If you farmed in the early 1990s era or before, you remember the different years when herbicide carryover was an issue. It's linked more so to residual herbicides. The dramatic switch to Roundup Ready crops in the late 1990s made it somewhat of a moot issue. Now, more farmers are applying residual herbicides again to combat weed resistance.
Enter the drought of 2012. It's prompting the need for farmers and their crop consultants to rethink herbicide chemistry related to potential carryover and how to avoid damage to the following crop. The dry conditions that prevented residual herbicides from providing effective weed control also make carryover a possibility, notes Mike Owen, an Iowa State University Extension weed scientist.
The active ingredients in herbicides vary dramatically in how fast they break down in the soil, he explains. For example, thifensulfuron has very little persistence. Picloram can hang around for several months. Even within the same chemical family, persistence varies. In the imidazolinone family, imazamox persists for a much shorter time in the soil than imazethapyr.
Carryover concerns for 2013, one of the consequences of this year's drought
But what if you get enough rainfall by spring? Won't that remove the carryover risk? Not necessarily, says Owen. Time is a big part of this too, as are other factors such as soil pH and the particular herbicide, application rate and date you applied it. Sensitivity of the rotational crop to injury risk is another consideration.
“Reduced herbicide degradation is one of the consequences of the current drought," Owen warns. “It could lead to damage in rotational crops next spring. The risk of carryover injury will vary widely from field to field, making it important to evaluate each field individually as you plan ahead this fall."