Released September 9, 2008

AMES, Iowa - Believe it or not, now is the time to begin thinking about weed management for the 2009 growing season. And as always, the watchword is “stewardship”.

As you begin to harvest the fields, note the weeds that are present and think back to what you did last spring and summer to control the weeds. If you only sprayed glyphosate or any other single herbicide, rethink the tactic. Recurrent application of herbicides will select rapidly for weeds that do not respond to that herbicide. Consider using many different tactics to manage as many weeds as possible and reduce the selection pressure.

Also observe patterns of weeds in the fields. I have observed many fields that have oblong-shaped patches of common waterhemp. The direction always follows the tillage pattern and typically there are dead plants next to the live plants. This should scream out impending concerns about resistant populations.

Unfortunately, most growers will not proactively manage issues until they become unmanageable. Remember that weeds produce an incredible number of seeds and these seeds have long lives in the soil. Once you “recognize” the problem, it is already too late to resolve the problem. Be observant and make adjustments before the problem gets out of hand.

Pay attention to this year's wet-holes that did not allow crop growth but did allow considerable weed growth. These areas will have extreme weed pressure next year, so adjust management accordingly. It is important to use a residual herbicide and follow with post herbicide applications and cultivation, as deemed appropriate when scouting these areas. Importantly, it may be necessary to implement additional weed control tactics on these areas. They should be managed differently than the rest of the field and each field should be considered separately when weed management plans are formulated. Simplicity and convenience in weed management has an economic and environmental cost.

As for stewardship, the first and foremost concern should be stewardship of profitability. The best way to hedge against yield losses attributable to early season weed competition is the use of an early pre-plant herbicide application. A preemergence application is also a good hedge against early weeds, but as we saw this year, is still risky. Remember, the reason herbicides are used is not to “kill” weeds but rather to protect crop yields.

A soil-applied residual herbicide can also provide stewardship by delaying or minimizing the risk of weeds evolving resistance to the post herbicides that are used recurrently, notably glyphosate. Soil-applied residual herbicide makes you money, it does not cost you money.

Finally, if you have fields where the control of common waterhemp has been a problem, please contact me. I would like to collect seeds from these fields. I can be contacted at mdowen@iastate.edu. Stewardship pays!

---30--

http://www.extension.iastate.edu/CropNews/2008/0909owen.htm