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Drought raises concerns about herbicide carryover into 2013
Iowa Correspondent

HARLAN, Iowa — While agronomists predicted the effects of severe drought that scorched most of the Midwest last summer would likely cross over well into 2013, one of the more notable repercussions concerning farmers is herbicide carryover.

“Yes, there is some talk about carryover being an issue,” said Clarke McGrath, Iowa State University extension field agronomist and partner program manager for the Iowa Corn & Soybean Initiative. “I might not characterize it as a widespread concern, but rather a sense of awareness based on experience.”

As a young agronomist in the late 1980s, McGrath still recalls some of the herbicide carryover issues that followed 1988’s drought.
“It was stressful for growers and their families, retailers and their families, and the local chemical reps as well,” he said. “Our experiences (as agronomists) heighten our awareness of the issue. I was just a kid, but I will never forget the long summer of 1989.”
When his dad was a retail co-op manager, McGrath said he watched him work hard with customers and chemical companies to help remediate damages from products applied in 1988 that carried over and damaged later crops. As a retailer himself for more than a decade, McGrath and his colleagues also saw carryover occasionally from smaller areas that were dry, even though “we hadn’t had a widespread drought since 1988.

“But in these dry pockets, overlap areas would occasionally show serious crop response to carryover the next year,” he said, “and then growers would question what was going on in the rest of the field.”

While there may not have been many visual symptoms, McGrath said “guys would still worry that yield could be impacted.
“Typically, there wasn’t a problem at harvest time other than the overlap areas, but once in a while there were bigger issues. So, with all that experience in mind for our clients, we will be watching.”
According to John Sawyer, ISU extension soil fertility specialist and professor of agronomy, “the rapid pace of planting in late April followed by rain has resulted in many fields having emerged corn before preemergence herbicides and nitrogen applications have been completed.

“Of particular concern are no-till fields where planting was completed prior to killing emerged weeds,” he said in a university report with Bob Hartzler, ISU extension weed specialist and professor of agronomy.

But McGrath said much depends on the type of herbicide used.
“A few of the herbicide active ingredients for corn and soybeans have characteristics that could lead to carryover in 2013,” he said. “The high-risk category includes atrazine, chlorimuron (Authority XL, Canopy, Envive, Valor XLT and others), imazaquin (Scepter) and simazine (Princep and others).”

Bill Johnson, Purdue University extension weed specialist, said in a drought year the lack of moisture slows carryover. However, he said the largest concern last year was the carryover of atrazine and its subsequent injury on wheat. “It is off-label to plant any crop other than corn or sorghum during the same calendar year of an atrazine application,” he pointed out.

While labels vary on exact rotational restrictions, Johnson said most atrazine premix labels range from 14-15 months. In addition, fomesafen applied post-emergence in soybeans could also injure wheat since the rotational restriction for fomesafen is only four months after application.

Johnson said the carryover time could be longer for areas with little, if any, rainfall. He said producers who applied a fomesafen product to soybeans last summer and haven’t seen significant rain following application “should be aware of the potential for injury on emerging wheat.”

For farmers suspecting carryover, Travis Legleiter, Purdue extension weed scientist, advised taking soil samples from suspected carryover fields and having them analyzed in a commercial lab – although this option could be costly.

“Both bioassays and lab analysis should either be done in late fall or early spring to allow for maximum herbicide degradation and provide a more representative result of potential injury at planting,” he said.

The best management practices from start to finish, McGrath said, are: good residue and soil management; optimal seedbed conditions; proper seeding depth; optimum plant populations and spacing; sound soil fertility programs; and integrated pest management of weeds, insects and diseases can minimize the effects of herbicide carryover.

“Evaluate the 2012 herbicide program for compounds that pose a carryover risk,” he said. “Consider rates, application date, soil characteristics and label restrictions. Keep in mind that if rainfall returns to normal, this rain will have much less effect on herbicide degradation than had it occurred near the time of herbicide application.”

While tillage should dilute herbicide residues within the soil profile, McGrath added “past experience has shown that this practice does not consistently reduce crop injury from herbicide residues.
“The bottom line is that there is no great way to predict what will happen,” he said, “so our role as growers is to implement the best management practices for the crop and do good scouting.”