Considerations for early weed control problems
The effectiveness of early herbicide programs can be affected by various environmental factors.
First is the effect of temperature on burndown applications. Michigan State University conducted trials on the control of chickweed and henbit when burndown herbicides were applied at various air temperatures ranging form 47 to 87 degrees F. Common chickweed control was consistent when glyphosate was used. When Gramoxone was used the control of chickweed was compromised at lower temperatures.
They also found that there was more of a temperature response with both glyphosate and Gramoxone for henbit control which is more difficult to control. The general conclusion was that increases in air temperature significantly enhanced weed control and reduced weed biomass.
With the fluctuating tempeeratures that we have experienced this year -- initial early weed control may not have been what was expected.
The trials also indicated that the treatment that was least affected by temperature was the burndown herbicide that was tank mixed with a soil-applied herbicide with residual activity. The improved weed control may have been the result of the residual activity and not the burndown herbicide.
Conditions in March and early April may have not been conducive for good performance of soil-residual herbicides. Soil applied herbicides that are sprayed onto a dry soil surface with no incorporation (mechanical or precipitation) for several days after application may be significantly reduced in effectiveness.
In general surface-applied herbicides need 1/2 to 1 inch of rain within 7 to 10 days.
In a few instances the soil residual herbicide might "reach back" to control small emerged weeds after precipitation.
It is more likely that emerged weeds are likely to survive a recently "activated" soil-residual herbicide. If emerged weeds are taller than 1 inch it would be bet to control these weeds rather than wait several days after precipitation to see whether a soil-applied herbicide will control them.
The residual herbicide may still provide weed control for weeds that have not emerged.
Any emerged summer annual weeds need to be controlled before planting. These weeds can be quite competitive with newly emerged seedlings.
If there are any herbicide resistant weeds in the field -- control before planting has more options than after planting.