Are you seeing any weeds becoming more problematic in your fields?
It is my hope that weed management in your fields is improving and that weed pressure is decreasing. If you are in that position, I encourage you to continue to be aggressive – but consider how you can adjust your herbicide program so it is different from what you did last time you grew soybeans to minimize risk of selecting for herbicide-resistant populations.
If you are in a position where you are seeing shifts in species or increases in population, do you have a robust weed management plan in place or are you in need of one?
You will be most successful in achieving excellent weed control if you include a full-rate PRE herbicide with 2 active ingredients that control the weed(s) that concern you. PRE herbicides need to move into the soil to be effective, so where there is heavy winter annual weed cover apply a burndown herbicide 2-4 weeks before planting, and then apply the PRE herbicide at planting and after the winter weeds have begun to deteriorate.
A robust weed management plan should include a POST herbicide application when weeds are less than 4 inches in height. You may consider scheduling your POST application 3-4 weeks after the PRE, especially if your POST herbicide includes an overlapping residual herbicide.
Below are a few things to consider regarding four troublesome species in Illinois.
Marestail (horseweed). The past winter was mild, and marestail is growing rapidly. Apply burndown herbicides before marestail stems begin to elongate. Effective options include 2,4-D+glyphosate or glufosinate (Liberty), Sharpen+MSO+glyphosate or glufosinate, 2,4-D+gramoxone+metribuzin. To manage for marestail that germinates after planting, apply a PRE mixture that includes flumioxazin, sulfentrazone or metribuzin at planting.
Giant ragweed. Apply a PRE that includes one of these ALS-inhibitors: chlorimuron-ethyl, cloransulam-methyl or imazethapyr. You will need to follow this with an effective POST 3-4 weeks after planting.
Waterhemp. PPO-resistance is increasing in prevalence in Illinois waterhemp! PPO-inhibiting herbicides are components of most PRE soybean herbicide mixtures. Fortunately, PPO inhibitors still work in most fields on germinating waterhemp. But they need help. Consider applying a chloroacetamide herbicide (metolachlor, pyroxosulfone, p-dimethenamid or acetochlor) as part of your PRE herbicide program to add a second mode of action effective on waterhemp.
Palmer amaranth. PPO-resistant Palmer amaranth has been reported in Tennessee, Arkansas, and Mississippi. We don’t want it in Illinois. Tank-mix metribuzin or a chloroacetamide with any PPO-containing product you use for a PRE.
A valuable resource for developing weed management plans is the 2016 Weed Control Guide. A link to download or order this book is available here.
Mark Bernards is a weed scientist at Western Illinois University. His areas of research include weed management in corn and soybeans, and evaluating herbicides and adjuvants for crop safety and weed control efficacy.