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Sulfentrazone and flumioxazin injury to soybean   
by
Bob Hartzler

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June 7, 2004 Sulfentrazone (Authority/Spartan/Gauntlet) and flumioxazin (Valor/Gangster) are preemergence PPO-inhibiting herbicides used in soybean for broadleaf control.  These herbicides inhibit the synthesis of a precursor of chlorophyll, causing rapid destruction of contacted tissue.  Although not in the same chemical family, they have the same mode of action as the diphenyl ether herbicides (Cobra, UltraBlazer, Flexstar).  Under most conditions soybean are able to metabolize these herbicides rapidly, therefore preventing the herbicide from accumulating at toxic concentrations..

Unfavorable environments may reduce a crop's ability to metabolize herbicides, therefore decreasing herbicide tolerance.  Injury with sulfentrazone and flumioxazin most frequently occur when heavy rains occur as soybean are breaking through the soil surface, splashing high concentrations of herbicide onto hypocotyls, cotyledons and growing points.  Injury is most pronounced when the rain at emergence is the first significant rainfall event since herbicide application, resulting in high herbicide concentrations on the soil surface.  This situation was fairly common throughout Iowa this year, resulting in more frequent injury with these herbicides than normally encountered.

Results from an experiment at Kanawha this spring demonstrate the potential for injury with these compounds (Table 1).  Soybean were planted on May 5 and herbicides were applied on May 6.  On May 21 the area was hit with 3.5" of rain, resulting in significant splashing of Valor and Authority on the soybean which probably were at the VE stage of development.   Valor caused 10 to 23% injury, whereas Authority resulted in only 3% injury.  The injury was characterized by severe crinkling and malformation of unifoliate and first trifoliolate leaves.  New growth appeared normal and I suspect the soybean will fully recover from the injury.  The conditions were ideal for activity for the herbicides, and the products not only provided excellent activity on broadleaf weeds but also gave more than 90% early-season control of foxtail. 

Table 1. Soybean injury one month after planting following treatment with PPO-inhibiting herbicides. 
Kanawha, IA.  2004.

Treatment Rate % injury
Authority 6.8 oz/A 3
Authority 7.9 oz/A 3
Valor 2.5 oz/A 10
Valor 3.0 oz/A 23
Gangster (Valor + FirstRate) 2.5 + 0.75 oz/A 13
Control - 0
LSD 0.05 - 6

I suspect the injury at Kanawha was somewhat reduced by the occurrence of approximately 1" of rain over several events prior to the 3.5" rain event, therefore moving much of the herbicide into the soil profile.  In addition, the soybean growing point probably was above the soil surface at the time of the major rainfall event (16 days after planting).  I am aware of two situations where the heavy rains occurred closer to when the soybeans were just breaking through the soil surface. In these situations the hypocotyl (Figure 1) or growing point (Figure 2) were severely damaged by the herbicide.  The difference in injury probably is related to the stage of soybean development at the time of rainfall.  The beans in Figure 1 were at the crook stage at the time of rain, resulting in a high concentration of Authority contacting the hypocotyl.  The majority of the soybean in this field had this damage and the field required replanting.   The  Valor injury was observed on numerous fields planted in the same time period in southeast Iowa.   A significant percentage of the plants had the growing point completely killed, leaving only the cotyledons and hypocotyl above the soil surface.   Injury was variable throughout the fields, with some areas having sufficient stand loss to warrant replanting. 

Figure 1.  Hypocotyl injury with Authority when major rainfall event       Figure 2.  Killed growing point due to Valor and heavy rainfall.
occurred at cracking.

Valor and Authority can provide effective control of many of our important broadleaves, including waterhemp.  Under most conditions they have an acceptable margin of crop safety and should not pose a significant risk to the crop.  However, when heavy rains occur at the time of soybean emergence the crop may absorb concentrations of herbicide that overwhelm it's ability to metabolize the chemical.   This risk can be reduced by a few management strategies.  Several seed companies provide varietal tolerance ratings to this class of chemistry since significant differences in varietal tolerance have been documented.  It would be prudent to check with your seed supplier to determine the sensitivity of varieties to the PPO inhibitors prior to purchasing seed.  The risk of injury can also be reduced by applying the product one to two weeks prior to planting.  This increases the likelihood that the herbicide will be moved into the soil profile by rain prior to soybean emergence  Finally, the potential for injury is greater on poorly drained soils where puddling and splashing may occur with heavy rain.  While there will always be a potential for crop injury with these herbicides, good management can minimize this risk.

 

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
http://www.weeds.iastate.edu
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Common chemical and trade names are used in this publication. The use of trade names is for clarity by the reader. Inclusion of a trade name does not imply endorsement of that particular brand of herbicide and exclusion does not imply nonapproval.