Seedling horseweed/maretail which often requires more than just glyphosate. Photo credit D. Lingenfelter.
The use of 2,4-D in combination with glyphosate in the burndown program before establishing either cash crops or cover crops can aid in the control of a number of weeds including horseweed/marestail as well as help to prevent the evolution of new glyphosate-resistant weeds. However, the injury risk and legality of preplant use of 2,4-D in front of many crops is less than clear. The plantback language used on 2,4-D labels varies by both manufacturer and formulation. Most of the interest in burndown is for 2,4-D ester products and with the exception of corn and soybean, we often have to rely on the “fallow ground label guidelines” which includes between crops or post-harvest. In general, this fallow ground label language states to “plant only labeled crops within 29 days following application”. “In addition, labeled crops may be at risk of crop injury or loss if planted soon after application, especially during the first 14 days”. “Under normal conditions, any crop may be planted without risk of injury if at least 90 days of soil temperatures above freezing have elapsed since application”. Most 2,4-D ester products are labeled for use in corn, sorghum, sorghum-sudan grass hybrids, soybean, small grains, fallow ground, CRP and pasture, noncrop areas, and turfgrass; again with the exception of corn and soybean, these crops would fall under the 29-day rule. Some labels are more specific and actually provide a table providing plantback restrictions such as the table that follows.
|Crop||Days following last application:|
|All other crops||30*|
*30 days for residue tolerance; under normal conditions any crop may be planted without risk of injury after 90 days.
So, here is the dilemma. For anything other than corn and soybean, if you want to include 2,4-D in the burndown, you should technically be waiting about 30 days after application although 14 days may be adequate for some labeled crops. We also know that the half-life of 2,4-D is about 7 days under normal growing conditions, so if you apply 1 pt or 0.5 lb ae/acre, within 14 days, you have gone through two half-lives with only about 0.125 lb ae/acre remaining which would pose little risk to grass crops with only the most sensitive species such as small seeded legumes potentially having a problem. This is one of the many issues we run into with herbicides – what is the practical reality vs. the label guidelines. Keep in mind, that cash crop rotation restrictions may be due to the concern for herbicide residues accumulating in forage or feed rather than carryover injury so the 29/30 day guideline should be followed. However, cover crops that are not harvested for forage can be planted after any herbicide program, but the grower assumes the risk of crop failure.