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Effect of Drill-Seeded Soybean Density and Residual Herbicide on Palmer Amaranth (Amaranthus palmeri) Emergence

  • Holden D. Bell (a1), Jason K. Norsworthy (a1) and Robert C. Scott (a2)


Palmer amaranth is the most troublesome weed of soybean in the southern United States. Field experiments were conducted at two Arkansas locations to determine the effect of drill-seeded soybean density on Palmer amaranth emergence. Experimental factors were multiple soybean seeding rates planted on a 19-cm-wide row spacing and the presence or absence of a PRE residual herbicide (flumioxazin plus pyroxasulfone). Soybean groundcover was measured throughout the growing season and daily soil temperature was recorded in selected soybean densities. In the absence of a PRE residual herbicide, at least a 1.7-fold reduction in Palmer amaranth emergence occurred when soybean were present. Differences in Palmer amaranth emergence occurred among soybean densities for both locations, suggesting the value of crop canopy in preventing Palmer amaranth emergence in the absence of an effective residual herbicide. In plots treated with the PRE herbicide, no difference in Palmer amaranth emergence occurred among soybean densities, except for the absence of soybean. Achievement of 95% groundcover by soybean reduced daily soil temperature fluctuations, which in turn reduced Palmer amaranth emergence. For both locations, soybean grain yields were greatest at the highest seeding rate (617,500 seed ha−1). In the presence of flumioxazin plus pyroxasulfone applied PRE, greater grain yields occurred compared to the absence of a PRE herbicide at both Fayetteville and Marianna. Based on this research, an effective PRE-applied residual herbicide has more influence on Palmer amaranth emergence than soybean density, and Palmer amaranth germination and emergence are dependent upon daily soil temperature fluctuations, which is a function of soybean density.

Amaranthus palmeri es la maleza más problemática en soja en el sur de los Estados Unidos. Se realizaron experimentos de campo en dos localidades en Arkansas para determinar el efecto de la densidad de la soja en siembra directa sobre la emergencia de A. palmeri. Los factores experimentales fueron múltiples densidades de soja sembrada en hileras espaciadas a 19 cm y la presencia o ausencia de herbicidas residuales aplicados PRE (flumioxazin más pyroxasulfone). La cobertura de la soja fue medida a lo largo de la temporada de crecimiento y la temperatura del suelo fue registrada diariamente en las densidades de soja seleccionadas. En ausencia de un herbicida residual PRE, se dio una reducción en la emergencia de A. palmeri de al menos 1.7 veces cuando la soja estaba presente. Hubo diferencias en la emergencia de A. palmeri entre las densidades de la soja en ambas localidades, lo que sugiere la importancia del dosel del cultivo para prevenir la emergencia de A. palmeri en ausencia de un herbicida residual efectivo. En las parcelas tratadas con herbicidas PRE, no hubo diferencias en la emergencia de A. palmeri entre las densidades de la soja, con excepción del tratamiento sin soja. El llegar a 95% de cobertura del suelo por parte del dosel de la soja redujo las fluctuaciones diarias de temperatura del suelo, lo que resultó en menor emergencia de A. palmeri. En ambas localidades, los rendimientos de la soja fueron mayores con la densidad de siembra más alta (617,500 semillas ha−1). En presencia de flumioxazin más pyroxasulfone aplicados PRE, hubo rendimientos de grano mayores al compararse con tratamientos sin herbicidas PRE en Fayetteville y Marianna. Con base en esta investigación, un herbicida residual PRE efectivo tiene más influencia sobre la emergencia de A. palmeri que la densidad de la soja, y la germinación y emergencia de A. palmeri dependen de las fluctuaciones diarias en la temperatura del suelo, las cuales están en función de la densidad de la soja.


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Associate Editor for this paper: Lawrence E. Steckel, University of Tennessee.



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Effect of Drill-Seeded Soybean Density and Residual Herbicide on Palmer Amaranth (Amaranthus palmeri) Emergence

  • Holden D. Bell (a1), Jason K. Norsworthy (a1) and Robert C. Scott (a2)


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