Weed control is an essential part of all crop production systems. Weeds reduce yields by competing with crops for water, nutrients, and sunlight. Weeds also may directly reduce profits by hindering harvest operations, lowering crop quality, and producing chemicals which are harmful to crop plants (allelopathy). In addition, weeds left uncontrolled may harbor insects and diseases and produce seed or rootstocks which infest the field and affect future crops. Despite large expenditures for weed control, it is estimated that losses in U.S. crops due to weeds left uncontrolled exceed $7.5 billion annually.
Years of research have shown that good weed control within the first 4 to 6 weeks after crops are planted is critical in order to avoid a yield reduction from weeds. The effectiveness of any weed control program depends largely upon one factor:TIMELINESS. There are many cultural, mechanical, and chemical methods of weed control which are extremely effective if applied at the correct time. Fields that are kept free of weeds for the first four to six weeks after planting give the crop a "head start" which enables it to shade out or otherwise out compete weeds that emerge later in the season.
Identify specific weed problems, because different weeds vary greatly in their ability to compete with crops and reduce yields. Is 100% control of all weeds necessary? Perhaps this can be answered by stating that most successful producers design control programs to maximize profit, not just weed control. While it is true that crops are able to tolerate a certain threshold number of weeds without suffering a yield reduction, it is first important to consider weed problems on an individual basis. There are some weeds for which 100% control may be desirable because they are particularly competitive, persistent, or difficult to control. These include some annual weeds such as giant ragweed, common cocklebur, burcucumber, or shattercane, and several perennial weeds such as johnsongrass, Canada thistle, bindweeds, Jerusalem artichoke, quackgrass, and hemp dogbane.
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Keith L. Smith, Associate Vice President for Ag. Adm. and Director, OSU Extension.
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