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Does Waterhemp Outcross with Powell Amaranth or Redroot Pigweed? (08/16/18)

Academicians teach agriculturalists to identify pigweed species by the presence or absence of hair on leaves and stem. Redroot and Powell pigweed have very small fine hairs throughout the plant.

Does Waterhemp Outcross with Powell Amaranth or Redroot Pigweed?

Academicians teach agriculturalists to identify pigweed species by the presence or absence of hair on leaves and stem. Redroot and Powell pigweed have very small fine hairs throughout the plant. There are no hairs on waterhemp and stem and leaf surfaces are smooth. Identification based on presence or absence of hairs is qualitative; yes or no, are there hairs on the plant.

There are occasions when Agriculturalists will identify plant samples with a few hairs leading to confusion in identification. Conversation ultimately leads to the question, is hybridization between waterhemp, redroot pigweed and Powell amaranth possible? The implication of this question is important, since glyphosate resistant waterhemp is rapidly advancing north and west from watersheds representing the confluence of the Bois de Sioux and Ottertail rivers and the beginning of the Red River in southern Richland and northern Travers counties in North Dakota and Minnesota. This has initiated speculation as to the potential transfer of this resistance trait among other Amaranthus species, like redroot pigweed and Powell amaranth, common in Minnesota and North Dakota.

Waterhemp plants are either male or female (dioecious). Thus, male plants produce only pollen, while female plants produce only seed. This type of biology leads to cross-pollination, or the fertilization of female plants with pollen from one or more male plants. Cross-pollination can greatly increase the genetic diversity of a population, and with genetic diversity comes a wide range of morphological and biological characteristics. Redroot pigweed and Powell amaranth plants have male and female flowers on the same plant (monoecious).

The literature usually focuses on waterhemp and smooth pigweed, another Amaranth species that is monoecious like Powell amaranth and redroot pigweed. Research has determined that hybridization is possible BUT in only one direction, from smooth pigweed to waterhemp and not from waterhemp to smooth pigweed. Thus, it may be difficult for smooth pigweed (or possibly redroot pigweed or Powell amaranth to acquire herbicide resistance from waterhemp despite the species’ cohabitation. By contrast, waterhemp probably has acquired genetic material from smooth pigweed, which may explain how waterhemp has been able to adapt to fields far removed from its origins in the floodplains along the Mississippi, Missouri and Ohio rivers in southern Illinois, Missouri and northern Tennessee.

 

Tom Peters

 Extension Sugarbeet Agronomist

NDSU & U of MN

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