Spread of weedy waterhemp

Spread of weedy waterhemp based on university weed scientists' estimates. Source: Hartzler 2003. Updated 2019.

Editor’s note: The following was written by Bob Hartzler, Iowa State University professor of agronomy and an Extension weed specialist, for the university’s Integrated Crop Management blog Nov. 11.


Waterhemp’s rise to the Corn Belt’s worst weed is one of the more fascinating weed-related events over the past 40 years.

Waterhemp (Amaranthus tuberculatus) is native to the United States but is a relative newcomer to crop fields. Prior to waterhemp’s ascent, redroot pigweed and smooth pigweed were the pigweed species most commonly found in crop fields.

This paper will briefly describe the history of the pigweeds in the region and discuss the management and genetic factors that accompanied the change in waterhemp’s behavior as a weed. I use the term “weedy waterhemp” to describe the biotypes that currently plague agricultural fields.

Rise to prominence

Waterhemp was not considered a significant agricultural weed until the early 1990s. At the 1982 North Central Weed Science Society annual meeting, no abstracts referenced waterhemp, whereas in 2003 there were 29 citations.

While weed scientists paid little attention to waterhemp until the 1990s, J.D. Sauer, an authority on Amaranthus taxonomy, stated the western bioype of waterhemp was invading crop fields as far east as Indiana as early as the 1950s. It is likely that waterhemp was present in crop fields prior to the 1980s, but rarely reached economic infestations and thus was not a focus of weed science research.

In 2003 I asked Extension weed scientists in the North Central region to estimate when waterhemp became a significant problem for farmers in their state. I recently did the same for colleagues in the eastern Corn Belt in order to complete the map.

Ground zero for weedy waterhemp was central Missouri and west central Illinois, and the weedy biotype seems to have radiated across the region from that area.

Agronomic causes

The 1980s were a period of rapid change for agriculture in the region, and several factors likely contributed to waterhemp’s evolution into a serious agronomic weed.

Increases in farm size, fuel prices and concern over soil erosion resulted in the adoption of conservation tillage. Waterhemp is best adapted to systems with less aggressive tillage where the majority of its seed remains near the soil surface.

In Iowa, farmers continued to use interrow cultivation on the majority of corn and soybean acres until the mid-1980s. Layby cultivation is an excellent control strategy for late-emerging weeds such as waterhemp. The reduction in tillage, both pre- and post-plant, created a better environment for waterhemp than existed previously, and likely contributed to an increase in waterhemp populations.

In addition to changes in tillage, the 1970s and ‘80s were the golden era of herbicide discovery. Farms became much more reliant on herbicides during the 1980s, and waterhemp is extremely well adapted to this control tactic.

In Iowa and Illinois, the arrival of weedy waterhemp coincided with the widespread adoption of Group 2 herbicides (ALS inhibitors) in the late 1980s. Within five years of introduction of these herbicides the majority of waterhemp in the two states was resistant to Group 2 herbicides.

Waterhemp became a problem in southern Iowa about three years prior to northern Iowa. Mike Owen and I attributed the arrival of weedy waterhemp in southern Iowa to the popularity of Extrazine (cyanazine + atrazine) in that part of the state. In northern Iowa, farmers relied more on Group 15 herbicides and dicamba than Extrazine, and those products were more effective on pigweeds.

While reductions in tillage and changes in herbicide use undoubtedly contributed to the increased problems posed by waterhemp, they don’t explain the expansion pattern of weedy waterhemp across the Corn Belt. Waterhemp is native to the entire region, thus if changes in production practices were the sole cause of the weedy biotypes it seems they would have appeared randomly across the region, rather than the observed migration pattern.

In addition, no-till is much more common in the eastern part of the Corn Belt than Iowa. If reduced tillage was a major factor in the weediness of waterhemp, it seems weedy waterhemp would have appeared earlier in Indiana and Ohio than in the west.

Genetic analysis

It is difficult to explain the spread of weedy waterhemp based on crop production practices alone. Sauer (1957) suggested the increased weediness of waterhemp was due to the movement of the western biotype across the Mississippi River, resulting in hybrids between the western and eastern species. Sauer stated the waterhemp hybrids were better adapted to agricultural fields.

Several scientists have used genetic analysis to identify changes that might have facilitated waterhemp’s movement from natural settings into crop fields. Trucco et al. (2009) evaluated hybridization between waterhemp and smooth pigweed, and determined that transfer of genetic material from smooth pigweed to waterhemp occurred much more easily than transfer of material from waterhemp to smooth pigweed.

The authors speculated that waterhemp may have acquired traits from a different Amaranthus species, and that the new traits facilitated expansion of waterhemp’s range from riparian habitat to agricultural fields.

Waselkov and Olsen (2014) evaluated the genetics of waterhemp populations across its range, looking for evidence of hybridization between the two biotypes as Sauer had proposed. Their data supported Sauer’s observation that the western population had moved eastward. However, they found no evidence that the weedy biotypes present in crop fields are hybrids of the eastern and western biotypes.

While evidence of hybridization between the two biotypes was found, mixing of genes between the two biotypes wasn’t linked to weediness. They concluded that the spread of weedy waterhemp is due primarily to the eastward movement of the western biotype that is better adapted to agricultural fields.

A recent paper evaluated the spread of glyphosate-resistant waterhemp in the U.S. and Canada (Kreiner et al. 2019). Their genetic analysis supports the presence of the two regional biotypes and the eastward movement of the western biotype that has weedy tendencies.

Summary

Changes in crop production and migration of the western biotype with weedy tendencies have contributed to the problems posed by waterhemp, but it seems some other, still unknown, factor must be involved based on how weedy waterhemp has spread across the region.

The combination of prolific seed production, prolonged emergence, persistent seed, and propensity for herbicide resistance creates a formidable foe. While all the causes of waterhemp’s rise to success are not fully understood, it is clear waterhemp is ideally suited to the production system that dominates the Corn Belt.

I appreciate my Extension colleagues’ willingness to estimate date of weedy waterhemp’s arrival in their state. Loyd Wax, University of Illinois, provided the information regarding the region of Missouri and Illinois where waterhemp first became a significant agronomic issue. Pat Tranel with the University of Illinois developed a much cleaner version of the map than my original.

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