Preemergence herbicides are the foundation of any excellent weed control program in soybeans. Using multiple effective residual herbicides is important to broaden the spectrum of controlled weeds, ensure herbicide activation in various environments, and guard against herbicide resistance. The basic “recipe” to control key weeds in soybeans is a Group 15 herbicide + a Group 14 herbicide + metribuzin or a Group 2 herbicide. Herbicides in each of these groups will be discussed below. Additional information can be found in the 2025 Chemical Weed Control for Field Crops, Pastures, Rangeland, and Noncropland (SRP 1190) at: https://bookstore.ksre.ksu.edu/pubs/SRP1190.pdf
Group 15 herbicides. Seedlings absorb these herbicides as they germinate and prevent the production of fatty acids needed for plant growth. The herbicides commonly used in soybeans are acetochlor (Warrant, others), dimethenamid-P (Outlook, others), pyroxasulfone (Zidua, others), and S-metolachlor (Dual, others). They control most annual grasses and small-seeded broadleaf weeds. These herbicides are very important across our crop rotations, and resistance has been reported in Palmer amaranth in Arkansas and waterhemp in Illinois. Therefore, it is very important to manage resistance by applying them in combination with other effective herbicides.
Group 14 herbicides. These herbicides inhibit an enzyme needed to make chlorophyll. The key residual herbicides in this group are flumioxazin (Valor, others) and sulfentrazone (Spartan, others). These herbicides contribute very little grass control to the mix but provide excellent control of pigweeds and morningglories. Group 14 herbicides can cause crop injury if seedlings are exposed to the herbicide due to poor furrow closure or rain splash.
Metribuzin. Metribuzin (Dimetric, others) is a Group 5 herbicide that inhibits photosynthesis. It provides good to excellent control of pigweeds and some large-seeded broadleaf weeds. It can cause crop injury, specifically if soybeans emerge slowly. However, soybean tolerance to metribuzin has generally increased across the industry. Recent research conducted in Kansas and 14 other states suggests that metribuzin rates up to 16 fl oz/A (0.75 lb a.i./A) can be safely used on soybeans.
Group 2 herbicides. Widespread resistance to ALS-inhibiting herbicides has reduced the usefulness of these products for pigweed control. However, products such as cloransulam (FirstRate) are still useful for controlling large-seeded broadleaf weeds like cocklebur, sunflower, and velvetleaf.
Another topic sometimes mentioned when discussing residual herbicide applications in soybeans is the interaction with row spacing. Generally speaking, residual herbicides must remain effective until the soybean canopy closes, so planting in row spacings less than 30 inches has advantages for weed control later in the season. However, narrow row spacing results in more soil disturbance, which can reduce the effectiveness of residual herbicides applied ahead of planting. Sometimes, logistics dictate that fields must be sprayed before being seeded. This means the herbicide layer will be disrupted, and an effective herbicide concentration may not be present in the zone where weed seeds germinate. In these situations, a general recommendation is to plant as soon as possible after spraying.
The use of trade names is for clarity to readers and does not imply endorsement of a particular product, nor does exclusion imply non-approval. Always consult the herbicide label for the most current use requirements.
Sarah Lancaster, Extension Weed Science Specialist
slancaster@ksu.edu
Tags: soybeans weed control pre-plant herbicide pre-emergence residual herbicides