A wet early spring has made it difficult for crop producers to use soil-applied – or residual – herbicides to their fields ahead of planting. Producers should not push the envelope on herbicide treatments while fields are holding too much water, Johnson says.
"If it's dry enough to plant, it's dry enough to spray," Johnson says. "We don't plant our crops when soil conditions are such that we're dropping the seed into mud, so we don't want to apply our herbicides onto muddy ground either.
"We run into challenges when we apply to fields that are too wet. One, we're going to leave ruts in the field. Two, all herbicides are labeled such that they cannot be applied to standing water anyway. If we want to get the most out of our soil-applied herbicides, we need to put them on in conditions similar to those in which we would plant the crops."
As their name suggests, soil-applied herbicides are sprayed on the soil before, during or just after crops are planted. While too much water nullifies their effectiveness, most residual herbicides need some soil moisture in order to work properly.
The herbicides are activated by moisture within the soil and are then absorbed by weed seedlings. Weed growth stops or is stunted, leading to plant death shortly after it emerges.
"A residual herbicide will have activity in the soil anywhere from about one to six weeks after application, depending on the product that you use," Johnson says. "So what you essentially get, then, is herbicide activity that you don't really see because you have fewer weeds coming up. The weeds that do emerge from that residual system are going to be a little bit easier to control with your post-emerge herbicides."
Excessive wetness isn't the only challenge to using soil-applied herbicides. Johnson says farmers should be careful to:
"Using the appropriate residual herbicide on your worst weed problems and then saving glyphosate for the postemerge cleanup treatment is going to be the best way to protect crop yields and slow the development of glyphosate-resistant weeds," he says.
For more information on weed control in agricultural crops, visit Purdue Weed Science.