ISSUE  13   August 12, 2010

RESISTANCE TO HPPD INHIBITORS IN AN ILLINOIS WATERHEMP POPULATION

Waterhemp looks similar to redroot pigweed and is found in many fields in eastern ND and western MN. It has become resistant to ALS herbicides, PPO herbicides, and glyphosate. From several phone calls from growers and consultants it appears that waterhemp have escaped control from these herbicides in this area in ND and MN. Few herbicide modes of action are left to control resistant biotypes. One effective mode of action are the HPPD herbicides like Balance, Callisto, Laudis, and Impact. Below is information about waterhemp populations escaping control from HPPD herbicides in Illinois. If it can happen there, it can certainly happen here.
Waterhemp resistance in Illinois first occurred to ALS-inhibiting herbicides in 1997. Since then, Illinois waterhemp populations resistant to triazines, PPO inhibitors, and glyphosate also have been confirmed. While resistance to any one herbicide family can introduce significant management challenges, biotypes with resistance to more than one herbicide family are becomingly increasingly common.

Weed scientists at the University of Illinois are currently working with a type of herbicide resistance that has not previously been reported. Basic and applied research is underway with an Illinois waterhemp population that is resistant to herbicides that inhibit 4-hydroxyphenyl pyruvate dioxygenase, generally referred to as HPPD-inhibiting herbicides. Foliar-applied HPPD inhibitors are commonly used for control of annual broadleaf and grass weed species in corn.

Initial greenhouse experiments conducted at the University of Illinois confirmed anecdotal reports from the field. Plants grown from field-collected seed and treated with HPPD herbicides survived, whereas treated plants from two known sensitive populations (used for comparison) were completely controlled. Tank-mixing atrazine with each HPPD inhibitor improved control of the resistant population over that provided by each HPPD herbicide alone, but survival was still much greater than with the sensitive controls.

Field research conducted in 2010 confirmed the greenhouse results. Foliar-applied HPPD inhibitors, alone or tank-mixed with atrazine, provided poor control of this waterhemp population. Crossing experiments have confirmed reduced sensitivity to HPPD inhibitors can be transferred to progeny, providing additional evidence that this population is resistant to this herbicide site-of-action family.

Sources: University of Illinois Weed Scientists - Aaron Hager, Dean Riechers and Pat Tranel

Rich Zollinger
NDSU Extension Weed Specialist
r.zollinger@ndsu.edu

OPPORTUNITY STRIKES FOR FALL WEED CONTROL

Now is the time to begin applying herbicides for the control of winter annual weeds, simple perennials such as curly dock and dandelion, biennials such as biennial wormwood, and in some cases cool-season perennial weed species.  This is especially true for no-tillage fields, but also for those fields receiving tillage other than moldboard plowing.  For fields in which tillage is planned, apply herbicides at least 5 days prior to tillage.  Herbicides may be applied within a few days of crop harvest or until the soil is frozen.  Based upon research across the Midwest, the most consistently effective control of dandelions is obtained with fall herbicide applications.  The best way to drastically reduce or stop seed production of winter annual species is with fall herbicide applications or effective fall tillage.  It is always better to apply herbicides in the fall under less than ideal conditions, than to wait until spring to achieve marginal control of these types of weed species, especially dandelion.

The most effective fall herbicide treatment with the most cropping flexibility next spring is an application of glyphosate at 0.75 pound acid equivalent/acre (lb ae/A) [Roundup at 22 ounce/acre {oz/A} or glyphosate products containing 3.0 lb ae/gallon at 32 oz/A] plus 2,4-D ester at 0.5 lb ai/A.  The addition of 2,4-D is most important for dandelion control and will antagonize glyphosate's activity on Canada thistle and perennial grass species.  Another herbicide option, would be the addition of Valor at 2 to 3 oz/A to the glyphosate plus 2,4-D mixture.  Fall applications including Valor will be most beneficial west of the Red River Valley where spring rains are not consistent enough to properly activate Valor.  Activation of Valor is almost certain with fall applications in the drier areas of the state.  Preliminary studies with fall-applied Valor have shown potential to control or suppress weeds such as kochia, seedling dandelion, canola, and chamomile.  However, NDSU and Valent are conducting additional research to determine proper timing of application of Valor and efficacy on spring-emerging weeds.  Valor should only be applied in no-tillage fields and any substantial soil movement next spring during planting will reduce the effectiveness of Valor on spring emerging weed species.  Read the Valor label and follow the crop rotation guidelines when applying Valor in the fall.  Only certain crops can be planted in the spring following fall-applied Valor.

Jeff M. Stachler
Assistant Professor - Sugarbeet Weed Science
NDSU and University of Minnesota