Abstract Background Crop tolerance to herbicides and the effectiveness of weed control are key considerations of a chemical management program. Crop tolerance levels and the mechanism involved is of great importance in ensuring the safe use of an herbicide. Objective This study aims to evaluate the effect of mefenpyr-diethyl as a safener and metabolic inhibitor (organophosphorus insecticides) on the penoxsulam tolerance of white oat cultivars. Methods We determined the relative tolerance (%) of URS Guará and URS Guria white oat cultivars to different doses of penoxsulam with and without prior application of organophosphorus metabolic inhibitors (malathion + chlorpyrifos). We then determined the relative tolerance (%) of the white oat cultivar URS Guria with and without prior application of a protective metabolization stimulator (mefenpyr-diethyl). Results Increased penoxsulam doses resulted in lower relative tolerance of both cultivars. The application of organophosphorus insecticides reduced the penoxsulam tolerance of white oat cultivars, with a 14.4 and 4.8 fold reduction of C50 in the URS Guará and URS Guria cultivars, respectively. The use of mefenpyr-diethyl increased the penoxsulam tolerance of URS Guria by a factor of 2.62. Conclusions The penoxsulam tolerance of white oat cultivars was reduced by organophosphorus insecticides and increased by mefenpyr-diethyl. Metabolization is the likely mechanism of tolerance of white oats to this herbicide.

Abstract Background The limited number of selective herbicides for white oat (Avena sativa L.) cultivation makes it managing weeds a challenge. Currently, few herbicide options are available for selective weed control in winter crops. Previous studies have indicated that ALS-inhibiting herbicides could be potential alternatives, so studies were conducted. Objective To characterize the tolerance level of white oat cultivars to ALS-inhibiting herbicides and to evaluate the level of weed control by applying penoxsulam and bispyribac-sodium. Methods Tolerance (%) was determined for white oat cultivars UFRGS 14, URS Guará, URS Guria, UFRGS 18, and URS Taura for imazethapyr, iodosulfuron, penoxsulam, and bispyribac-sodium dose-response. Also, the control efficiency of different doses of penoxsulam and bispyribac-sodium was determined on weed species of white oat, namely crabgrass (Digitaria horizontalis), siberian motherwort (Leonurus sibiricus), italian ryegrass (Lolium multiflorum), and wild radish (Raphanus raphanistrum). Results Imazethapyr and bispyribac-sodium were the least selective herbicides, whereas iodosulfuron had intermediate selectivity, and penoxsulam was the most selective for the white oat cultivars tested, with average control of 100, 72, 62.7 and 40%, respectively. Overall, bispyribac-sodium had a higher level of weed control (77.1%) than penoxsulam (62%). Label doses of penoxsulam and bispyribac-sodium resulted in low control levels of crabgrass. Conclusions All doses of bispyribac-sodium effectively control wild radish and italian ryegrass. The recommended dose of penoxsulam was selective for some cultivars of white oat and effectively controlled italian ryegrass and wild radish.