum for the year.
Rice: Roundup + 2,4-D +Sharpen is an excellent burndown option in front of rice. However, the plant-back window for 2,4-D is 21 days. Roundup + Sharpen (2oz/A) will control most annual weeds including glyphosate-resistant horseweed if it is small. If horseweed has any size, especially once it starts to bolt; it will most likely regrow if 2,4-D is not included with the Sharpen. Roundup + FirstShot is also an excellent option with an immediate replant interval, and this combination will control most all winter annuals with the exception of glyphosate-resistant horseweed. Gramoxone is an option with no pre-plant interval to rice, and is good on most winter annuals as long as they are small. Following the initial burndown application, Roundup tankmixed with Command and/or other PRE herbicides will help finish off winter annuals or early grass flushes at planting.
Grain Sorghum: Most options available for burndown prior to planting corn will work for grain sorghum, with the exception of Leadoff. The re-plant interval for grain sorghum is 10 months following a Leadoff application. Roundup + 2,4-D + Sharpen or FirstShot, or Roundup + Verdict are excellent options for burndown applications in grain sorghum. I am partial to Roundup + 10 oz/A Verdict especially if there is a tight planting window. Verdict provides excellent POST control of henbit and horseweed and adds residual activity on many broadleaf and grass weeds. Growers should consider spiking the Verdict with more Outlook or Dual to increase residual grass activity, or follow up with Dual Magnum PRE.
Soybean: We still have some time before soybean planting, but that doesn’t mean we need to delay burndown applications. Roundup and Gramoxone are generally key components in burndown for soybean. Roundup +1.5oz/A Sharpen + 1.5pt 2,4-D, or Roundup + 7.5oz/A Verdict is a really good option if horseweed is present. FirstShot can be substituted for Sharpen if no horseweed is present. With most of these mixes the replant is 7-14 days. I have had a lot of calls regarding dicamba for use as a burndown in beans or a Roundup + 2,4-D + dicamba three way mix. Soybeans are very sensitive to dicamba and if it is used in a burndown program, 1in of rain has to accumulate prior to counting days until planting. For soybean the re-plant interval for 8oz/A Clarity is 14 days following 1inch of rainfall. If the Clarity rate is increased, the waiting period must also be increased. Gramoxone should be included at planting with a preemerge herbicide to kill glyphosate-resistant pigweed that have potentially emerged prior to planting.
Cotton: A three way mix of Roundup + 2,4-D + dicamba, or Roundup + FirstShot+ dicamba are still the cadillac burndown programs for cotton. Sharpen and Verdict are not used on many cotton acres due to the lengthy plant-back interval (1.5 months). Consider using a residual like Valor when making burndown applications ahead of cotton planting to keep fields clean and provide residual glyphosate-resistant horseweed control. Including Gramoxone in the pre-plant or preemerge application is very important to remove glyphosate-resistant pigweed especially if a residual was not included with the first burndown application.