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Managing weeds poses a challenge for farmers

By D. Bruce Bosley
CSU Extension agent, cropping systems

For crop producers, managing weeds represents the most complex and expensive pest management challenge. Weeds frequently reduce crop yields by 20 to 50 percent in their competition for water, nutrients, and sunlight. Weed control is complicated because there are many weeds with different life cycles and various strategies for competing successfully with field crops and native rangeland. Herbicide resistance also plays a significant role in producers' decisions regarding weed control options. Each producer must evaluate the economics and efficacy of the various weed control options in order to make the best choice for his farming operation.

Weed species vary in how they compete with different crops. Broadleaved spring and summer weeds can cause significant yield reductions in warm season grass crops such as field corn. For example, sunflowers, averaged at one plant per square yard, are expected to reduce corn yields by over 28 bushels per acre if left uncontrolled. Grassy weeds, while harder to control, generally have a lower impact on corn yields. One green or yellow foxtail per square yard will reduce yields by only 3 bushels per acre. An exception is shattercane, which grows tall in corn and will reduce expected yields by 10 bushels per shattercane plant per square yard.

Winter annual weeds are the most damaging weeds in wheat. These broadleaved or grassy weeds germinate in the fall like winter wheat, overwinter, and re-grow in the late winter and spring. One blue or tansy mustard per square yard can reduce dryland wheat yields by nearly 4 bushels. These winter annual broadleaved weeds compete directly with wheat and cause yield losses. Cheat grass (downey brome), jointed goatgrass, and feral rye also cause yield losses, however, both goatgrass and rye contaminate the grain that is harvested and reduce wheat's market quality.

Weeds, like sunflower, that cause problems in corn are not generally damaging in winter wheat and vice-versa. Sunflower's growth cycle doesn't fit the crop cycle of the wheat. Planning crop rotations can help reduce weed problems by altering the cropping growth cycle. Using a crop rotation complements tillage and/or chemical weed control efforts resulting in greater weed reductions.

Integrated weed management programs take advantage of the gained synergy when using more than one method of weed control. Use of multiple strategies is also the key to avoiding or eliminating herbicide resistant weeds.

Many weeds have developed resistance to glyphosate containing herbicides in the Midwest Corn Belt. Of particular concern is that glyphosate has significant advantages over other available control practices for these weeds. The spread of these resistant weeds could have serious repercussions on cropping systems highly reliant on Roundup Ready crops and glyphosate. Using herbicides with different modes of action, along with crop rotation and tillage, all help reduce the potential for weeds to develop resistance to one form of herbicide.

Using pre-plant and pre-emergent herbicides to control weeds early in the growth of a Roundup Ready crop can be very useful in reducing multiple glyphosate applications and controlling weeds that could develop resistance. Avoiding continuous planting of Roundup Ready crops is the simplest approach to managing resistance since this approach eliminates selection pressure from glyphosate in the 'off' years.

Please contact me if you would like more information on this or other cropping system topics. Call me in Sterling at 970-522-3200 ext. 285, or Fort Morgan at 970-542-3542.

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