Pristine beans or good weed control, take your pick

May 23, 2024 9:20 AM
Blog Post

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

HG 14 products are associated with occasional injury to emerging soybean plants, especially when planted in cool and wet soils, poorly drained soils, or if the soybean is planted shallow or furrows are not fully closed. In particular, the HG 14 herbicides 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 occurring at a similar time to 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 HG 14 injury from preplant and preemergence herbicide applications.

HG 14 injury identification

HG 14 products like flumioxazin, sulfentrazone, and saflufenacil can all cause injury to emerging soybean. Symptoms are typically browning (necrosis) of the cotyledons or hypocotyl from herbicide splashing or washing onto tissue (Figure 1). In worse 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. In some very rare cases, splashing of HG 14 herbicides onto the growing point can cause more significant injury, like malformed trifoliate leaves, stunting, or even death of the apical meristem (Figure 2).

Two newly emerging soybean seedlings from the brown soil. Both seedlings have browned tissue on the hypocotyl, a result of a preemergence PPO herbicide and untimely rainfall.
Figure 1. HG 14 injury on emerging soybean seedlings. Note the necrotic tissue stops at the soil line, the result of herbicide contact on the arch of the hypocotyl. Photo courtesy of Meaghan Anderson.

Two small soybean seedlings against a brown background of soil and crop residue. One has malformed leaves and is stunted, while the other has browned leaves and is stunted.
Figure 2. Soybean seedlings with malformed leaves, burned tissue, and stunting from HG 14 splashing. The herbicide in this field was applied approximately two weeks after planting. Photo courtesy of Rebecca Vittetoe.

HG 14 injury look-a-likes

HG 14 herbicide injury may be mistaken for other early-season issues. It differs from seedling disease in that the injury generally stops at the soil line, while seedling diseases often affect the stem and root, extending below the soil line (Figure 3). 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 (Figure 4).

A person's hand holding two soybean seedlings. One is healthy, with green tissue on the stem, cotyledons, and root. The other seedling is a mirror image with a rotted and brown stem and root system.
Figure 3. A comparison of a healthy soybean seedling and one dying of a seedling disease. Photo courtesy of Meaghan Anderson.

A green soybean plant with a halo of brown tissue around the rim of the cotyledon.
Figure 4. The symmetric "halo effect" caused by ILEVO seed treatment on soybean seedlings. Photo courtesy of Brandon Kleinke.

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, though 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.

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Meaghan Anderson Field Agronomist in Central Iowa

Meaghan Anderson is a field agronomist in central Iowa and an extension field specialist at Iowa State University Extension and Outreach. Educational programming is available for farmers, agribusinesses, pesticide applicators, certified crop advisors, and other individuals interested in...