Soybean Crop Staging and Herbicide Application Timing

Cool temperatures, experienced by much of the province this spring, can often result in the soybean crop developing at a slower pace than many annual grass and broadleaf weeds. You may be faced with a scenario where there is an abundance of weeds at or near the maximum leaf-stage for effective control, yet the soybean crop is not at the growth stage specified on the product label. As a producer you are now faced with the ultimate risk/benefit analysis. Do I sacrifice the level of weed control, or risk possible crop injury?

Is there any risk in applying an herbicide before its specified label stage?

In general, yes. The application stage specified on the label is there because:

1. There is an increased chance of crop injury when the product is applied outside the application window, or
2. Research on the product was conducted at the stages specified on the label. Therefore crop tolerance to applications outside those stages are unknown. 

In any case, there is a level of risk to applying a herbicide before the recommended crop stages Refer to Table 1 for a breakdown of ideal crop application stages. The decision to “pull the trigger” will ultimately come down to what growth stage the weeds are at. For example, lets assume you scout your field and the soybean crop is at the cotyledon stage, yet common lamb's-quarters is at the 4-6 leaf stage and at extremely high densities. In this situation, it is probably more advantageous to spray a suitable herbicide as soon as possible versus waiting until the specified crop stage. Refer to Table 2 for a list of maximum weed leaf stages where a specific herbicide will still be effective.

Table 1. Soybean application stages for a number of post-emergent herbicides.

Table 2. Maximum weed leaf stage where the herbicide application is still effective according to the product label.