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Resistant Palmer amaranth spreads

In Mississippi, glyphosate-resistant Palmer amaranth was first confirmed following the 2007 growing season in Tunica and Coahoma counties in the northwest corner of the state.

Since that time, glyphosate-resistant Palmer amaranth has been spreading to the south and east. After the spring floods of 2008, I assumed that 2009 would be the year Palmer amaranth exploded across the Mississippi Delta.

Through surveys and greenhouse screening, weed scientist Vijay Nandula and his group at the Delta Research and Extension Center in Stoneville, Miss., confirmed the presence of glyphosate-resistant Palmer amaranth in nine counties in the Mississippi Delta following the 2009 growing season.

To date, a majority of these glyphosate-resistant Palmer amaranth populations have occurred north of U.S. Highway 82.

Although nine counties represents a significant portion of the crop production area in the Mississippi Delta, glyphosate-resistant Palmer amaranth does not seem to have spread as quickly across Mississippi as it has Tennessee or Arkansas. However, based on the calls I have received over the past two weeks, early indications from 2010 are that infestations have increased this year.

The problem with glyphosate-resistant Palmer amaranth is intensifying in the northern Mississippi Delta counties. Growers in that part of the state are doing all they can to manage the problem, and some of them are into their third or fourth year of trying to control it. I spoke with a consultant who checks multiple crops over a wide area in northwest Mississippi this morning (May 14), and he told me that glyphosate-resistant Palmer amaranth was out of control in many areas.

Mother Nature has also done us no favors in that part of the state. Between rain and wind, growers have had a tough time applying herbicides so far this spring. The consultant said he had seen one field with emerged cotton and 12-inch tall glyphosate-resistant Palmer amaranth. The grower had planned to treat the field with Ignite, but had been unable to due to wind. Unfortunately, situations like this have been common this year.

Glyphosate-resistant Palmer amaranth is spreading, and those of us in areas that were previously not impacted had best get prepared to deal with it. Over the past week, I have received several questions from the southern part of the Mississippi Delta about glyphosate-resistant Palmer amaranth and/or waterhemp. A couple of those growers and consultants had not had resistance problems in previous years and said they had no idea the Palmer amaranth/waterhemp was glyphosate-resistant until they made their first glyphosate application, and the weeds did not die.

Our new soybean weed scientist/agronomist, Tom Eubank, found a field just off the station at Stoneville this week that was infested with glyphosate-resistant Palmer amaranth, glyphosate-resistant horseweed, and glyphosate-resistant Italian grass, a three-headed monster that will impact all facets of the grower’s weed control program for years to come.

I have seen several glyphosate “misses” on Palmer amaranth in cotton and corn weed control plots at Stoneville this week. I do not know that these plants are resistant, but in one case, I found several Palmer amaranth plants that survived two, 26-ounce burndown applications of glyphosate.

No crops are immune to glyphosate-resistant Palmer amaranth. One of the questions I have received most frequently in the last month concerns how to control glyphosate-resistant Palmer amaranth in rice. We do not have an effective residual herbicide for glyphosate-resistant Palmer amaranth in rice. Furthermore, emerged glyphosate-resistant Palmer amaranth survives the glyphosate plus Command treatment that is popular immediately after planting.

After rice is emerged, there are very few options for control of Palmer amaranth. Propanil plus Grandstand performs most consistently for me at Stoneville on Sharkey clay soil. However, rice must have at least two true leaves before Grandstand may be applied, and it cannot be used on recently land-formed fields. Also, many growers and consultants avoid using Grandstand on silt loam soil due to injury potential.

In most cases, a flood should eventually kill Palmer amaranth infesting rice. However, even if the flood kills the Palmer amaranth plants in the field, chances are the levees will be covered in Palmer amaranth that will produce seed and cause problems in the soybean crop the following year. So even though glyphosate is not a component of in-crop weed management in rice, the glyphosate resistance problem can impact rice production.

If you have not dealt with glyphosate-resistant Palmer amaranth yet, please try to understand how fast this problem can arise and how vicious it is after you have it. Do you want to risk assuming that a glyphosate-only program in cotton, corn, or soybeans will control the Palmer amaranth in your fields? Do you want to risk making a second glyphosate-only treatment if the first one fails?

Those of us who work within the university system often hear growers say they will deal with glyphosate-resistant Palmer amaranth when it hits their farm. Another common thought among growers is they cannot justify the cost of adding residual herbicides to their weed control programs because they have no resistant weeds, yet.

Will a residual herbicide cost more than the yield lost from season-long Palmer amaranth competition? Will it be more expensive to manage a few scattered patches of glyphosate-resistant Palmer amaranth this year or wait and try to control a field full of it next year?

If you are one of the fortunate people that works in an area that is not infested with glyphosate-resistant Palmer amaranth, then the adage “an ounce of prevention is worth a pound of cure” is something you need to think seriously about. Glyphosate-resistant Palmer amaranth is here to stay. It will continue to spread across the Mid-South until everyone is impacted. Because the glyphosate resistance trait moves through wind-blown pollen, you will not be able to keep it out of your fields forever. However, you can utilize programs to manage the populations that are likely starting to develop in your fields right now. It is not easy. It certainly is not as easy as spraying glyphosate two or three times over the top and then rolling out poly-pipe. It is not cheap.

In his Delta Farm Press article last week, weed scientist Ken Smith posed the following question, “Are we leaving more weeds in the field each year than the year before?” His answer was that in many cases we are. He also proposed that by doing this we may someday reach a level of weed infestation that will become too great to farm. Even though it may be more expensive in the short term, the added cost may be worth achieving a sustainable level of weed control. Fram sold a lot of oil filters with their slogan, “Pay me now or pay me later.” The same holds true for managing glyphosate-resistant Palmer amaranth.


TAGS: Soybean
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