Competitiveness of Herbicide-Resistant Waterhemp (Amaranthus tuberculatus) with Soybean

Abstract

Waterhemp [Amaranthus tuberculatus(Moq.) J. D. Sauer] is a troublesome weed occurring in cropping systems throughout the U.S. Midwest with an ability to rapidly evolve herbicide resistance that could be associated with competitive disadvantages. Little research has investigated the competitiveness of differentA. tuberculatuspopulations under similar environmental conditions. The objectives of this study were to evaluate: (1) the interspecific competitiveness of three herbicide-resistantA. tuberculatuspopulations (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 [Glycine max(L.) Merr.]; and (2) the density-dependent response of eachA. tuberculatuspopulation within a constant soybean population in a greenhouse environment.Amaranthus tuberculatuscompetitiveness with soybean was evaluated across five target weed densities of 0, 2, 4, 8, and 16 plants pot−1(equivalent to 0, 20, 40, 80, and 160 plants m−2) with 3 soybean plants pot−1(equivalent to 300,000 plants ha−1). At the R1 soybean harvest time, no difference in soybean biomass was observed acrossA. tuberculatuspopulations. AtA. tuberculatusdensities <8 plants pot−1, the 2AGP-S population had the greatest biomass and stem diameter per plant. At the R7 harvest time, the 2AGP-S population caused the greatest loss in soybean biomass and number of pods compared with the other populations at densities of <16 plants pot−1. The 2AGP-S population had greater early-season biomass accumulation and stem diameter compared with the otherA. tuberculatuspopulations, which resulted in greater late-season reduction in soybean biomass and number of pods. This research indicates there may be evidence of interspecific competitive fitness cost associated with the evolution of 2,4-D, atrazine, glyphosate, and PPO-inhibitor resistance inA. tuberculatus. Focus should be placed on effectively using cultural weed management practices to enhance crop competitiveness, especially early in the season, to increase suppression of herbicide-resistantA. tuberculatus.

Influence of multiple herbicide resistance on growth in Amaranthus tuberculatus

“…Butts et al . () reported that A. tuberculatus populations resistant to two SOA competing with soyabean in pots exhibited slightly lower growth rates than a population susceptible to the same SOA at early growth stages, but at harvest all populations had accumulated the same biomass. Dioecious species, such as A. tuberculatus , can benefit from their genetic diversity and quickly evolve other life‐history traits that could potentially compensate for any fitness penalties during vegetative growth.…”

“…The fitness cost of herbicide resistance can be defined as a reduction in plant productivity and reproduction in a herbicide‐free environment caused by pleiotropic effects of the herbicide resistance mutation (Gressel, ; Vila‐Auib et al ., ). Examples of a fitness penalty are (i) the reduction in offspring that contribute to the next generation by a particular weed population relative to other weed populations (Primack & Kang, ) and (ii) reductions in competitive ability (Butts et al ., ). The fitness cost may be the result of mutations that impede normal plant function, causing a diversion of resources from growth and development processes to the evolved resistance mechanism, or pleiotropic effects from resistance alleles in absence of the herbicide (Vila‐Auib et al ., ) or changes in genome architecture caused by the resistance evolution process (Darmency et al ., ).…”

“…Butts et al . () reported that A. tuberculatus populations with resistance to two SOA produced the same biomass as a population susceptible to the same SOA. The significance of the absence of herbicide resistance fitness penalties is that simply revoking herbicide applications will not cause the herbicide‐resistant condition to be transient within the population, and a cessation in herbicide applications will not restore efficacy within that agroecosystem.…”

Particle drift potential of glyphosate plus 2,4-D choline pre-mixture formulation in a low-speed wind tunnel

“…in which b is the slope at the inflection point, c is the lower limit of the model (μg cm −2 ), d is the upper limit (μg cm −2 ), x is the downwind distance from the nozzle (m), and e is the inflection point distance (m). A variance ratio (F-test) was conducted to determine whether individual or pooled nozzle-type models best fit the data (P ≤ 0.05) (Butts et al 2018d;Oliveira et al 2018).…”

Cultural weed management practices shorten the critical weed-free period for soybean grown in the Northern Great Plains

“…were the prominent broadleaf weeds. Amaranthus retroflexus and other members of the genus have been found to be highly competitive with soybean(Butts et al 2018;Van Acker et al 1993b), which likely influenced the results in 2016. At St-Adolphe in 2017, few broadleaf weed species were observed; nevertheless, low densities of volunteer B. napus were present in the row-spacing experiment.…”