Seedling soybeans felt the burn of protoporphyrinogen oxidase (PPO) inhibitor herbicide in 2017. The herbicide may have caused the actual physical injury, but put the blame on the sustained cold, wet weather following planting for the situation. In a normal, sunny spring, the soybean plant should metabolize the herbicide before injury symptoms develop.
University of Illinois weed scientist Aaron Hager hopes this season's injuries don't cause growers to pull away from use of soil residual herbicides. Group 14 PPO chemistry has become increasingly important in managing tough-to-control weeds such as Palmer amaranth, common waterhemp and marestail.
"We have known for decades if you put a soil-applied herbicide close to the seed at planting, and it turns rainy and cool, there is a risk," Hager said. "This is not anything that is unique to the PPO herbicides that are commonly used today."
There are many commercial products containing PPO-inhibitor mode of action including Authority products, Enlite, Envive, Fierce, OpTill, Sharpen, Sonic, Spartan and Valor. They work by creating oxygen radicles at toxic levels that destroy the lipids of cell membranes and create the necrotic spotting and burning.
Under good growing conditions, the soybean is capable of rapidly metabolizing the herbicide. However, under stressful growing conditions, soybean plants are unable to metabolize these herbicides as quickly.
"Lesions on cotyledons and the hypocotyl are often very apparent when the soybean is under stress," Hager said. "But early-season injury often looks worse than it is."
Injury to the crook of the cotyledon and lesions on the cotyledons can cause seedling death. "However, fields can usually incur significant seedling loss and not impact yield since soybean plants compensate where gaps appear in stands," he added.
Growers looking to avoid PPO-inhibitor herbicide damage have some control. Certain soybean varieties can be more sensitive to some chemistries than others. The trick is knowing how a variety will respond, since most seed companies don't commonly screen for PPO tolerance.
"For example, when we first worked with Sharpen, we saw varieties that were very sensitive, but they were not always as sensitive to Valor or Authority," Hager noted.
"None of the soil-applied PPOs are more potent than another, but pay attention to the label and use rate. And, remember the overriding factor is often the environment as the soybean plant begins to emerge. Adverse weather can sometimes overcome the natural tolerance of a variety to a PPO inhibitor," Hager said.
"When a PPO is laid over the top after soybeans are planted and followed by a rainfall event and cool temperatures as the plants begin to emerge, it's not if there will be injury but how bad will the injury be," he said.
PPO injury can also increase in marginal soil or no-till conditions if the seed is left on the surface or planters don't close the slot, and more herbicide leaches down.
The risk of damage is reduced by applying PPO inhibitor herbicides seven to 10 days before planting. However, there are trade-offs. "When a PPO is laid down a week before planting, you reduce the risk of damage; but, the trade-off is that waterhemp will continue to emerge after your (PPO) residual has run out of steam," Hager said.
While PPO damage is a risk, it's rarely economic, and the seedling often recovers with the onset of warm and dry weather. It's more important to protect the crop from early-season weed pressure, and that requires keeping residuals in the herbicide program.
If you have a question, email Dr. Daniel Davidson at email@example.com.
The information provided is general only and should not be taken as a professional recommendation.
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