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Selection by herbicides has resulted in widespread evolution of herbicide resistance in agricultural weeds. In California, resistance to glyphosate was first confirmed in rigid ryegrass in 1998. Objectives of this study were to determine the current distribution and level of glyphosate resistance in Italian ryegrass, and to assess whether resistance could be due to an altered target site. Seeds were sampled from 118 populations and seedlings were treated with glyphosate at 866 g ae ha−1. Percentage of survivors ranged from 5 to 95% in 54 populations. All plants from 64 populations died. One susceptible (S) population, four putatively resistant (R) populations, and one S accession from Oregon were used for pot dose–response experiments, shikimic acid analyses, and DNA sequencing. Seedlings were treated with glyphosate at eight rates, ranging from 108 to 13,856 g ae ha−1. Shoot biomass was evaluated 3 wk after treatment and fit to a log-logistic regression equation. On the basis of GR50(herbicide rate required to reduce growth by 50%) values, seedlings from putatively R populations were roughly two to 15 times more resistant to glyphosate than S plants. Shikimic acid accumulation was similar in all plants before glyphosate treatment, but at 4 and 7 DAT, S plants from California and Oregon accumulated approximately two and three times more shikimic acid, respectively, than R plants. Sequencing of a cDNA fragment of the EPSPS coding region revealed two different codons, both of which encode proline at amino acid position 106 in S individuals. In contrast, all R plants sequenced exhibited missense mutations at site 106. Plants from one population revealed a mutation resulting in a proline to serine substitution. Plants from three R populations exhibited a mutation corresponding to replacement of proline with alanine. Our results indicate that glyphosate resistance is widespread in Italian ryegrass populations of California, and that resistance is likely due to an altered target enzyme.

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Companion crops have been used in alfalfa (Medicago sativa L.) establishment to increase forage yield and decrease weeds. When oat (Avena sativa L.) is used as a companion crop, the typical seeding rate is 50 to 75 kg ha−1. In irrigated situations this seeding rate has depressed alfalfa yield and stand density. This study was conducted under irrigated conditions to evaluate alfalfa yields, forage composition at first harvest, and alfalfa and weed densities at four oat seeding rates; 0 to 36 kg ha−1. Two field experiments were established at Madera and one at Courtland, CA. The interplanting of oat with alfalfa increased first‐harvest forage yields 2.45 to 8.62 Mg ha−1, relative to alfalfa planted alone. Alfalfa yields at subsequent cuttings during the first season were reduced by the oat companion crop. By the last cutting in the first season and all cuttings in the second season, yields were equal for all treatments. First‐season forage yields were increased 1.54 to 5.05 Mg ha−1 by an oat companion crop. The oat component of the first cutting ranged from 71 to 98%. Alfalfa and weed biomass were both reduced by the oat companion crop. The oat companion crop reduced alfalfa stand density during establishment, but density was higher at the beginning of the second season at two of the three sites when the oat seeding rate was 18 kg ha−1. Alfalfa established with an oat companion crop had an average of 30% fewer weeds in the second year when 9 kg ha−1 were used and almost 50% fewer weeds at 18 kg−1. Thus, the seeding rate of an oat companion crop considered best for optimizing yields and reducing weeds under irrigation in California is approximately 18 kg ha−1.

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