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Giant ragweed control with FirstRate
by Bob Hartzler

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February 2 -  2000  Giant ragweed is a serious weed problem in soybeans, particularly in eastern Iowa.  Giant ragweed is one of the earliest emerging summer annuals and grows relatively rapidly, even with the cool temperatures experienced in early spring.  The rapid growth of giant ragweed often allows it to reach sizes that result in incomplete control with many postemergence herbicides.  FirstRate (cloransulam) quickly became the leading choice for postemergence giant ragweed control in soybeans because of its ability to control large giant ragweed plants1

Researchers at the University of Illinois recently published results of research investigating the influence of FirstRate application timing on giant ragweed control (Franey, R. J. and S.E. Hart.  1999.  Time of application of cloransulam for giant ragweed control in soybean.  Weed Technol.  13:825-828.).   Research was conducted at DeKalb, Illinois during 1996 and 1997.  All treatments included a non-ionic surfactant (0.25%) and UAN (2.5%).  Grasses were controlled with a separate application of Poast Plus.  The soil was a Drummer silty clay loam with 6% OM and a pH of 6.1.  Rainfall patterns were different in the two years:  in 1996 there was approximately 7 inches of rain in the first 30 days after planting compared to only 1.5 inches in 1997.

The PPI treatment of 0.6 oz FirstRate provided unacceptable (68%) giant ragweed control in both years (Table 1). The PRE application provided 95% control in 1996 but in the dry year (1997) only 25% control of giant ragweed was obtained.  The label recommends
0.75 oz of FirstRate on soils with greater than 3% organic matter, thus the inconsistent performance of the soil applications may be associated with the relatively low rates used in the study.  All EPOST and POST treatments of FirstRate provided greater than 80% control of giant ragweed, but control dropped below 80% control with the LPOST applications.  The addition of 4 oz Cobra did not improve giant ragweed control with the EPOST or POST application.  While Cobra did increase the activity of 0.3 oz FirstRate at the late post application, control with this treatment was below 80% in both years.  The Pursuit + Cobra treatment provided acceptable control in 1996, but only gave 60% control in 1997.

Table 1.  Influence of rate and application timing on effectiveness of FirstRate for giant ragweed control.

Treatment Timing G. Ragweed
Soybean Injury
G. Ragweed
Control (%)
1996 1997 1996 1997 1996 1997
0.6 oz FirstRate PPI - - 0 0 68 68
0.6 oz FirstRate PRE - - 0 0 95 25
0.3 oz FirstRate EPOST 2-4" 2-6" 0 0 87 88
0.45 oz FirstRate EPOST 2-4" 2-6" 15 0 93 88
0.3 oz FirstRate + 4 oz Cobra EPOST 2-4" 2-6" 9 12 93 85
0.3 oz FirstRate POST 2-8" 4-10" 2 0 97 82
0.45 oz FirstRate POST 2-8" 4-10" 0 0 98 90
0.3 oz FirstRate + 4 oz Cobra POST 2-8" 4-10" 10 12 97 82
0.3 oz FirstRate LPOST 12-18" 4-24" 0 0 53 47
0.45 oz FirstRate LPOST 12-18" 4-24" 3 0 75 67
0.3 oz FirstRate + 4 oz Cobra LPOST 12-18" 4-24" 12 10 78 67
1.44 oz Pursuit DG + 4 oz Cobra EPOST 2-4" 2-6" 9 13 90 60
LSD (0.05)       3 3 7 11

Adapted from:  Franey, R. J. and S.E. Hart.   1999.   Weed Technol.  13:825-828.


This research documents the effectiveness of FirstRate on giant ragweed.  Postemergence applications were more consistent than soil-applied treatments, especially in a year with limited rainfall.  The effectiveness of FirstRate dropped off when giant ragweed exceeded 10 inches in height, demonstrating the importance of timeliness.  Addition of Cobra did not improve the activity of FirstRate on giant ragweed; however, in fields with other weeds there may be a benefit to this tank mix. 

FirstRate is an ALS inhibitor, and in the same chemical family as flumetsulam (Broadstrike).  Numerous weed species have developed resistance to this class of chemicals, thus it is important to develop an effective resistance management program when using these herbicides.  ALS-resistant giant ragweed has been identified in Illinois and is suspected in Iowa.  Since resistance in this species is not widespread at this time, careful planning should limit its spread.  The simplest resistance management program is to avoid using ALS-inhibiting herbicides for giant ragweed control in consecutive years.  If FirstRate fails to control giant ragweed in a field and application errors can be ruled out as a cause of the problems, we suggest alternative control strategies be used in dealing with the escapes. 

1The label rates for soil applied FirstRate are 0.6 oz/acre on soils with 3% or less organic matter and  0.75 for soils with more than 3% organic matter.  The higher rate may be used in low OM soils with moderate to heavy giant ragweed infestations.  Postemergence applications are recommended on soils with greater than 5% OM due to potential for decreased weed control.  The rate for postermergence applications is 0.3 oz/acre.  The maximum height for giant ragweed specified on the label is 10 inches.

Prepared by Bob Hartzler, extension weed management specialist, Department of Agronomy, Iowa State University.

For more information contact:
ISU Extension Agronomy
2104 Agronomy Hall
Ames, Iowa 50011-1010
Voice: (515) 294-1923
Fax: (515) 294-9985
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