Palmer amaranth is once again a hot topic this growing season as the noxious weed continues its relentless spread into more of the country. Considered one of the most destructive weeds in agriculture, stakeholders have been frantically working on new solutions but as the old saying goes, an ounce of prevention is worth a pound of cure. That’s why agronomists and ag extensions are also trying to spread the word about what steps producers can take to prevent it from taking root on more farms.
Palmer amaranth is native to the desert regions of northern Mexico and the southwestern U.S. Over the last two decades, it has been spreading further north and now poses a major threat to row crop growers in southern and Midwest growing regions.
A single Palmer amaranth plant can produce half a million seeds, grow 2-4 inches in a day, reaching an average of 6 to 8 feet tall or more. The fast-growing and aggressive weed can easily outcompete crops for nutrients, moisture, and sunshine, and has shown the potential to wipe out up to 91% of corn yields, 68% of soybean yields, and 54% of cotton yields.
Similar to waterhemp, Palmer amaranth also has the ability to rapidly evolve resistance to common herbicides. In areas of the country where the weed has taken hold, Palmer populations have developed resistance to at least five major herbicide classes, including ALS-inhibitors, PPO-inhibitors, glyphosate. More recently, Arkansas scientists say confirmed Palmer amaranth resistant to glufosinate, the active ingredient in Liberty herbicide.
Palmer amaranth can enter farms through several pathways, including feed ingredients and seeds derived from infested areas. A couple of years ago, the weed found its way into Minnesota fields via manure from cattle that had been fed screenings from contaminated sunflower seed. In many states, Palmer amaranth is on their prohibited noxious weed list so it is illegal to sell seed that contains any traces of the weed. Palmer amaranth seeds are tiny and extremely difficult to visibly distinguish from other pigweed species, so DNA testing is viewed as the most effective testing measure. Growers are encouraged to double-check with both seed and feed suppliers for more information on how their products are screened. Growers should also investigate where manure providers are getting their feed products.
Like most weeds, Palmer amaranth can also be spread on equipment. Some experts recommend that producers avoid buying combines that come from Palmer-infested areas entirely. Although, with it’s rapid spread, that’s becoming a tougher challenge. Producers should also investigate where custom harvesting equipment has previously been used.
Like waterhemp, Palmer amaranth emerges throughout the growing season, which complicates detection and management. In some areas, emergence comes later in the season than waterhemp, which means Palmer amaranth may avoid early season control tactics. Some agronomists recommend reapplying preemergent herbicides through the season, at least until crops are big enough to starve weeds of sunlight. Be aware that waterhemp and Palmer amaranth found in the same field can be resistant to different herbicides, meaning what works on one won’t necessarily work on the other. Agronomists also warn that what worked last year may not be effective this season or the ones that follow.
Scouting fields throughout the growing season is important to detect any Palmer that escapes prevention methods. Again, it’s tough to tell the difference between Palmer and other pigweeds at early growth stages so DNA testing is recommended. Timing is everything for postemergence control considering how fast Palmer amaranth grows. Agronomists say applying herbicide to too large Palmer plants is likely the number one cause of control failures. Applications should be targeted for weeds that are less than three inches in height. Prolonged emergence may not necessarily impact yields in the current season, but they increase the seedbank, setting producers up for hefty management bills down the line.
The North Central Integrated Pest Management Center (NCIPMC) recommends growers adopt a diversity of herbicide groups as the repeated reliance on a single program will inevitably result in new resistance. They also encourage producers to rotate those over time. Non-chemical practices such as cover crops, narrow-row spacing, and interrow cultivation can be effective tools to incorporate. Growers can check with their state ag departments or local university extensions for more details about Palmer amaranth in their specific area. There is also more info and links to additional resources over at the NCIPMC website HERE. (Sources: NCIPMC, USDA, Iowa State)