The Enid News and Eagle, Enid, OK

July 13, 2013

Herbicide-resistant weeds can be managed

By Roger Don Gribble, Extended Forecast
Enid News and Eagle

ENID, Okla. — As the costs and scale of farming operations in northwest Oklahoma have increased, so has the use of herbicide-resistant crops like corn, soybeans and canola.

These crops provide a weed management tool to meet the needs of our modern agriculture system. As we look at these herbicide-tolerant crops, the use of soil-applied herbicides producers have relied upon heavily in the recent decade has been reduced. Because of weeds developing resistance to post-emergence herbicides, producers need to once again look at other weed control management strategies to overcome herbicide-resistant weeds.

As we look at fields in northwest Oklahoma, our biggest problems are with tall waterhemp, marestail and pigweed. These weeds most likely will be resistant to glyphosate, and some will be resistant to an ALS-inhibiting herbicide. Some of these weeds will be resistant to both modes of action, making them even tougher to control.

Oklahoma Cooperative Extension Service would like to suggest some additional thoughts producers could use to battle herbicide resistant weeds:

• Diversify your management approach. Forget the simple approach. Producers are used to diversifying hybrid or variety choices and fertility treatments to mitigate risks. The same approach for weeds means knowing what you have done historically, knowing if you are seeing the same issues in specific fields, and scouting and managing weeds for as small a unit as possible. Do not treat all fields alike.

• Use herbicide group numbers to diversify weed management. There still are too many producers relying solely on one post-emergent herbicide for weed control. Even if producers are applying a product with multiple modes of action, if the weeds are resistant to those modes already, poor herbicide performance will result.

• It seems that no one herbicide treatment will provide season-long control. If there was a herbicide product like that, it likely would cause a carryover problem to the crop coming in behind the herbicide treatment. The best thing to remember about carryover is to know what was applied, when it was applied, if you got enough rainfall after the application and then make an informed decision on what crop to follow that herbicide treatment with. The highest risks of herbicide carryover would be with atrazine, chlorosulfuron, trisulfuron, sulsulfuron and maybe imazamox in northwest Oklahoma.

• Use residual soil-applied herbicides with multiple modes of action. Producers seem to be leaving money on the table in lost yield potential from early weed competition. Most OSU test plot data signals that early weed control is the most important money that can be spent. Soil-residual herbicides will provide multiple modes of action to control early weeds and get you to a point five and six weeks later where the crop canopy can sometimes hold weeds back from causing yield loss. There is tons of research from various part of the country to indicate performance of the herbicides you are intending to use.

• Scout, scout and scout. Know what your problem weeds are and where they are likely to occur. Develop a diverse plan of action to provide adequate control without suffering yield losses. Study a herbicide mode of action chart to recognize products that could be used if resistance is a major concern in your fields. The only way that a producer will know if a problem exists is to scout, scout and scout some more. Shadows in the field are your best solution to a building problem.

There are herbicide modes of action charts at your county OCES Extension office. Your county educators can assist you with developing a plan to avoid herbicide resistant weeds in your fields.

Gribble is Oklahoma Cooperative Extension Service northwest area agronomist.