Palmer Amaranth: In The Midwest, It’s Not Your Father’s Pigweed

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PHOTO: BOB HARTZLER, WEED SCIENTIST, IOWA STATE UNIVERSITY

Editor’s Note: Bob Hartzler, Iowa State University weed scientist, used this slide (see below) in a recent presentation, and it caught our eye when he recently posted it on Twitter.

The slide says plenty about this history and progression of pigweeds in his state over several decades and it illustrates how much of a hit farmers might take when this weed gets out of hand – particularly Palmer pigweed/amaranth.

To further illustrate how pigweed challenges have evolved, Hartzler breaks down 3 pigweeds – redroot pigweed, waterhemp and Palmer – by the generation of farmers most affected by these species. Much like the car commercial – “It’s not your father’s Oldsmobile” – Hartzler shows that Palmer isn’t your grandfather’s pigweed or, for that matter, your father’s pigweed.

The table was based on research in 2000 at Kansas State University.

Here’s a brief summary from Hartzler that expands on the information on the chart.

The pigweeds are fascinating since they demonstrate how small biological differences can make a huge difference in the success of a weed. Both redroot pigweed and waterhemp are native to the Cornbelt. Redroot was the primary pigweed of ag fields of the Cornbelt prior to the 1980s.

However, there were other pigweeds (primarily smooth pigweed and Powell’s amaranth) in the region and early agronomists rarely took time to differentiate them.

All three of these early pigweeds are monoecious (both male and female flowers on same plant). 

In the late 1980s we saw a rapid shift from the monoecious pigweeds to waterhemp. The rise of waterhemp as an agronomic problem occurred at the same time as widespread use of ALS inhibiting herbicides – to which the species rapidly evolvled resistance.

By the mid-1990s it became difficult to find redroot pigweed in agricultural fields, although it still is present in other disturbed habitats. We don’t fully understand what led to this shift in pigweed species. A paper from Illinois speculated that hybridization between waterhemp and smooth pigweed may have contributed to the adaptation of waterhemp to ag habitats.

The emergence of waterhemp as a problem seemed to originate in southern Illinois and then spread across the Cornbelt. 

Unlike redroot and waterhemp, Palmer amaranth is not native to the Cornbelt, but originated in the Southwest U.S. The expansion of its range began in the early 1900s, although it wasn’t until the mid-1970s when it was reported to be a problem ag weed in the southeastern U.S.

In the past 10 to 15 years it began moving northward into the Cornbelt. Iowa’s first detection was in 2013. At this time, it doesn’t appear to be well adapted to soils/climate of the northern Cornbelt (my personal observations, not based on any data).

I think it will be a decade or so before it begins to compete with waterhemp as the primary pigweed.

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