Last year, resistance to the herbicide propanil was confirmed in populations of smallflower umbrella sedge and ricefield bulrush. Considering that propanil is a common “clean up” herbicide, used in almost 400,000 rice acres every year, propanil resistant sedges are a significant threat to the rice industry. To make matters worse, populations of these propanil resistant sedges were also found to be resistant or partially resistant to several ALS-inhibiting herbicides (Londax, Sandea and Granite).

Herbicide trials conducted last year showed sedge populations resistant to propanil and ALS-inhibiting herbicides were susceptible to the herbicide Shark H2O. For control of propanil resistant sedge, Shark H2O herbicide can be applied at one of two timings: 

• Early, at the 2 to 4 leaf stage of rice, for control of submerged weeds, at a rate of 7.5 oz/a, or

• Twenty to 45 days after seeding, to the foliage of exposed weeds at a rate of 4 oz/a. 

Additionally, the herbicides Bolero and Abolish used at standard rates and timings also control propanil and ALS-resistant smallflower umbrella sedge. 

Other herbicides will need to be used to control the whole spectrum of weeds present. For example, programs could include an early application of Shark H2O followed by Regiment or propanil; Cerano or Bolero can be followed by a later application of Shark H2O to control escapes. Remember to read and follow the label. 

When dealing with herbicide resistant weed populations, do your best to control all weed escapes and late season flushes. Harvest infested checks last, so your equipment does not spread seeds of resistant weeds to uninfested checks. 

Growers who are not experiencing reduced efficacy of propanil should implement these well known strategies to delay the development of propanil resistance: 

• Avoid repeated use of herbicides with the same mode of action. 

• Use different modes of action in mixtures and sequences. 

• Use label rates and avoid low rates. 

If you suspect propanil resistance, collect mature sedge seeds in problem fields and bring them to the Rice Experiment Station, where they will be tested during the winter. Control failures are not necessarily due to resistance, but can be caused by application problems such as incorrect timing, dosification errors, mixture incompatibilities, etc.

Smallflower umbrella sedge in rice field