Reduced control of Italian ryegrass in California with herbicides has raised concerns about the evolution of populations with resistance to multiple herbicides. The goal of this study was to investigate variation among populations in plant response and resistance to glyphosate and glufosinate in Italian ryegrass from vineyards and orchards in northwest California. Population resistance screening using field-collected seed revealed up to 56.9% of individuals surviving glyphosate treatment at 1,678 g ae ha^-1, and 53.5% of individuals surviving glufosinate treatment at 2,242 g ai ha^-1 in the same population. Frequencies of surviving plants within populations varied among screening times, particularly for glufosinate. Treating vegetatively propagated, genetically identical tillers with each herbicide pointed to separate mechanisms of resistance rather than cross-resistance to glyphosate and glufosinate. Dose—response experiments were conducted for each herbicide at two different screening times using a subset of populations, field-collected seed, and 10 herbicide rates. Plant survival and biomass were evaluated for each population at 3 wk after treatment and for plant regrowth 1 wk thereafter. Log-logistic regression models fit to the data were used to estimate LD₅₀, GR₅₀, and RD₅₀ values and calculate resistance indices (R/S ratios). Based on LD₅₀ values, the most highly resistant population was 14.4- to 19.2-fold more resistant to glyphosate than the most susceptible population tested but only 1.6- to 2.0-fold more resistant to glufosinate than the most susceptible population tested. Levels of resistance to both herbicides varied with screening time period and variable measured. Results indicate high frequencies of glyphosate-resistant plants but an early stage in the evolution of glufosinate resistance in some Italian ryegrass populations of northwest California.

Nomenclature: Glyphosate; glufosinate; Italian ryegrass, Lolium perenne L. ssp. multiflorum (Lam.) Husnot LOLMU.