For various reasons, each year herbicides applied preemergence fail to control weeds. Timely application is critical for optimum weed control. Residual preemergence herbicides must be applied before crop or weed emergence. While moisture is needed to activate residual herbicides, too much rain after a preemergent application increases the likelihood of herbicide leaching.

Weather variability is a major cause of disappointing performance of herbicides applied to the soil surface, notes Bob Hartzler, Iowa State University Extension weed management specialist. If weeds have germinated and emerged before the herbicide is activated, your residual herbicide program will fail.

This spring in Iowa, planting has occurred at a fast pace in much of the state due to lack of rain in late April. The string of dry-weather days also allowed most herbicide applications to be made in a timely fashion, but there’s concern about herbicide performance after a prolonged time on the soil surface. Hartzler provides the following information and observations to help farmers evaluate their preemergence herbicide performance.

Preemergence herbicides must be present within the soil profile when weed seeds begin to germinate to have maximum effectiveness, Hartzler says. Preemergence herbicides are inactive while on the soil surface. It typically takes a half inch of rain to move herbicides into the soil profile where weeds germinate; there are relatively small differences among herbicides in the amount of rain required to activate. The most consistent performance occurs when it rains within five to seven days of application, especially if soils are warm enough for weed germination.

The second role of rain is to keep a portion of the herbicide dissolved in the soil so the chemical can be absorbed by germinating seeds. Residual herbicides are less active in dry soil due to less herbicide being in solution. The terms reactivation and recharge are used to describe the activity of herbicides on weeds that escaped control due to dry conditions during germination. Group 27 herbicides display this property more frequently than other herbicide groups, but these products still perform best when available while seeds germinate.

Fate of herbicides on soil surface

Herbicides typically are more persistent in dry soil due to lower degradation rates, Hartzler explains. But in some cases, herbicide may be lost from the soil surface. Volatilization is a concern since the soil surface reaches temperatures much higher than the air temperature. Several older herbicides (such as trifluralin, EPTC, propachlor) had relatively high vapor pressures, and if left on the soil surface, enough of the herbicide could be lost to compromise its weed control performance. 

Photodegradation (breakdown by light) can also be a source of loss for certain herbicides from the soil surface. For most products, Hartzler doesn’t think enough herbicide will be lost from photodegradation or volatilization to compromise activity.

Another concern is wind erosion — blowing of surface soil. Much of Iowa experienced very strong winds during the last week of April, and that undoubtedly moved herbicide on the soil surface. Hartzler suspects many fields have areas where enough herbicide was moved to limit herbicide activity once rain occurs to activate it. This risk will be influenced by field topography and tillage practices, as crop residue on the soil surface should reduce movement. He says wind movement will be more of a concern than losses due to volatility or photodegradation in most fields.

Evaluating herbicide performance  

Herbicides on the soil surface aren’t absorbed by germinating seeds, thus any weeds that germinate will be unaffected by this herbicide, Hartzler says. Fields at greatest risk for weed escapes are those planted as soon as they were fit to work. In this situation, there probably was enough soil moisture in the upper soil surface for weeds to germinate. Fortunately, soil temperatures during early planting were cool enough that many of Iowa’s important weeds (foxtails, waterhemp) hadn’t begun to emerge. However, early-season weeds such as giant ragweed and common lambsquarters could easily escape due to lack of activation.

In some areas, surface soils might have dried sufficiently at time of planting and preemergence herbicide application that few if any weeds will germinate until more rain is received. In these situations, the main concern is the loss or movement of herbicide on the surface.

When rain occurs, the herbicide on the soil surface will be moved into the profile where it will be active. It takes slightly more rain to activate herbicides with dry soil since the initial rain wets the soil rather than percolates through the soil, carrying the herbicide into the profile.

What actions can be taken?

What can you do to minimize herbicide performance failures? Some weed scientists promote the use of rotary hoes or harrows when rain fails to provide timely herbicide activation. These tools move some of the herbicide off the surface, but more importantly they control weeds that have germinated and escaped the preemergence herbicide.

For maximum effectiveness, a rotary hoe or harrow should be used while weeds are in the white root stage — germinated but not yet emerged. While they are operated at high speeds and thus cover acres quickly, these tools are not as effective in fields with high amounts of crop residue on the soil surface.

Probably the most important thing to do is to systematically scout fields beginning about 14 days after planting, Hartzler says. Preemergence herbicide performance across fields likely will be more variable this year than in other years, so be sure to cover the entire field.

Some fields will require an earlier postemergence application than normal to control weeds that escaped the preemergence treatment due to lack of activation. Consider including a layered residual herbicide with these postemergence applications since there will be a longer interval between the post application and crop canopy closure.  

“Since it’s impossible to know whether a significant amount of the preemergence herbicide was lost from the soil surface, there is no need to ‘blindly’ try and supplement the herbicide with additional residual herbicide,” he says. “Use scouting to determine when to make postemergence applications at the optimum time, and knowledge of the field’s weed history to guide the need for using a layered residual with the postemergence herbicide treatment.”