Wallaces Farmer

Take your pick: Pristine soybeans or good weed control?

Group 14 herbicides occasionally injure emerging soybean plants. The good news is, plants often outgrow injury with good growing conditions. Meanwhile, Group 14 herbicides provide excellent residual weed control.

Meaghan Anderson

June 3, 2024

4 Min Read
injuries to soybean plants
EMERGING INJURY: Herbicide Group 14 injury on emerging soybean seedlings. Note the necrotic tissue stops at the soil line, which is the result of herbicide contact on the arch of the hypocotyl.Courtesy of Meaghan Anderson

While the title is written with some jest, the sentiment is one I’ve been thinking about a lot this spring. With few effective postemergence herbicide options to control waterhemp, farmers must prioritize residual herbicides as a first line of defense. While numerous residual herbicides are available, most soybean residual programs are based on the PPO inhibitors (Herbicide Group 14, or HG14).

HG 14 products are associated with occasional injury to emerging soybean plants. This occurs especially when soybeans are planted in cool and wet soils, poorly drained soils — or if the soybean is planted shallow or furrows are not fully closed.

Group 14 herbicides particularly pose the greatest risk when a significant rainfall occurs just before, or as, soybean emerge. Most of these products must be applied no more than three days after planting due to the risk they pose to emerging soybean.

While rainfall typically moves these products into the soil and safely away from emerging soybean, the likelihood of a rainfall coinciding with soybean emergence increases as the window between application and crop emergence decreases. Due to the combination of early soybean planting and timing of rainfall events, field agronomists have received phone calls regarding soybean exhibiting Group 14 injury from preplant and preemergence herbicide applications.

HG 14 injury identification

HG 14 products such as flumioxazin, sulfentrazone and saflufenacil can all injure emerging soybean. Symptoms are typically browning (necrosis) of the cotyledons or hypocotyl from herbicide splashing or washing onto tissue In worst-case scenarios, the hypocotyl can be girdled, resulting in plant death. Only tissue contacted by the herbicide turns brown, resulting in spots or uneven lesions (see photo above).

In rare cases, splashing of Group 14 herbicides onto the growing point can cause more significant injury, such as malformed trifoliate leaves, stunting or even death of the apical meristem.

Courtesy of Rebecca Vittetoe

HG 14 injury look-a-likes

HG 14 injury may be mistaken for other early-season issues. It differs from seedling disease in that the injury generally stops at the soil line. Seedling diseases often affect the stem and root, extending below the soil line.

Courtesy of Meaghan Anderson - A comparison of a healthy soybean seedling (left) and one dying of a seedling disease

HG 14 injury also differs from the characteristic “halo effect” of the Ilevo seed treatment, which causes a slight brown ring around the outer margin of soybean cotyledons.

Courtesy of Brandon Kleinke - Symmetric

What to do

This kind of herbicide injury is only 100% preventable by avoiding these herbicides, which is not a recommended practice. We must manage herbicide programs to maximize waterhemp control and tolerate injury when it occurs. However, we can take steps to minimize the risk of injury.

Follow herbicide label recommendations for rate and timing of application based on soil type and potential risk to crop.

Plant sufficient plant populations to withstand small losses from herbicide or other early season injury.

Damaged plants usually outgrow early injury with good growing conditions. Injury often looks much worse than it is during initial evaluations. Carefully evaluate the plant population once all plants have emerged to determine whether filling in a soybean stand is necessary.

Replant is usually unnecessary if the stand is uniform and more than 70,000 plants per acre remain. This is approximately four plants per foot of row on 30-inch rows and two plants per foot of row on 15-inch rows. If replant is necessary, fill in soybeans at a partial seeding rate to compensate for the original losses; this is usually easiest to accomplish by seeding perpendicular or on an angle from the original stand.

Anderson is an Iowa State University Extension field agronomist.

About the Author(s)

Meaghan Anderson

Meaghan Anderson is the Iowa State University Extension field agronomist for east-central Iowa. He areas of expertise include weed management, weed biology, cover crops, corn and soybean management, and Integrated Pest Management.

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