As it has started to warm up, folks are ready to implement early-spring weed management programs, especially in fields with infestations of winter annuals like field pennycress and marestail (horseweed). Spring burndown (preplant) herbicide programs should target winter annual plants at the rosette stage, prior to stem elongation (bolting), for consistent control. Cold nights (<40°F) will reduce activity of herbicides, particularly for glyphosate. With cool weather conditions, it is recommended to wait for a few days until it warms up before applying herbicides.
Marestail populations in Iowa have developed resistance to glyphosate (HG 9) and ALS-inhibitor (HG 2) herbicides. Therefore, a diverse herbicide program is needed. Adding 0.5 lb acid equivalent (ae) 2,4-D LVE, 0.25 lb ae dicamba, or saflufenacil (Sharpen @ 1 oz/a) to glyphosate will increase the consistency of marestail control. Based on the results from our field trials, dicamba and 2,4-D both provided greater control of marestail when applied at the rosette stage compared to the bolting stage. Glufosinate (Liberty @ 32 oz/a) plus 2,4-D or dicamba is an effective spring burndown option to control marestail even if plants have started bolting. Paraquat + atrazine/metribuzin + 2,4-D would also be an effective spring burndown program to control glyphosate-resistant marestail in the field. Most 2,4-D labels have a 7-14 day planting restriction for corn or soybean, and dicamba has a longer replant restriction than 2,4-D when used preplant in soybean (14 days at 0.25 lb ae and 28 days for 0.50 lb ae). Dicamba-tolerant (Xtend/Xtendflex) and 2,4-D choline-tolerant (Enlist E3) soybean varieties allow use of approved formulations of dicamba (Xtendimax) and 2,4-D choline (Enlist One/Enlist Duo), respectively, for preplant burndown without any planting delays. Use the full labeled rate along with a full load of recommended adjuvant(s) to prevent any control failures especially when weeds are stressed by cold conditions or are larger in size.
Volunteer corn management
With the derecho-affected fields in Iowa, spring burndown programs may also be targeted to control early-emerging volunteer corn. Meaghan Anderson wrote a blog last month entailing options for burndown of volunteer corn prior to planting corn. Rotating to soybean would provide flexibility to use HG 1 herbicides for in-season control of volunteer corn. It is important to remember that antagonism between HG 1 products and several broadleaf herbicides like 2,4-D and dicamba may make volunteer corn control more challenging. Specific details and recommendations to overcome antagonism can be found from a previous ICM news article.
Diverse weed control approach
While fields planted to a cereal rye cover crop previous fall often have a reduced infestation of winter annuals, fields should be scouted prior to burndown applications to determine appropriate herbicide mixtures to effectively terminate cover crops and control winter annuals. Based on our field trials, residual herbicides should be included in spring burndown programs for controlling late-emerging cohorts of marestail and other challenging weed species such as common lambsquarters, giant ragweed, and waterhemp. A high biomass cereal rye cover crop (thick stand and tall growing) may provide good weed suppression, but residual herbicides (HG 5, HG 14, HG 15) at the time of cover crop termination (spring burndown) will still be important to minimize weed interference during the soybean growing season. Regardless of weeds targeted, using herbicides with multiple, effective sites of action should be a priority for all applications. These diverse strategies (cover crop and soil residual herbicides) will reduce the load on POST products and slow down the development of herbicide-resistant weed populations.