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Should Tillage be a Tool to Control HR Palmer Amaranth?

We all know that herbicide-resistant (HR) weeds have become arguably the greatest production problem for Midsouth crop producers. This has generated renewed interest in using tillage as a tool for controlling these problematic weed pests.

Dr. Jason Norsworthy, Professor and Weed Scientist at the Univ. of Arkansas, explores this subject in a recent webcast titled “Seed Burial and Persistence of Palmer amaranth and Waterhemp” that was posted by the Plant Management Network in its “Focus on Soybean” series. In his presentation, some of which is based on results from research supported by the USB, Dr. Norsworthy discusses the seed production potential of these weeds along with depth of seed burial by tillage, and the likelihood of these weeds to emerge following a one-time deep tillage operation or in a continuous no-till production system. He discusses weed management strategies that include tillage, along with recommendations for its use based on current infestation level in production fields. Specific points from the presentation related to Palmer amaranth follow.

•   Palmer amaranth has developed resistance to eight herbicide sites of action (8 WSSA groups). It has been deemed the most problematic weed in the US.

•   A single Palmer pigweed escape in a soybean field can produce over 1.75 million seed.

•   Normally, a Palmer amaranth plant will produce more than 100,000 seeds. Some of these are moved by wind and/or water from the site of production, some germinate and die, some are non-viable, some are dormant, and some are destroyed by predation. However, even with this normal attrition, many seeds are left to produce new weed seedlings that must be controlled the next crop year.

•   The goal for management of Palmer pigweed is to lessen the input into the soil weed seedbank, and to ultimately have seedbank inputs that are less than seedbank losses.

•   Palmer amaranth seed that are in the top 1 in. of soil have a high germination percentage (>35%). Thus, the goal is to bury seed so that they are in soil depths >2 in. to prevent or lessen their germination and emergence (~5%).

•   In a normal no-till production system, weed seed remain on or just below the soil surface. In a system that uses a moldboard plow (deep till), the vast majority of weed seed are buried to depths >2in.

•   Research has shown that deep tillage buries most weed seed (only 28% remain in the top 2 in. of soil), whereas >75% of weed seed remain in the top 2 in. in conventional, minimum, and no-till systems. There is about a 70% reduction in Palmer amaranth emergence from deep till vs. no-till systems.

•   Thus, a one-time deep tillage operation with a moldboard plow should take selection pressure off herbicides that are used to control Palmer amaranth populations.

•   Research has shown that Palmer amaranth persistence is not related to latitude in the US. Thus, burial conditions–e.g. depth–are more important for viability than is latitude.

•   Research has shown that Palmer amaranth does not form a highly persistent seedbank–i.e., after 36 months of burial or left on the soil surface, only about 5% of seed remain viable.

Dr. Norsworthy concludes that 1) Palmer amaranth does not form a highly persistent seedbank, and 2) the greatest influence on viability of seed that are produced is burial condition. This, then, leads to the implication that a one-time moldboard plow operation should promote deep burial of seed and result in subsequent loss of viability of those seed.

Dr. Norsworthy proffers the following practical implications from the above.

•   If Palmer amaranth populations in a production field are low, consider a no-till or conservation tillage system of production. This should result in greater seed decay and loss of viability, and increased predation of exposed seed.

•   If Palmer amaranth populations are high, consider a one-time moldboard plow operation to promote deep burial of seed and their subsequent loss of viability.

•   Of course, all weed management options and operations should be geared toward zero-tolerance of Palmer amaranth populations since it is difficult to drive the population to extinction once a population is established. The ultimate goal should be to reduce the weed seedbank in soil using the most effective means.

Readers are encouraged to view this webcast in its entirety to learn about this promising tool to aid in the control of this problem weed. Also, check a previous article that presents a summary of results from Dr. Norsworthy’s research using the integrated Harrington Weed Seed Destructor to destroy weed seed during the crop harvesting operation. The equipment is described in a video seen here.

Composed by Larry G. Heatherly, June 2019, larryheatherly@bellsouth.net