Giant ragweed is a growing headache in soybeans, but there are new ways to nail it.
FirstRate, a herbicide introduced last year, is getting good marks from weed scientists for its ability to control this towering pest.
Giant ragweed is tough because it germinates early, can thrive over a wide range of temperatures, soil moistures and soil depths, and can regrow from axillary buds. That's according to Rebecca Franey, University of Illinois weed science graduate student.
Ohio and Kentucky are especially hard hit by giant ragweed, reports Jeff Stachler, Ohio State University extension weed scientist. The weed is getting worse in other states as well, he says.
"If giant ragweed is the major problem, the most consistent control program we are seeing is something applied pre-emergence at a reduced rate, such as Scepter, Squadron, Canopy or Canopy XL, followed by FirstRate," Stachler reports. "With our heavy pressure, we need a two-pass program."
Since all the above products are ALS inhibitors, Stachler recommends that growers avoid ALS inhibitor herbicides in alternate years on corn.
At the University of Illinois, Franey and weed scientist Steve Hart used FirstRate in preplant incorporated (PPI), pre-emergence, early postemergence, postemergence and late postemergence applications for giant ragweed control. They also looked at FirstRate plus Cobra postemergence.
"Soil-applied FirstRate will provide suppression of giant ragweed and complete season-long control if rainfall is adequate following herbicide application," says Franey. "However, we observed the most consistent control with early post or postemergence application."
Adding Cobra to FirstRate did not improve giant ragweed control, says Franey.
"FirstRate is one of the best products I have seen for giant ragweed control if you apply it when the ragweed is 6-8" tall," says Hart. "Since it has some residual activity, it may make a good mix with Roundup Ultra on Roundup Ready varieties."
Ohio's Stachler points out that FirstRate, when applied pre-emergence, is weak on black nightshade. When applied postemergence it's weak on nightshade, plus lambsquarters and pigweed/waterhemp.
To control those weeds, he suggests that it be tankmixed with Dual II Magnum, Frontier, Lasso, Python or a reduced rate of Scepter if applied pre-emergence. It should be mixed with Cobra, Flexstar, Pinnacle, Pursuit or Raptor, depending on the targeted species, when applied postemergence.
"Raptor would be the best single tankmix partner for broad-spectrum weed control," Stachler states.
John Oesch, with Terra at Atlanta, OH, reports that FirstRate cleaned up 2"- to 3"-tall giant ragweed last summer in rescue treatments.
"In extreme cases we spiked it with Reflex to speed up the activity. It was the best giant ragweed control we have seen.
"This year, where we have a history of giant ragweed problems, we will apply a pre-emergence product at a reduced rate, followed by a full rate of FirstRate postemergence," Oesch explains. "We like the way FirstRate also suppresses Canada thistle."
University of Illinois weed scientist Marshall McGlamery has compiled a herbicide rating scale that includes ratings from Illinois and up to 11 other states. The combined rating shows FirstRate to be the most effective product against giant ragweed among the various alternatives in both the PPI-pre-emergence and postemergence categories.
Some of the states in the survey use a numerical system (1-10), while others use a letter scale (E, G, F & P for excellent, good, fair and poor).
FirstRate, applied postemergence, had a mean score of 9.0-E. Roundup Ultra, which had the second-best rating, had a score of 8.4-GE.
In the pre-emergence category, FirstRate posted a mean score of 8.2-G. Canopy XL, with the second-best score, was rated at 8.0-G.
Besides its positive ratings for effectiveness, FirstRate appears to have good crop tolerance, says Ohio's Stachler.