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Staying Ahead of Palmer Amaranth in the Dakotas

Wednesday, January 29, 2020
filed under: Weeds

       Sunflower growers in parts of the High Plains — especially Kansas — have wrestled with Palmer amaranth for a number of years.  The sight of seed-loaded Palmer plants rising above nearby maturing sunflower plants is, unfortunately, not all that uncommon in the area.  Growers and Kansas State University weed scientists, along with those in adjacent districts of neighboring states, have scrambled to keep this troublesome weed in check in sunflower and other rotational crops.  It has not been easy.

       Now, within the past few years, the multi-state Palmer nemesis has generated headlines in the Northern Plains as well.  This species of pigweed has been discovered in several Minnesota counties, in some cases coming in through grain and seed screenings used for cattle feed.

       In North Dakota, “we have found Palmer in nine counties so far,” reports Brian Jenks, weed scientist with North Dakota State University’s North Central Research Extension Center, Minot.  Palmer was discovered in five North Dakota counties in 2018 and in two of those same counties again in 2019.  The weed also was identified in another four counties in 2019.  “In Emmons and Grant, Palmer was found in multiple millet fields,” Jenks says.  “In Sioux County, a single plant was found in a sunflower field; and in Morton County it was found in an alfalfa field.”

       A single plant of a particular weed species may not sound alarms in most cases.  But when that one plant — as is the case with Palmer amaranth — can produce upwards of a half million seeds under the right conditions, it’s definitely worrisome.

       In South Dakota, “Palmer has been found at least once in the past few years in almost every county along the Missouri River,” says Gared Shaffer, South Dakota State University extension weeds field specialist.  “Also, it has been found in counties bordering Nebraska, such as Bennett.” While those specific populations may not still exist, that certainly doesn’t mean the problem has gone away, Shaffer emphasizes.  Understanding, awareness, monitoring and action are the orders of the day both for South Dakotans and for their neighbors to the north

       Both NDSU and SDSU published extension bulletins on Palmer amaranth in 2019.  The NDSU one — titled “Identification, Biology and Control of Palmer Amaranth and Waterhemp in North Dakota” — can be viewed and downloaded through publications/crops/ .  A PDF of the SDSU bulletin — “Identification and Management of Palmer Amaranth in South Dakota” — can be accessed via an online search of its title.
       Here are some highlights from these two university bulletins regarding the identification and control of Palmer amaranth.

Identifying Palmer Amaranth — Its stems are smooth with no hair.  Petiole length is long — the only pigweed species whose petiole is longer than the plant’s leaf blade.  This trait becomes more noticeable as the plant matures.  Palmer’s leaf shape is wider than waterhemp and not as long. 

      Seed heads of Palmer Amaranth are open, unbranched and very long.  It is a dioecious plant, meaning there are both males and females.  Male plant heads are smooth; those of the females are prickly, with spiny bracts at leaf axils.  The flowering structure of Palmer can be from one to three feet long.  The female seed-producing plants are fertilized via wind-borne pollen from the males.

Plant Biology — Palmer amaranth seeds, like those of all pigweed species, are very small — which in turn makes them easier to be transported, be it via equipment, in cover crop seed, by migratory birds or other means. 

      Palmer amaranth, like waterhemp, can germinate throughout the growing season.  That complicates management in slow-canopying crops and makes for harvest challenges in early maturing crops.  Palmer plants also grow rapidly if conditions are favorable — up to two to three inches per day. 

      NDSU’s Brian Jenks says that at present, he is most concerned about the number of millet fields where Palmer has taken root.  “Because millet is planted late (June and July), Palmer doesn’t have a chance to get real tall — so growers may not see it at all or won’t think it’s a big problem,” Jenks remarks.  “We walked several millet fields with growers [in 2019] and pulled what we could see.  However, I’m concerned about all the other millet fields that may have had Palmer, but growers just didn’t know or recognize it.”

Management  / Chemical — Given its extraordinary seed-producing capability, even 90-95% control of Palmer amaranth is not adequate to prevent future infestations.  “Zero tolerance” is often touted as the needed goal— though that is obviously difficult to achieve.  Still, it is the target whenever possible. 

     When it comes to Palmer amaranth control in sunflower, Jenks says there are several soil-applied herbicide that have activity on pigweed species, of which Palmer is one.  “The best product may be Authority®Supreme, which contains Spartan and Zidua (1:1 ratio),” he observes.  Both of these products generally provide good pigweed control. 

      “Another is Spartan®Elite or BroadAxe® (same active ingredients).  Spartan Elite contains Spartan and Dual, which usually provide good pigweed control.”  There’s also Spartan and Spartan®Charge, the latter of which contains Spartan + Aim.  “However, Aim only controls weeds that are emerged and provides no residual control,” Jenks points out. 

      AuthorityEdge, a new FMC product, contains the same products as Authority Supreme but has a higher ratio of Spartan to Zidua (1.8 to 1).  Jenks says Authority Edge is intended for use in eastern North Dakota, where higher rates of Spartan can be used due to heavier soils with higher organic matter.  Other standalone products would be Prowl H2O and Zidua. 

      “There are two big challenges with these soil-applied herbicides,” Jenks cautions.  “First, these herbicides need a good half-inch rain to be activated soon after application.  We don’t always get that needed rain in a timely manner.  And second, Palmer tends to emerge throughout the growing season, [so] it is possible that Palmer could emerge after the soil-applied herbicides break down and lose their effectiveness later in the season.” 

      SDSU’s Gared Shaffer adds these observations.  “First, always use a burndown and pre-emergent herbicide.  Herbicide mixtures have proven better than stand-alone options.”  For pre-emergents, he recommends the following or an off-brand with the same active ingredient(s):
  • Spartan Charge
  • Authority Supreme
  • BroadAxe, Authority Elite or Spartan Elite
       “Zidua could be applied later after crop emergence if needed and depending on what was used pre-emerge,” Shaffer adds.

Other Management Aids — 
  • NDSU &?SDSU Weed Specialists Address Its Control  An effective control program in other rotational crops like corn, soybeans and small grains goes a long way toward controlling Palmer amaranth in sunflower.  The availability of more effective herbicides in corn and soybeans is key.  With small grains, their early row closure helps prevent late-season germination of Palmer prior to harvest.
  • Hand Weeding:  Hand weeding is time and labor intensive and not always practical on a commercial field basis.  However, if “zero tolerance” is the goal and the Palmer population is limited and sporadic, it’s a very viable option to prevent a “seed rain” and avoid major issues down the road.
  • Tillage: It’s not an option for no-till producers, of course, but thorough preplant tillage will control emerged plants, and inter-row cultivation likewise controls Palmer and other weeds in the emerged, growing crop.  Deep tillage helps control areas with a large population of emerged weeds. “The small seed size of Palmer amaranth and waterhemp restricts germination to the top one-half inch of the soil profile,” notes the 2019 NDSU bulletin.  “If seed is buried deeper than a few inches, Palmer and waterhemp cannot emerge.  Deep tillage should be a one-time event to avoid bringing buried seed back near the surface.” Pigweed seed can be expected to remain viable for at least three to five years on the soil surface, note NDSU weed scientists, and research has shown the buried seed can remain viable for as long as 16 years.
  • Adjacent Areas & Sanitation:  Scouting for and controlling Palmer amaranth in nearby areas such as field edges, borders, ditches and fence rows is very important since weeds in those spots can contribute to the spread of resistant populations of Palmer via pollen.  
         Since equipment is among the easiest ways to move pigweed seeds,  it’s imperative to thoroughly clean combines and other harvest equipment, if Palmer amaranth is present, prior to moving to another field.  Fields known to be infested with Palmer preferably should be combined last.  Likewise, avoid combining through known patches of Palmer if feasible. 
— Don Lilleboe                
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