by Rodrigo Werle (UW-Madison Extension Cropping Systems Weed Scientist)
and Shawn Conley (UW-Madison Extension Soybean and Small Grain Specialist)
Spring has finally arrived in Wisconsin and so has the planting season. Before getting your soybeans in the ground, make sure you have plans for your PRE-emergence herbicide program. According to our recent SURVEY, several Wisconsin soybean fields get only treated with a one-pass POST-emergence herbicide program. Because of the spread of weed species resistant to glyphosate and/or other POST-emergence herbicides (e.g., ALS- and PPO-inhibitors), a one-pass POST-emergence program in soybeans is no longer a viable strategy.
Moreover, research conducted across the Midwest has demonstrated the importance of keeping your soybean crop weed-free from establishment through the V3 growth stage (3rd trifoliate). Weeds emerging after the V3 growth stage will likely not impact soybean yield; however, they should still be proactively managed to prevent them from reproducing and replenishing the seedbank (remember “no seed, no weed”).
Thus, PRE-emergence herbicides can help farmers maintain their fields weed-free during initial establishment of the crop (= achieve full yield potential) and also reduce the selection pressure on POST-emergence herbicides (due to fewer weeds to be controlled POST-emergence), helping on the fight against herbicide resistance.
Several PRE-emergence herbicides are available for soybeans. Most of them can be applied before or up to 3 days after planting. To maximize their residual activity in-season, PRE-emergence herbicides should be sprayed at or shortly after planting. PRE-emergence herbicides need moisture for incorporation and activation in the soil. If there are established weeds at the time of PRE-emergence application and no additional pre-plant field cultivation will take place, it’s important to have an effective burndown herbicide in the tank-mix. A PRE-emergence herbicide containing 2 or more effective modes of action (MOA) will likely provide control of a wider range of weed species when compared to the use of a single MOA. Using multiple MOA during each pass is also a proactive strategy for herbicide resistance management.
It’s important to note that under cool and wet conditions, PRE-emergence herbicides containing metribuzin (Group 5) and/or PPO-inhibitors (Group 14; e.g.: flumioxazin, saflufenacil, sulfentrazone) may cause some crop injury, particularly in lighter soils with low OM and/or higher pH. Our preliminary research in Nebraska has demonstrated that early-season crop injury caused by metribuzin (group 5) and sulfentrazone (group 14) did not lead to yield reduction (soybeans are indeed very resilient). Moreover, the benefit of an early-season weed-free field outweighs the concerns of early-season crop injury (assuming a herbicide is applied according to the label and no significant stand reduction is observed).
Metribuzin injury in soybeans (note healthy new growth):