Drs. Daniel Stephenson and Josh Copes
I have received many calls concerning burndown over the past couple of weeks. The warm weather we have experienced recently has most farmers itching to get started. Research has shown that the optimum time to burndown winter vegetation is 4 to 6 weeks prior to planting. This is primarily to reduce the risk of insect damage to seedling crops. Think of it this way, winter vegetation in the field is like a buffet for the worms. Removing the buffet 4 to 6 weeks prior to planting will cause the worms to die or move on to another food source. If a crop is planted into green or dying vegetation, the possibility of those worms feeding on the seedling crop is very high. Also, removal of winter vegetation 4 to 6 weeks prior to planting reduces the risk of physical competition between the weed and the crop. Corn, for example, is determining its yield as it is spiking and if growth is hampered by any physical competition, i.e. weeds, then yield will be reduced. Acreage that had a burndown application greater than 6 weeks prior to planting may need to be sprayed again prior to planting, especially if a residual herbicide was not applied with the burndown. The take home message is simple, plant into a clean, weed-free seed bed.
Use of residual herbicides tank-mixed with the burndown application is pretty common. Many herbicides utilized for residual control in a burndown application need to contact bare soil to provide residual. If a field is completely covered by winter vegetation (cannot see much bare soil), that vegetation will intercept the burndown application cocktail, thus the “residual” herbicide may only act as a foliar herbicide and offer little to no residual herbicide. Metolachlor or S-metolachlor are examples of herbicides that are tightly bound by plant biomass, so don’t expect residual control if it doesn’t reach the soil surface.
Research has shown that glyphosate plus 2,4-D at 1 lb ae/A is the best broad spectrum burndown treatment. Notice I wrote 1 lb ae/A, not 0.5 or 0.75 lb ae/A. For a 4 lb ae/gal 2,4-D formulation, 1 lb ae/A of 2,4-D equals 1 quart/A. In my opinion, this holds true no matter if you add another herbicide like Sharpen, Goal, LeadOff, etc. to the burndown application. Essentially, if you are going to make the trip to apply the herbicide, why not apply enough 2,4-D that research has shown will kill almost all of the winter weeds Louisiana farmers deal with.
If a producer does not want to or can’t use 2,4-D in their burndown application, then the choice of burndown herbicide depends upon weed spectrum. I’m not going to go through every scenario because there can be many options. Give your local county agent a call for help in this situation.
In conclusion, the main item all consultants and producers need to strive for with burndown is to ensure that any crop is planted into a weed-free seedbed. Good luck and please call us if you need any help.