Improved rainfall patterns and an early spring have provided corn and soybean producers with opportunities for planting and the application and activation of soil residual herbicides.
Soil residual herbicides that target your most troublesome weeds provide an excellent start in managing herbicide-resistant weeds by targeting them when they are most vulnerable. They also provide the added benefit of reducing crop yield-loss due to weed competition from delayed post-emergence herbicide applications.
"Remember, Minnesota is noted for its diverse weather patterns," said Jeff Gunsolus, an agronomist with University of Minnesota Extension. "Extreme shifts to wet, hot, dry or windy weather patterns can result in untimely post-emergence application and poor weed control."
Some weeds will escape control even under ideal rainfall activation conditions. Gunsolus advises checking your fields for such escapes 10 to 14 days after herbicide application. "Weeds that are susceptible to the herbicide can still escape control by emerging from deeper in the soil profile or they may be present at high densities in the soil," he said.
Your next step will be targeting post-emergence herbicides to small weeds, ideally weeds 1-3 inches in height and no more than 4 inches tall, to ensure good weed control and limit weed-crop competition.
For all post-emergence herbicide applications, use full rates, high quality adjuvants, and always add ammonium sulfate with glyphosate and Liberty. Due to difficulties in controlling early-emerging common lambsquarters, include additional nonionic surfactant at 0.25 percent volume/volume to all fully loaded glyphosate formulations to improve lambsquarters control.
Use the proper spray volume, based upon the type of herbicide(s) being applied. For example, glyphosate is a translocating herbicide (glyphosate moves within the plant) and should be applied at less than or equal to 10 gallons per acre of spray volume for maximum effectiveness.
Liberty and Flexstar are contact herbicides (they do not move within the plant) and should be applied at greater than 15 gallons per acre of spay volume for maximum effectiveness. If contact herbicides are mixed with translocating herbicides, apply the spray volume for contact herbicides.
Scout your fields for effective control five to 10 days after your post-emergence herbicide application. Scouting will allow you to determine if any timely rescue management practices will be necessary.
For those of you discouraged by dry soil conditions that resulted in poor activation of soil residual herbicides, Gunsolus asks you to remember that later rainfall events will activate the herbicide and reduce the likelihood of late-emerging weeds (waterhemp, for example) from emerging and going to seed in the fall.
"Keep in mind that the long-term goal of weed management is to deplete weed seed reserves and to reduce selecting for herbicide-resistance by diversification of weed-control tactics," Gunsolus said. This would include timely rotary hoeing as weeds begin to germinate in the spring and inter-row cultivation of weed escapes.