Amaranthus Palmeri in South Dakota and Herbicide Options.

Palmer amaranth (Amaranthus palmeri) is a native southern weed that has been extending its range northward. In its southern range, Palmer is problematic due to fast growth rate, poor control once it reaches a 4 cm height, high seed production, and herbicide resistance to many different herbicide modes-of-action. It has recently been found in South Dakota, introduced as a seed contaminant of agricultural products. There is little known about the basic biology of Palmer in South Dakota. Seeds from an infestation of Palmer amaranth near Corsica, SD were planted into pots of potting mix or sand in a greenhouse and grown to the 3- to 4-leaf stage. Plants were split into 14 treatments based on soil type, herbicide, and timing (pre- or post-emergence). Herbicide applications included atrazine, S-metolachlor, dicamba, glufosinate, glyphosate, mesotrione, and thifensulfuron. All treatments were rated based on percentage of Palmer amaranth controlled 23 days after application (post-emergence) and 48 DAA (pre-emergence). S-metolachlor showed the most control over both soil types pre-emergence. Atrazine displayed minimal control pre- and post-emergence in sand and potting mix as well as thifensulfuron, which was applied post-emergence. Glyphosate had the third lowest control (60 percent) and all others rating at 90 percent or higher. Complete control was observed from the dicamba application. From this study, the infestation of Palmer amaranth found in Corsica, SD may be resistant to atrazine. Glyphosate does offer some control but may not be a viable option. S-metolachlor and dicamba offer the greatest chance of control in soybeans of the herbicides tested. Some herbicides tested may not be used in soybeans but do offer good control in other crops such as corn.