Italian ryegrass – Lolium multiflorum (Photo by Joe DiTomaso)

Italian ryegrass (Lolium multiflorum) is an annual grass that can sometimes behave as a biennial or short-lived perennial in California. The species is an upright grass (to about 3 feet in height) that germinates in the late fall and grows vigorously through the winter and early spring. The species can be identified by its dark green, glossy and hairless leaves that are rolled in the bud. Auricles are well-developed and the ligules are long and membranous. Once flowering occurs, ryegrass is easily distinguishable by alternating spikelets that run along the length of the main flowering stem (April through September). Additional details regarding identification can be found on the UCIPM website. Italian ryegrass has become problematic in the Sacramento Valley, particularly in tree and vine systems and in rainfed wheat acreage. This is in part due to its biology. Italian ryegrass is an obligate out-crossing wind-pollinated species. This means resistance genes can be shared easier than would be the case with self-pollinating populations of grass species. With respect to winter cereals, Italian ryegrass germinates and matures approximately at the same time as the crop and can be extremely competitive for soil nutrients, causing up to 80% reductions in winter wheat grain yield. 

Across the United State, the control of Italian rygrass in wheat has been compromised by the development of biotypes resistant to acetolactate synthase (ALS) and/or acetyl Co-A carboxylase (ACCase) inhibiting herbicides including Osprey (mesosulfuron-methyl) and Axial (pinoxaden) (http://weedscience.org/). In California, populations of ryegrass collected from alfalfa and orchards have been formally confirmed with resistances to multiple herbicides, which include Roundup (glyphosate), Gramoxone Inteon (paraquat), Raptor (imazamox), Osprey, and several ACCase inhibitors. Last year, Dr. Mariano Galla, a former Farm Advisor in Glenn County, reported that some cereal growers in the Sacramento Valley were also having difficulty controlling Italian ryegrass with herbicides currently labeled for use in small grains (https://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=28285). UCCE trials have since been initiated in the Esparto area west of the Dunnigan Hills to compare both chemical and non-chemical strategies for the control of herbicide resistant ryegrass biotypes in wheat.

The take-home from the grower's perspective is simply that growers should be strategic and vigilant in their approach to ryegrass.

• Destroy emerged ryegrass seedlings before planting (following initial fall rains) using cultivation or a burndown herbicide. Make sure that clods are crumbled to prevent ryegrass survival. 

• Apply in-crop herbicides like Osprey, Axial, and Simplicity (pyroxsulam) before ryegrass plants produce more than two tillers to maximize control. Remember to follow label rates, registrations, and instructions. 

• Check sprayer nozzles to ensure complete coverage and minimize sub-lethal doses to ryegrass seedlings (which could inadvertently accelerate selection for herbicide resistance).

• Scout fields following herbicide applications to assess herbicide effectiveness and to identify possible resistant plants.

• Manage ryegrass in ditches and field edges by cultivation and/or spraying to prevent putative herbicide resistant individuals from becoming established.