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Now is best time to control wheat weedsBy D. Bruce Bosley Extension Agent/Cropping Systems Colorado State University Cooperative Extension I talked about top-dressing nitrogen fertilizer on wheat early to enhance yields in last week's column. I had calls from producers waiting to double apply both the nitrogen and herbicides in one application to control broad-leafed weeds. My answer was twofold: Now is still the best time to control weeds in wheat when using residual herbicides, and furthermore, broadcast spreading nitrogen will tie up on field residues. Kochia, prickly lettuce, mustards and many other winter annuals and very early spring broad-leafed weeds are already up and growing. The kochia is still very small and is very resistant to glyphosate containing herbicides but it should be susceptible to those containing dicamba and a sulfonylurea class of herbicide. These herbicides remain in the soil long enough during our cool late winter early spring to be effective. Herbicide injury and efficacy studies done in the mid 1990s on wheat showed that early treatments with dicamba applied alone or combined with other wheat herbicides in a tank mix performed very well in controlling winter and early spring broadleaves and had much less chance for crop injury than when the dicamba treatments were applied around the jointing stage of growth. In Northeast Colorado jointing normally occurs around April 10. However, the date jointing begins varies from year-to-year and by variety. Then the weeds are easier to see but also harder to kill and there is a much higher chance for herbicide caused crop damage. Note that to avoid developing resistance in weeds, use a combination of two or more chemicals from different classes or different modes of action when applying herbicides on wheat. Using combinations of herbicides and alternating herbicides is considered a good practice for pest control in all crop and landscape settings. Moving on to the second point, nitrogen fertilizer can be bound up on previous crop residue and the percent that is held out of the soil depends upon the amount of the residue cover and the method of fertilizer application. Broadcast sprays of liquid fertilizer spread the nitrogen evenly over the field surface and is therefore the most likely to be caught on the residue before getting into the ground. The best way to apply liquid or dry nitrogen is to knife it into moist soil close to the seed row. Dribbling nitrogen over the surface is an improvement over broadcasting it but it still needs precipitation or irrigation to move it into the soil where the wheat roots can get to it. Please contact me, Bruce Bosley on this or other cropping systems or natural resources topics. Call me at 970-522-3200 ext: 285. B 6 3/26/07 3 Star CO Date: 3/21/07
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