Soybean density and Palmer amaranth (Amaranthus palmeri) establishment time: effects on weed biology, crop yield, and economic returns

Authors: KORRES, NICHOLAS - Oak Ridge Institute For Science And Education (ORISE); NORSWORTHY, JASON - University Of Arkansas; MAUROMOUSTAKOS, ANDY - University Of Arkansas; Williams, Martin

Submitted to: Weed Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/24/2020
Publication Date: 6/2/2020
Citation: Korres, N.E., Norsworthy, J., Mauromoustakos, A., Williams II, M.M. 2020. Soybean density and Palmer amaranth (Amaranthus palmeri) establishment time: effects on weed biology, crop yield, and economic returns. Weed Science. 68(5):467-475. https://doi.org/10.1017/wsc.2020.41.
DOI: https://doi.org/10.1017/wsc.2020.41

Interpretive Summary: The development of an integrated weed management system requires detailed information on weed:crop interactions, including the relative competitive ability of the crop during various phases of development on weed growth. Manipulation of soybean density as a cultural approach to increase crop competitiveness and suppress Amaranthus palmeri is promising. Nevertheless, little is known if increases in soybean density can affect the growth and seed production of A. palmeri populations established at various timings during the growing season. Furthermore, can higher soybean densities increase the producer’s income knowing the high seed cost of genetically modified soybean? We found, by conducting two-year field experiments, that medium soybean density (i.e. approximately 220,000 plants ha-1) surpassed the effects of low (110,000 plants ha-1) while equally suppressed A. palmeri, compared to high density (i.e. appr. 350,000 plants ha-1) through biomass and fecundity reductions of the early established weed populations. No effects were observed for late established weed populations. Grain soybean yield reductions were recorded for early established A. palmeri populations (i.e. 0 and 1 weeks after crop emergence (WAE)) compared to late weed populations (i.e. 6 and 8 WAE) for both 2014 and 2015 experimental years. Medium and high soybean densities resulted in higher soybean yields compared to low soybean density, but no grain yield benefits were observed between medium and high soybean densities. The financial benefits of moderate crop density compared either to low or high densities, in terms of crop revenue, net income returns, and break-even price were revealed when crop budget analysis was conducted. Differential performance of A. palmeri gender regarding the higher biomass production of females under crop presence merits further investigation.

Technical Abstract: Knowledge of crop-weed interference, as affected by cultural practices such as crop density manipulation and temporal weed establishment, on yield and weed biology can be used for the development of integrated weed management systems. Two field experiments were conducted in 2014 and 2015 to investigate the effects of soybean density and Amaranthus palmeri establishment timings on height, biomass and seed production of the weed and crop yield. Higher dry weight and seed production was recorded at early A. palmeri establishment timings (i.e. 0, 1, and 2 weeks after crop emergence (WAE)) compared with later establishment timings (i.e. 4, 6, and 8 WAE). Seed production reductions between 31-68%, 57-89% and 70-95% at 0 and 1 WAE for low, medium or high crop densities respectively in comparison to seed produce in weedy monocultures were recorded in 2014 and 2015 respectively. These were proportional to greater biomass production at earlier A. palmeri establishment timings and lower crop density compared to later establishment timings and higher soybean densities. Differential performance of A. palmeri gender regarding the higher biomass production of females under crop presence merits further investigation. Grain yield reductions between 15-21% 0 WAE and 14-20% 1 WAE were recorded in 2014 and 2015 respectively compared to 8 WAE establishment timing. Medium and high soybean densities resulted in higher soybean yields compared to low soybean density, but no grain yield benefits were observed between medium and high soybean densities. Crop budget analysis revealed the benefits of moderate seeding rate increases in comparison to lower (i.e. 125,000 seeds ha-1) or high (i.e. 400,000 seeds ha-1) seeding rates on crop revenue, net income returns and breakeven price. Earlier A. palmeri establishment timings (i.e. 0, 1 and 2 WAE) resulted in lower crop revenue and net income returns compared to later establishment timings of the weed.