Comparative analysis of Palmer amaranth (Amaranthus palmeri) and waterhemp (A. tuberculatus) in Iowa

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Baker, Rebecca
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Hartzler, Robert
Jha, Prashant
Nair, Ajay
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Waterhemp (Amaranthus tuberculatus [Moq.] J.D. Sauer), a native to the Midwestern United States, is difficult to control in crops and has become resistant to several herbicides. Emergence and growth of waterhemp have been studied extensively in Iowa. Palmer amaranth (A. palmeri) is a relative of waterhemp, but it is native to the southwestern United States. It has evolved resistance to several herbicides and was recently introduced to Iowa in its northward expansion through the United States. The objectives of this research were to understand seed bank dynamics, growth, and competitive ability of Palmer amaranth compared to waterhemp. Short-term seed persistence of Iowa Palmer amaranth, Kansas Palmer amaranth, and Iowa waterhemp populations were compared, with seeds left on the soil surface or buried at 15-cm depth in the soil. The Kansas Palmer amaranth population and the Iowa waterhemp population were more persistent than the Iowa Palmer amaranth population at the soil surface, with up to 55% of seeds remaining viable after one year; all populations maintained over 50% viability at 15 cm burial depth. Emergence patterns of the populations were studied using an artificial seed bank. Emergence differed between years, with differences between populations only occurring in 2019. Waterhemp reached 50% emergence 1 to 2 weeks earlier than Palmer amaranth. Both species demonstrated abilities to establish persistent seed bank and sustain season-long emergence. A common garden study compared the growth, biomass, and seed production capabilities of Iowa waterhemp and the two populations of Palmer amaranth, Iowa and Kansas. Across years, females were 24% taller than males. Plant growth rate in 2019 showed differences between species early in the season with Iowa waterhemp having higher relative growth (0.05 cm cm-1 day-1) than Palmer amaranth (0.03 to 0.035 cm cm-1 day-1). Maximum growth occurred on July 9 at around 0.07 cm cm-1 day-1. Palmer amaranth produced 50 to 100% more biomass than waterhemp in 2019 with values of 1,000 g plant-1 for waterhemp and up to 2,000 g plant-1 for Palmer amaranth. Palmer amaranth reached 50% flowering 2 weeks earlier than waterhemp. Seed production was similar between the species due to high variability. Average seed production for Iowa Palmer amaranth, Kansas Palmer amaranth, and Iowa waterhemp was 60,000, 120,000 and 100,000 seeds plant-1, respectively. Waterhemp produced more seed per gram of dry biomass compared to Palmer amaranth, 45,000 and 25,000 seed g biomass-1, respectively, indicating larger seed size for Palmer amaranth than for waterhemp. A greenhouse replacement study compared the competitive abilities of Iowa waterhemp and Iowa Palmer amaranth in five ratios: 100% Palmer amaranth; 100% waterhemp; 75% Palmer amaranth, 25% waterhemp; 50% Palmer amaranth, 50% waterhemp; and 25% Palmer amaranth, 75% waterhemp. After six weeks of growth, Palmer amaranth produced twice as much biomass than waterhemp, 3 and 1.5 g plant-1, respectively, regardless of planting ratio. Relative yield of the species was calculated by dividing biomass produced in mixture with a similar proportion of biomass produced in monoculture. When the species grow similarly in mixture and in monoculture, relative yield is equal to one. Values greater than one indicate greater productivity in mixture, and values less than one indicate decreased productivity in mixture relative to monoculture. Biomass of Palmer amaranth was not affected by competition with waterhemp (relative yield = 1.24), but waterhemp biomass was decreased by competition with Palmer amaranth (relative yield = 0.75). Greenhouse temperatures were greater than early-season field conditions which might have favored the growth of Palmer amaranth over waterhemp. These studies provide evidence that Palmer amaranth is not yet well-adapted to growing conditions in Iowa, especially early in the growing season, and indicate that further introductions and spread of the species should be avoided and carefully managed.
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