Management of glyphosate-resistant johnsongrass (Sorghum halepense) and junglerice (Echinochloa colona) in soybean in northwestern Argentina

OLEA, I.L.; SABATE, S.; VINCIGUERRA, H.; DEVANI, M.R.; PLOPER, L.D.

Durban

Congreso; World Soybean Research Conference IX (WSRC IX); 2013

Protein Research Foundation

Two glyphosate-resistant biotypes, Sorghum halepense (GR johnsongrass) and Echinochloa colona (GR junglerice), were reported in northwestern Argentina in 2005 and 2009, respectively. This paper presents different strategies developed for their management in soybean crops. Since 2005, different herbicides have been tested in field trials at different scales, for defining management programs for resistant weed biotypes. Also, biological and ecological aspects of these species have been under study, considering their dispersion and distribution in fields. The control of GR johnsongrass with non-selective herbicides (imazapir, imazapic, and nicosulfuron), applied with knap-sap sprayers for patch control at pre-planting, was efficient in reducing rhizome biomass. In highly infested fields, MSMA applied in tank mix with glyphosate is recommended in burndown; however, some antagonisms have been observed for this mix and thus a double knock sequence becomes necessary when other perennial grasses are present. After planting and 20 to 30 days after burndown, the weed re-growths get to an optimum size for the use of imazethapyr or a fop or dim graminicide. In both cases, a tank mix with glyphosate is considered necessary to control other weeds in the field. Generally, one last application with a graminicide may be necessary for controlling late-season´s re-growths which appear over the closed canopy. For GR junglerice, the best burndown treatment was achieved with a double knock treatment, consisting in a fop or dim graminicide application, followed by a glyphosate application one week later. Pre-emergence herbicides diclosulam, clomazone, s-metolachlor, and imazethapyr were efficient as well. Imazethapyr and fop or dim graminicides were also effective in post-emergence control of new seedlings. Their mix with glyphosate allowed the control of other weeds. Rotation with maize is important for diversifying herbicides. In this crop, atrazine + s-metolachlor or isoxaflutole + thiencarbazone-methyl mixtures are recommended for controlling GR junglerice in preemergence. Nicosulfuron and foramsulfuron plus iodosulfuron are the choices for controlling GR johnsongrass in postemergence. The annual or perennial nature of the resistant biotypes, their reproductive behavior, and the availability of registered herbicides, have determined different crop management strategies. Nonetheless, chemical control is still based on the use of old herbicides. For the moment and due to economic reasons, growers rely on graminicides for controlling both biotypes, regardless of the risks of not rotating herbicide modes of action.