By Drs. Donnie Miller and Daniel Stephenson
Each year producers in Louisiana fight a constant battle against weed species bent on robbing yields and shrinking profit margins. Due to the widespread adaptation of the Roundup Ready technology and subsequent overuse of only glyphosate for weed management, confirmation of weed resistance to glyphosate takes the battle to an entirely different level. For several years, producers in Louisiana were fortunate to watch glyphosate resistance issues with Palmer amaranth played out in articles of Delta Farm Press regarding neighbors to the north in Mississippi, Tennessee, and Arkansas. The old adage “Mother Nature Always Wins” has unfortunately proven true again and resistance cases have been confirmed to glyphosate here in Louisiana. The majority of parishes in Northeast and Northwest Louisiana boarding the Mississippi and Red Rivers, as well as St. Landry Parish, now have documented cases of glyphosate resistant Palmer amaranth. Seed from resistant populations in neighboring states is carried along these main waterways and distributed in adjacent fields with back water flooding and seep water in spring. Three characteristics that make glyphosate resistance in Palmer amaranth such a problem are a very aggressive growth habit, tremendous seed producing capability, and a 73% germination rate. In our perfect growing conditions in mid to late summer, it is not uncommon to see Palmer amaranth plants put on 8 to 10 inches of growth in a week’s period and produce viable seed. Studies in Arkansas have shown that female plants are capable of producing up to 1.77 million seed. At that rate, managing the weed becomes a numbers game. Take for instance an area with 50 female plants that each produce 500,000 seed. Let’s say that 90% of those seed are lost to predation or rot or other means. Also, let’s say a producer implements a management strategy that provides 99% control. He/she is still left with 4.975 million seed. With a 73% germination rate, that means there is a potential to have 3.63 million plants, each capable of producing 1.77 million seed!!!
The main recommendation for preventing introduction of or managing an established population of glyphosate resistant Palmer amaranth is to start clean with a planned rotation of herbicides that are effective on the weed species and offer a completely different mode of action, or means of controlling the weed, than glyphosate. Switching from Roundup Powermax to Touchdown or Glyfos Xtra does no good as all are glyphosate products. In corn, products like Callisto, Laudis, or products that contain atrazine are effective on Palmer amaranth and offer a differing mode of action that is not available in soybean or cotton. In soybean, a number of products are effective soil applied on Palmer including Prefix, Boundary, Authority MTZ, Valor SX, Envive, Valor XLT among others. In cotton, programs including preplant applications of Valor SX, Reflex, or Direx coupled with at planting application of Cotoran, Caparol, or Prowl have been proven effective on Palmer amaranth. A number of postemergence options are also available within each crop. Effective options for control of Palmer and other pigweed species are listed in the Louisiana Suggested Chemical Weed Control Guide which can be accessed at http://www.lsuagcenter.com/en/communications/publications/management_guides/Louisianas+Suggested+Chemical+Weed+Control+Guide.htm. An effective strategy for management of this weed is to overlay residual herbicides to never let the weed off the mat. This includes application of residual herbicides preplant, at planting, in early season over-the-top applications, and at layby. You always want an effective material present at peak strength when another is playing out. This will ensure season long control of the weed. Liberty 280 SL herbicide, used within the Liberty Link system and offering a different mode of action to glyphosate, is effective on pigweed species, including Palmer, if applied to very small weeds 1 inch or less. Control of pigweed 2 to 4 inches can be somewhat erratic. Addition of residual herbicides mentioned before at planting or included with Liberty in early over-the-top applications (Dual Magnum, Staple LX etc.) are needed for season long control. Since Palmer amaranth has a light requirement for germination, burial of the seed utilizing deep tillage can prove effective in its management. Germination and soil life are drastically reduced with seed burial of depths 2 inches are greater. Re-hipping of row following deep tillage, however, negates this advantage by bringing seed up to light exposure and subsequent germination. Prevention of seed production after removal of the crop is also essential to prevent replenishment of the seed bank. Utilize whatever means necessary, be it hand rouging, tillage, or herbicides to prevent plants from producing seed. Also remember that Palmer amaranth seed can be carried by equipment between fields so make sure it is thoroughly clean prior to entering/exiting fields, especially if custom harvesting is being utilized.
Another area to manage Palmer amaranth is on field turn-rows and ditch banks surrounding fields. Allowing Palmer amaranth to grow on turn-rows and ditch banks and produce seed is an excellent way to infest the field. Therefore, if Palmer amaranth is present on a turn-row prior to planting, the turn-row needs to be either tilled or paraquat should be applied to control it. For best management, a soil-applied residual herbicide, such as the one a farmer will be applying to the field for Palmer amaranth management, should be tank-mixed with the paraquat. Palmer amaranth management on a ditch bank is not as easy because of aquatic restrictions for many herbicides that are effective for Palmer amaranth control. Herbicides that can be applied in aquatic situations, such as diuron and diquat, are alternatives for management of Palmer amaranth on ditch banks. However, be sure to read the label to verify that a herbicide is labeled for use on in these situations.
Devastating effects of glyphosate resistant Palmer amaranth are many and include loss in value of the technology on your farm, possible decreased land value, outright field abandonment, and increased production costs due to incorporation of additional herbicides and tillage needed for successful management of the weed. In other words, all of the benefits associated with the Roundup Ready technology such as reduced herbicide use, reduced tillage, faster applications, are no longer realized. Therefore, use all available tools to prevent this problem from causing you to have to fight the battle within the battle.