Waterhemp (Amaranthus tuberculatus) is a troublesome weed with an ability to rapidly evolve herbicide resistance. It occurs in cropping systems throughout Nebraska and the Midwest. Little research has investigated the competitiveness of different waterhemp populations, specifically herbicide-resistant populations, under similar environmental conditions.
The objective of this study was to evaluate the competitiveness of three herbicide-resistant Nebraska waterhemp populations:
- 2,4-D- and atrazine-resistant (2A-R),
- glyphosate- and protoporphyrinogen oxidase (PPO)-inhibitor-resistant (GP-R), and
- 2,4-D-, atrazine-, glyphosate-, and PPO-inhibitor-susceptible (2AGP-S)
with soybean in a greenhouse environment.
Additionally, plant biomass and stem diameter of each waterhemp population was measured to identify self-competition effects between waterhemp plants within a constant soybean population. Waterhemp competitiveness with soybean was evaluated across five target weed densities of 0, 2, 4, 8, and 16 plants per pot (equivalent to 0, 2, 4, 7, and 15 plants per ft2) with three soybean plants also growing in each pot (equivalent to 121,500 plants per acre) (Figure 1).
Soybean and waterhemp plants were harvested at two timings (R1 and R7 soybean growth stages). Data collected included soybean biomass, number of soybean pods (R7 growth stage-only), waterhemp biomass, and waterhemp stem diameter. At the R1 soybean harvest time, no difference in soybean biomass was observed across waterhemp populations (Figure 2).
At waterhemp densities of less than eight plants per pot, the 2AGP-S population had the greatest biomass and stem diameter per plant (Figure 3); therefore, the 2AGP-S population was more competitive early-season, but that did not translate to greater soybean yield loss (biomass) early-season compared with the other waterhemp populations.
At the R7 harvest time, the 2AGP-S population caused the greatest loss in soybean biomass (Figure 4) and number of pods (Figure 5) compared to other waterhemp populations at densities of less than 16 plants per pot.
At the R7 harvest time, all waterhemp populations had achieved similar biomass and stem diameters across the waterhemp densities (Figure 6). Therefore, the 2AGP-S waterhemp population had greater biomass and stem diameter early-season compared to the other waterhemp populations, which resulted in greater soybean yield loss (soybean biomass and number of pods) late-season.
This research indicates there may be evidence of a competitive fitness cost associated with the evolution of 2,4-D, atrazine, glyphosate, and PPO-inhibitor resistance in waterhemp. Weed management strategies should effectively use cultural weed management practices (narrow row widths, early planting dates, and other agronomic strategies to increase plant vigor) to enhance crop competitiveness, especially early in the season, to increase suppression of herbicide-resistant waterhemp.
For more details about this research, please view the full journal article in Weed Science.