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):