It's getting near the end of April and fieldwork is running behind because of the wet spring. Getting the crop in the ground becomes a priority for many farmers. Some want to skip the preplant burndown herbicide application in no-till, and apply the herbicide later on, after planting. "This puts the crop at added risk of early season competition and yield loss since the weeds have a head start over the crop and can impact yield potential rather quickly," says Brian Lang, Iowa State University Extension field agronomist at Decorah in northeast Iowa.
If you're using 2,4-D as that burndown, ignoring the planting delay between application of 2,4-D and planting also carries the risk of crop injury, he notes. The magnitude of that risk depends on the rate, soil type and other factors.
Planting into standing weeds is risky
No-till farmers may plant fields prior to killing weeds with a burndown herbicide, with the intent of returning later to control the weeds with an early postemergence application. "While this strategy can be effective, it is important to realize that planting into established weeds greatly shortens the time required for weeds to impact crop yields," says Bob Hartzler, an ISU Extension weed management specialist. "That is, it shortens the "critical period.'"
When crops are planted into a weed-free seedbed, weeds typically compete with the crop for three to four weeks after crop emergence without impacting yield, but this isn't he case when planting into established weeds. Factors that will influence the critical period include:
• density of weeds
• size of weeds
• cultural practices such as row spacing, seeding rate, etc.
• environmental conditions
"Because of the many variables involved, it is impossible to accurately predict when weeds begin to affect yields," says Hartzler. "However, weeds present at planting may affect yields as early as the V1 stage of crop development. That's the first leaf stage. Thus, appropriate herbicides should be applied as soon after planting as possible to remove these weeds."
Due to the increased cost of glyphosate this year, an increase in the use of preemergnece herbicides is anticipated in order to reduce the number of postemergence glyphosate applications. Certain preemergence herbicides can't be applied if the crop has emerged, so be sure to determine label restrictions and crop stage of development prior to application.