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Dealing With Problem Weeds
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Last week’s column dealt with the weed named the number one weed problem for 2016 – Palmer amaranth, a pigweed species. This week let’s broaden the focus a bit and include not just this weed species but all common problem weeds, especially those that have developed resistance to herbicides, especially Roundup® (glyphosate).
• The first, most important item is to start with a weed free seedbed. There are many ways to accomplish this, including tillage, but minimizing tillage to leave crop residue on the soil surface is vital for moisture conservation. We already know glyphosate, atrazine and other herbicide resistance issues are a problem. However, Glyphosate may better activity on newly emerged weeds. Paraquat also make an effective burndown treatment and can easily kill newly emerged broadleaf seedlings and will burn down grasses but not necessarily kill the growing point. Also paraquat cannot control perennial problem weeds such as bindweed but control/suppression of these is a subject for another day.  
• More and more producers are using Preemergence herbicides to prevent germinating seedlings to get a foothold. It’s a different mode of action than glyphosate and helps the crop get a head start to outcompete and shade out weeds, especially in narrow rows.
• While this is already being done by many – tank mix glyphosate with other herbicide modes of action such as bronate, dicamba, 2,4-D. This isn’t an option in all cases. But soybeans have been developed which are tolerant of dicamba and special formulations of 2,4-D along with glyphosate which may help.
• There are other newer herbicides on the market that will control problem weeds like marestail but they are pricey and some have crop/cropping restrictions.
• Don’t skimp on the rates of herbicides. Use the full labelled rate of the product. Read the label and use the appropriate adjuvants for conditions. Finally don’t skimp on the spray volume. Use the recommended volume to provide adequate coverage, especially with nonsystemic herbicides.
• Do everything possible to mitigate plant stress and promote growth – proper fertility, plant population, select hybrids/varieties best adapted to your conditions and where possible, varieties and hybrids with good resistance or tolerance to insects and diseases.
• Identify weed problems, especially those with herbicide resistance and if possible alter cropping patterns to allow a wider array of herbicides. And if able to move to another crop, don’t stick to glyphosate tolerant crops and primarily rely on Roundup. That’s what created this problem in the first place.  
• Although already being done by many, narrow rows for crops like soybeans help fight weed pressure. This can also be effective in grain sorghum, working best under irrigation. Plant the highest practical plant population for the crop and row spacing.
• If no-tilling, be as religious as possible in controlling weeds. If you can keep those weeds from setting seed and not till the ground, eventually the seed bank starts to deplete and weed control can actually become easier. But this can be easier said than done.
• If the problem is bad enough or you aren’t a no-tiller, dust off the cultivator. Remember though to do just enough tillage to control the weeds. This is most effective for weed seedlings.
• Proper sanitation also helps. Use only weed free seed. Don’t carry weed seed into fields on tires or equipment. If possible, a well-timed burn can help. Keep water ways and ditches mowed.
While producers are dealing with weed issues, with time, money, and good management skills, they can be dealt with.