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Early Season Weed ManagementPosted Monday, January 31, 2011, at 7:49 PM
Even though there is still snow on the ground, planning for spring planting is not too far away. Weed resistance to various herbicides is a popular topic. One of the more troublesome problems for row crop producers is glyphosate resistance. There are 16 weeds listed as resistant to glyphosate. These include Palmer amaranth, common waterhemp, common ragweed, giant ragweed, hairy fleabane, horseweed, junglerice, sourgrass, goosegrass, wild poinsettia, Italian ryegrass, rigid ryegrass, ragweed parthenium, buckhorn plantain, johnsongrass and liverseed grass. We don't have all of them in Missouri now. Overall there are more than 330 resistant weeds worldwide.
Waterhemp is of particular concern because of it's multiple herbicide resistances. A study conducted in Illinois on glyphosate resistant waterhemp found that all populations resistant to glyphosate were also resistant to ALS inhibitors and 40 percent contained resistance to PPO inhibitors. Recently , waterhemp has been found with resistance to HPPD-inhibiting herbicides. Waterhemp is not a weed species that can be adequately managed with one or two different herbicides. It will take a much integrated approach.
Bill Johnson, Purdue University weed scientist, indicates that multiple Modes of Action (MOA) should help corral glyphosate-resistance problems. Using a residual herbicide preplant or preemergence and appropriately timed Roundup postemergence to help protect crop yields from early season weed competition and reduces the number of weeds exposed to glyphosate.
The first key in controlling resistant weeds, if you have resistant weeds, is to start clean. That means not planting into any existing vegetation unless you are 100 percent confident you can burn it down. If you are not sure, use tillage to insure that you start clean.
The next key is to use multiple applications of residual herbicides. Timing is always an issue. If you apply too early they can play out too soon, but if you apply too late you miss some of the emerged vegetation, which can be worse.
If you get to planting time with scattered, small vegetation, then a burn-down may start you clean. At planting is another place to combine a burn-down herbicide with the residual program.
If you use a preplant tillage as a means of starting clean, incorporating a preplant herbicide is an option, especially on the sandy to silt loam soils.
If you plant into glyphosate-resistant weeds and fail to kill them, you are behind to start with. Starting with a clean field is necessary to improve your chances of controlling resistant weeds.
Weed control in glyphosate-resistant crops will continue to increase in complexity due to development of resistance or selection of weed species that can avoid glyphosate, late emergence or just inherent tolerance to the rates we are using, according to Bill Johnson.
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As our local University of Missouri Agronomy Extension Specialist, Crook has been writing a column for the print edition Agriculture page for the past three years and we will now be sharing it on our web version. Crook has a bachelor and masters degree in agronomy from University of Missouri and received his doctorate in Agronomy from Kansas State University. He was in soybean variety development research for 22 years for various seed companies and has been Saline County's agronomy specialist for 10 years.