Be aware of the potential for herbicide carryover

The effectiveness of soil-residual herbicides at controlling weeds has been reduced due to the dry soil conditions in most of Illinois during the 2012 growing season. Associate professor of weed science Aaron Hager warned that these conditions also slow the rate of herbicide degradation, increasing the likelihood that herbicide carryover could damage rotational crops.

"Once a herbicide is applied, there is little that can be done to shorten the time it remains active in the soil environment," Hager said. "Herbicide persistence depends on the interaction of the herbicide's characteristics, the soil, and climactic conditions"

Characteristics of the herbicide molecules affect its persistence in the soil. Some herbicides, such as thifensulfuron, have very little soil persistence while others, such as picloram, can persist for several months.

"Soil persistence can also vary among herbicides within a particular chemical family," Hager noted. "For example, within the imidazolinone herbicide family, soil persistence of imazamox is much shorter than that of imazethapyr."

Chemical, physical, and microbial soil properties also influence herbicide persistence. For some herbicides, soil pH is important because it impacts how much herbicide is available for plant uptake and how quickly it is degraded by hydrolysis.

"For example, atrazine is more available for plant uptake when the soil pH is approximately 7.0 or higher," Hager explained. "It also persists longer because the rate of hydrolysis slows at higher soil pH values. The degradation rate of many sulfonylurea herbicides slows with high soil pH values. Imidazolinone herbicides persist longer in soils with low pH values."

Physical properties of soils, such as the sand, silt, clay, and organic matter content, affect herbicide persistence. Soils with high clay and organic matter content adsorb more herbicide onto soil colloids than coarse-textured soils or soils with less organic matter.

"Herbicide bound to soil colloids cannot be taken up by plants, move downward through the soil profile, or be degraded by microbes," said Hager. "Rotational crops can be injured when water molecules displace these bound residues late in the season or in the following spring."

Soil microorganisms degrade many herbicides; the speed of degradation depends on the levels, species and activity of microorganisms. They work most efficiently when there is adequate soil moisture. Under extremely dry conditions, microbial degradation can slow enough to allow the herbicide to persist into the next growing season.

What can be done to minimize the risk of injury to rotational crops from herbicides applied this season?

"Apart from a return to normal or above normal precipitation amounts, there is little that can be done to accelerate herbicide degradation at this point," said Hager. "Degradation requires time and moisture."

Shallow tillage can help distribute herbicide more evenly across a field, but it will do little to accelerate herbicide degradation in extremely dry soil. If planting is early next season, the likelihood of crop injury from herbicide carryover could increase.

Planting the same crop that was planted in 2012 in the following season would eliminate the potential for crop injury from herbicide residues. This option may not be feasible for every situation where herbicide carryover is possible, but it warrants some consideration. If crop rotation must occur where there is a risk of herbicide carryover, delaying planting as long as possible could allow time for additional herbicide degradation.

"Several producers have asked about the feasibility of planting a cover crop, fall forage crop, or small-grain crop after the 2012 corn crop is harvested or destroyed," Hager said. Most herbicide labels indicate the amount of time that must elapse between application and planting the rotational crop. Labels of most corn herbicides also indicate an interval that must elapse between application and harvest for grain, silage, or grazing.

"In many instances, the interval between application and winter wheat planting will not be long enough this year," Hager warned. "Small-grain crops can be sensitive to even small amounts of herbicide residue, such as atrazine, that remain in the soil."