Key Takeaways

• Time applications carefully: Post-emergence herbicide timing should be based on soybean growth stage, pre-harvest interval (PHI) or both to avoid crop injury and herbicide residue concerns. 

• Follow the most restrictive label: When using herbicide mixtures, always follow the most restrictive application restrictions among the products in the tank mix. 

• Know the soybean growth stage: Properly identifying vegetative and reproductive growth stages is essential for determining whether a herbicide can still be legally and safely applied. 

• Confirm trait compatibility: Before spraying, verify that the soybean variety's trait package matches the herbicides being applied.

Early-season weed control is crucial to prevent crop-weed competition and potential yield loss, especially in soybean, where effective post-emergence herbicides are limited. When adequate soil moisture and warm temperatures promote rapid weed germination and growth, soybean fields can quickly develop dense weed infestations (Figure 1) that require timely post-emergence herbicide application for effective control in no-till soybean production system.

When applying post-emergence herbicides, it is essential to consider the soybean growth stage or pre-harvest interval to avoid soybean injury and herbicide residue in the seeds. Herbicide labels specify the maximum soybean growth stage at which broadcast herbicide applications can be made (see Table 1), and these guidelines should be followed closely. When using a mixture of herbicides, always adhere to the most restrictive label instructions among all products included.

How to Determine Soybean Growth Stage

• Restrictions on postemergence herbicide applications in soybean are based on the growth stage or the pre-harvest interval (PHI).
• These restrictions are typically expressed as a specific soybean growth stage or PHI, and sometimes both are provided (see Table 1). When both are listed, follow the most restrictive requirement.
• After the emergence of the first pair of unifoliolate (single) leaves, all subsequent leaves are trifoliate leaves (compound leaves with three leaflets). Vegetative (V) growth stages are determined by counting the number of fully developed trifoliate leaves on the main stem, not on branches.
• The V1 stage occurs when the first trifoliate has fully unrolled and the leaf edges no longer touch. New trifoliates typically emerge every three to five days through the V5 stage. For example, a plant with five fully developed trifoliates is at V5. Beyond V5, trifoliates may emerge more rapidly, approximately every two to three days, under favorable growing conditions.
• The R1 (beginning bloom) stage is defined by the appearance of at least one open flower at any node on the main stem. This typically corresponds to plants in the V6 to V8 range and approximately 15–18 inches tall. The R2 (full bloom) stage occurs when open flowers are present at one of the top two nodes of the main stem with a fully developed leaf.
• When the first pod on one of the four upper nodes reaches 3/16-inch long, the plant is at the R3 stage. Most post-emergence soybean herbicides have restrictions up to the R3 growth stage. Certain herbicides can be applied later in the season when the restriction is based on pre-harvest interval. For example, Select MAX can be applied as long as you can maintain 60 days of pre-harvest interval.
• At the R4 stage, the soybean plant reaches the full pod stage.
• Before applying, ensure the trait package in the soybean variety matches the herbicides.

When using herbicide mixtures, always follow the most restrictive label directions among of the products being used in the mixture.

Use labeled adjuvants with post-emergence application of herbicides. Refer to page 71 in the Nebraska Extension NebGuide EC130, “Guide for Weed, Disease, and Insect Management in Nebraska.”