The spread of multiple herbicide resistant weeds brought an end to the era of total postemergence programs in soybean. Unfortunately, a prolonged rainy period prevented applications of preemergence treatments on many planted soybean fields in certain areas of the state. By the time fields are fit for field operations soybeans likely will be emerging and limit herbicide options in those fields. Without careful management and a bit of good fortune, this situation will make it difficult to stay ahead of the weeds for the remainder of the season. Controlling emerged weeds and applying an effective residual herbicide in these fields should be a top priority moving forward.

If soybeans are already in the ground, there is a good likelihood the planned preemergence program cannot be used once fields dry out. HG 14 products (e.g. flumioxazin, sulfentrazone) are the backbone of many preemergence programs; labels of most products containing these active ingredients restrict applications to within three days of planting. Delayed applications greatly increase the risk of injury due to the likelihood of the hypocotyl being exposed to high concentrations of the herbicide on the soil surface (Figure 1). In severe cases, this results in widespread seedling death from girdled hypocotyls (Figure 2).

Figure 1. Necrotic lesions on cotyledons associated with applications of HG 14 products

Figure 2. Girdling of hypocotyl with HG 14 product, usually associated with heavy rain at time of emergence

Table 1 lists herbicides that can be applied following soybean emergence for residual weed control. Herbicide common names are used due to the abundance of products on the market. Read the herbicide label prior to use as restrictions may vary on products. Although the products available for application after crop emergence are not as broad-spectrum as materials used before emergence, they will increase the odds of staying ahead of weed problems throughout the season.

Table 1. Residual herbicides approved for use following soybean emergence

Even with the recent cool temperatures, weeds are still emerging in fields. Scout fields prior to application of the preemergence product to determine if a postemergence product is necessary. Control these weeds as soon as possible with an appropriate herbicide (e.g. glyphosate if glyphosate-resistant beans), and include the preemergence herbicide with this application. Many winter annuals (e.g. marestail) may be too large for consistent control in no-till fields. Adding multiple products to try to improve control is usually unproductive.

Corn does not pose as significant a challenge due to the wider availability of products and generally wider windows for application. However, treating these fields with a combination of products to both control emerged weeds while they are small and provide residual control is also critical.

Originally published by Iowa State University.