Management Considerations for Palmer Amaranth in a Northern Great Plains Soybean Production System

Abstract

Core Ideas Soybean yield loss in South Dakota was dependent on Palmer amaranth density if it emerged before canopy closure. Soybean yield losses were not Palmer amaranth density dependent when it emerged at the R2 soybean growth stage. The relative growth rate of Palmer amaranth was rapid and suggests a limited period for effective post‐emergence control. The South Dakota biotype was insensitive to several broadleaf herbicides that typically are recommended for Palmer amaranth control. Palmer amaranth (Amaranthus palmeri S. Watson) was first observed in a South Dakota field in 2015. This study assessed Palmer amaranth growth based on planting date (PD), impact on soybean [Glycine max (L.) Merr.] yield, and response of seedlings of South Dakota biotype seedlings to herbicides with different mechanisms of action (MOA). Soybean yield loss was influenced by Palmer amaranth density in 2016 (p = 0.001), with yield losses of 33% at densities greater than 15 plants m−2 (R2 = 0.65), although yield losses at low densities were greater than predicted by the fitted model. In 2017, yield loss was not correlated to Palmer amaranth density when Palmer amaranth established later in the season. Relative growth rates (RGR) of Palmer amaranth (based on plant volumes) were rapid just after transplanting, irrespective of the initial PD (ranging from mid‐May to mid‐June). Late‐planted cohorts had lower final volumes (0.23 m3) at August harvest compared with early planted cohorts (6.5 m3), but even late‐planted cohorts were two to three times larger than other common South Dakota Amaranth species [A. retroflexus L. and A. tuberculatus (Moq.) Sauer], which emerged at similar times. In greenhouse studies, labeled rates of atrazine (6‐chloro‐N‐ethyl‐N‐(1‐methylethyl)‐1,3,5‐triazine‐2,4‐diamine), glyphosate (N‐(phosphonomethyl)glycine)), and mesotrione (2‐[4‐(methysulfonyl)‐2‐nitrobenzoyl]‐1,3‐cyclohexanedione) did not control Palmer amaranth plants grown from SD biotype seed, but were controlled with S‐metolachlor (2‐chloro‐N‐(2‐ethyl‐6‐methylphenyl)‐N‐[(1S)‐2‐methoxy‐1‐methyethyl]acetamide), dicamba [3,6‐dichloro‐2‐methoxybenzoic acid], and glufosinate (2‐amino‐4(hydroxymethylphosphinyl)butanoic acid). However, Palmer amaranth biotypes resistant to S‐metolachlor, dicamba, and several other herbicides have been reported, so techniques to limit future herbicide resistance should be followed.