Palmer amaranth and waterhemp have become increasingly troublesome weeds throughout the United States. Both species are highly adaptable and emerge continuously throughout the summer months, presenting the need for a residual PRE application in soybean. To improve season-long control of Amaranthus spp., 19 PRE treatments were evaluated on glyphosate-resistant Palmer amaranth in 2013 and 2014 at locations in Arkansas, Indiana, Nebraska, Illinois, and Tennessee; and on glyphosate-resistant waterhemp at locations in Illinois, Missouri, and Nebraska. The two Amaranthus species were analyzed separately; data for each species were pooled across site-years, and site-year was included as a random variable in the analyses. The dissipation of weed control throughout the course of the experiments was compared among treatments with the use of regression analysis where percent weed control was described as a function of time (the number of weeks after treatment [WAT]). At the mean (i.e., average) WAT (4.3 and 3.2 WAT for Palmer amaranth and waterhemp, respectively) isoxaflutole S-metolachlor metribuzin had the highest predicted control of Palmer amaranth (98%) and waterhemp (99%). Isoxaflutole S-metolachlor metribuzin, S-metolachlor mesotrione, and flumioxazin pyroxasulfone had a predicted control ≥ 97% and similar model parameter estimates, indicating control declined at similar rates for these treatments. Dicamba and 2,4-D provided some, short-lived residual control of Amaranthus spp. When dicamba was added to metribuzin or S-metolachlor, control increased compared to dicamba alone. Flumioxazin pyroxasulfone, a currently labeled PRE, performed similarly to treatments containing isoxaflutole or mesotrione. Additional sites of action will provide soybean growers more opportunities to control these weeds and reduce the potential for herbicide resistance.

Nomenclature: 2,4-D; dicamba; isoxaflutole; metribuzin; S-metolachlor; Palmer amaranth, Amaranthus palmeri S. Watson; waterhemp Amaranthus tuberculatus (Moq.) Sauer; soybean, Glycine max (L.) Merr.

Amaranthus palmeri y Amaranthus tuberculatus se han convertido en malezas que son cada vez más problemáticas a lo largo de los Estados Unidos. Ambas especies son altamente adaptables y emergen en forma continua durante los meses de verano, lo que hace que sea necesaria la aplicación PRE de herbicidas residuales en soja. Para mejorar el control de Amaranthus spp. durante toda la temporada de crecimiento, se evaluaron 19 tratamientos PRE sobre A. palmeri resistente a glyphosate en 2013 y 2014 en sitios en Arkansas, Indiana, Nebraska, Illinois, y Tennessee; y sobre A. tuberculatus resistente a glyphosate en sitios en Illinois, Missouri, y Nebraska. Las dos especies de Amaranthus fueron analizadas en forma separada; los datos de los dos años fueron combinados para cada especie, y el efecto sitio-año fue incluido como un efecto aleatorio en los análisis. La disipación del control de malezas a lo largo del curso de los experimentos fue comparada entre tratamientos con el uso de análisis de regresión, donde el porcentaje de control de malezas fue descrito en función del tiempo (el número de semanas después del tratamiento [WAT]). En la media (i.e., promedio) WAT (4.3 y 3.2 WAT para A. palmeri y A. tuberculatus, respectivamente), isoxaflutole S-metolachlor me