Alternative herbicides for the control of clethodim-resistant rigid ryegrass (Lolium rigidum) in clearfield canola in Southern Australia

Abstract

Rigid ryegrass is the most-troublesome, herbicide-resistant weed in cropping systems of southern Australia. Field experiments were undertaken at Roseworthy, South Australia, in 2013 and 2014, to identify effective herbicide options for the control of clethodim-resistant rigid ryegrass in Clearfield canola. PPI trifluralin + triallate followed by (fb) POST imazamox + imazapyr + clethodim + butroxydim had the lowest plant density of rigid ryegrass in 2014 and provided superior control compared with the standard grower practice of PPI trifluralin + triallate fb POST imazamox + imazapyr + clethodim in 1 of 2 yr. Propyzamide either alone or as a split application (PPI fb POST) or in combination with clethodim provided similar rigid ryegrass control to that of the standard grower practice (38 to 553 plants m⠻²). Rigid ryegrass treated with PPI dimethenamid-P, pethoxamid, pethoxamid + triallate, and PPI trifluralin fb carbetamide POST produced significantly more seeds than the standard grower practice, which would lead to reinfestation of subsequent crops. Canola yield responded positively to effective herbicide treatments, especially in 2014, when rigid ryegrass density was greater. PPI dimethenamid-P and pethoxamid alone or in combination with triallate and propyzamide were ineffective in reducing rigid ryegrass density and seed production to levels acceptable for continuous cropping systems.Nomenclature: Butroxydim; clethodim; dimethenamid-P; imazamox + imazapyr; pethoxamid; propyzamide; triallate, trifluralin; rigid ryegrass; rigid ryegrass, Lolium rigidum Gaudin; Clearfield canola; Brassica napus L.