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.

Two field experiments were undertaken at Roseworthy, South Australia from 2006 to 2007 to evaluate the performance of herbicide application strategies for the control of herbicide-resistant rigid ryegrass in faba bean grown in wide rows (WR). The standard farmer practice of applying postsowing PRE (PSPE) simazine followed by POST clethodim to faba bean grown in WR provided consistent and high levels of rigid ryegrass control (≥ 96%) and caused a large reduction (P < 0.05) in spike production (≤ 20 spikes m−2) as compared with nontreated control (560 to 722 spikes m−2). Furthermore, this herbicide combination resulted in greatest yield benefits for WR faba bean (723 to 1,046 kg ha−1). Although PSPE propyzamide used in combination with shielded interrow applications of glyphosate or paraquat provided high levels of rigid ryegrass control (≥ 93%), these treatments were unable to reduce ryegrass spike density within the crop row (20 to 54 spikes m−2) to levels acceptable for continued cropping. Furthermore, a yield reduction (13 to 29%) was observed for faba bean in treatments with shielded application of nonselective herbicides and could be related to spray drift onto lower leaves. These findings highlight that shielded interrow spraying in WR faba bean could play an important role in the management of rigid ryegrass in southern Australia. However, timing of shielded interrow applications on weed control, crop safety, and issues concerning integration with more effective early-season control strategies require attention.

Two field experiments were conducted during 2012 and 2013 at Roseworthy, South Australia to identify effective herbicide options for the management of clethodim-resistant rigid ryegrass in faba bean. Dose–response experiments confirmed resistance in both field populations (B3, 2012 and E2, 2013) to clethodim and butroxydim. Sequencing of the target site of acetyl coenzyme A carboxylase gene in both populations identified an aspartate-2078-glycine mutation. Although resistance of B3 and E2 populations to clethodim was similar (16.5- and 21.4-fold more resistant than the susceptible control SLR4), the B3 population was much more resistant to butroxydim (7.13-fold) than E2 (2.24-fold). Addition of butroxydim to clethodim reduced rigid ryegrass plant density 60 to 80% and seed production 71 to 88% compared with the standard grower practice of simazine PPI plus clethodim POST. Clethodim + butroxydim combination had the highest grain yield of faba bean (980 to 2,400 kg ha−1). Although propyzamide and pyroxasulfone plus triallate PPI provided the next highest levels of rigid ryegrass control (< 60%), these treatments were more variable and unable to reduce seed production (6,354 to 13,570 seeds m−2) to levels acceptable for continuous cropping systems.