Mike Stanyard

Italian ryegrass [ Lolium perenne ssp. multiflorum (Lam.) Husnot] (referred to as L. multiflorum hereafter) is one of the most problematic grass weeds infesting agronomic and specialty crops across the United States. In 2023 to 2025, inadequate control of L. multiflorum populations (NY_R1, NY_R2, and NY_R3) with glyphosate was reported in Livingston, Ontario, and Genesee counties, New York. This research aimed to (1) confirm and quantify glyphosate resistance in these suspected glyphosate-resistant (GR) populations, (2) evaluate the efficacy of alternative postemergence herbicides, and (3) determine whether EPSPS gene amplification confers glyphosate resistance. A known glyphosate-susceptible (GS) population (AR_S) from Arkansas was included for comparison. Glyphosate dose–response assays indicated that NY_R1, NY_R2, and NY_R3 populations were 13-, 4-, and 5-fold resistant, respectively, relative to the AR_S population. Alternative postemergence herbicides, including clethodim, glufosinate, paraquat, and pinoxaden, provided 96% to 97% control and reduced shoot dry weight by 91% to 97% at 21 d after treatment (DAT). In contrast, nicosulfuron provided reduced control (63% to 74%) and limited biomass reduction (51% to 56%), suggesting possible resistance to acetolactate synthase (ALS)-inhibiting herbicides in three tested populations. Quantitative PCR analysis revealed that NY_R1 and NY_R3 had approximately 30-fold higher EPSPS gene copy numbers than AR_S, indicating gene amplification as a mechanism of glyphosate resistance. This study confirms the first case of GR L. multiflorum associated with EPSPS gene amplification in New York, underscoring the need for integrated, diversified weed management strategies to mitigate its spread.

Recent reports of glyphosate-resistant (GR) waterhemp pose serious concerns among crop producers in New York. Field experiments were conducted in 2019 and 2020 near Waterloo, New York, to evaluate the effectiveness of preemergence, postemergence, preemergence followed by (fb) postemergence, and preemergence herbicides fb row cultivation (RC) to control glyphosate-resistant (GR) waterhemp in dicamba-resistant (DR) soybean. In 2019, all two-pass herbicide treatments resulted in significantly reduced GR waterhemp densities (2 to 13 plants m ⁻² ) compared with nontreated plots (160 plant m ⁻² ) at 8 wk after preemergence/2 wk after postemergence, reductions in shoot dry biomass (85% to 99% of nontreated), and greater soybean grain yields (2,659 to 2,936 kg ha ⁻¹ ). In addition, flumioxazin, acetochlor, chlorimuron + flumioxazin + metribuzin, acetochlor + fomesafen + metribuzin, carfentrazone + sulfentrazone + metribuzin, and S -metolachlor + sulfentrazone + metribuzin applied preemergence resulted in significant reductions in GR waterhemp density (3 to 19 plants m ⁻² ) and shoot dry biomass (60% to 98% of nontreated). In 2020, most preemergence (except cloransulam), postemergence only, and preemergence fb postemergence herbicide application strategies resulted in lower densities of GR waterhemp (1 to 46 plants m ⁻² ) compared with nontreated plots (104 plants m ⁻² ) at 8 wk after preemergence/2wk after postemergence and lower shoot dry biomass (66% to 99% of nontreated). All two-pass strategies as well as metribuzin, acetochlor + fomesafen + metribuzin, carfentrazone + sulfentrazone applied preemergence resulted in higher grain yields (3,343 to 4,244 kg ha ⁻¹ ). These results conclude that two-pass strategies tested in this research can consistently provide effective control of GR waterhemp in DR soybean in New York.