Italian ryegrass

Images above: Upper right: Italian ryegrass spike (Jack Clark, University of California). Upper left: Italian ryegrass plant in flower (Joseph DiTomaso, University of California, Davis). Bottom: Italian ryegrass seedling (Jack Clark, University of California).

Identification

Other common names:  annual ryegrass, Australian rye grass, Australian rye

Family:  grass family, Poaceae

Habit:  Summer annual, winter annual or biennial bunch grass or, in mild climates like the Pacific Northwest, a short-lived perennial (Hannaway 1999, UC SAREP).

Description:  The seed leaf emerges vertically.  The first true leaves of the seedling are rolled in the bud, dark green, have a 0.1” (0.25 cm) translucent ligule and small inconspicuous auricles, and leaf blades are 4-5” (10-13 cm) long by 0.1” (0.25 cm) wide with obvious, raised veins and glossy undersides.  Blades and sheaths both lack hairs.  Sheaths are red at the base.  Mature plants of upright, sturdy Italian ryegrass reach 20-50” (50-130 cm) in height and have a coarse texture.  Stems are pale yellow to red at the base; sheaths are round and vary in texture.  Older leaf blades measure 4-8” (10-20 cm) long by 0.2-0.4” (0.5-1 cm) wide and are rough with prominent, raised veins, smooth edges, and glossy undersides. The broad collar region has long, slender, claw-like auricles and a flat, translucent ligule that may exhibit light tearing or fringing.  Blades are rough, with prominent, raised veins, smooth edges, and glossy undersides.  Inflorescences are flat, 4-15” (10-38 cm) long spikes located at the end of stems.  Spikelets 0.4-0.7” (1-1.8 cm) long, containing 10-20 flowers are attached to the spike alternately so that the length of the spikelet runs parallel to the stem.  Spikelets have one thin, papery, 0.3-0.5” (0.8-1.3 cm) long glume attached at the base of the spikelet.  Each individual flower has two 0.4” (1 cm) chaffs and one long awn.  As with all grasses, seeds are covered by a thin, tight layer of fruit tissue.  They are, oval and 0.3” (0.8 cm) long, including the chaff but not the long awn.  The flattened seed base is encircled by a raised ridge.

Similar species:  Perennial ryegrass (Lolium perenne L.) has spikelets that consist of only 6-10 flowers and lacks awns. While the leaves of Italian ryegrass are rolled in the bud, those of perennial ryegrass are folded (Fischer et al. 1985).  Quackgrass [Elymus repens (L.) Gould], a rhizomatous perennial, also has auricles and a flat, terminal spike inflorescence.  However, it is distinguishable by its sharp, pointed rhizome tips, longer seedling blades (4-8” = 10-20 cm), very short ligule, and two glumes at the base of each spikelet in its inflorescence. 

Management

Italian ryegrass is a serious weed of winter wheat throughout the Southeast, southern Midwest and far West.  It grows relatively poorly in hot weather, and it can be controlled effectively in summer row crops with a combination of tine weeding, inter-row cultivation and hilling up of the crop at lay-by.  Consequently, rotation of winter grains with summer row crops interrupts the life cycle of Italian ryegrass and reduces populations. 

Since the seeds die off rapidly and must be close to the soil surface for successful emergence, plowing under the seeds can also aid long term control.  Tilling under ryegrass seeds on the soil surface greatly reduced density of the weed in a subsequent winter grain crop relative to use of minimum tillage (Trusler et al. 2007).  However, the seeds produced by a bad infestation may be so numerous that rotating into a hay crop for several years may be required to substantially reduce the seed bank, provided the mowing regimen prevents seed production. 

To reduce infestations within grain crops, tine weed before emergence and again when the crop has three leaves.  Tine weeding can be done more aggressively if the crop is planted at a consistent depth and the density of plants in the row is sufficient to deflect the weeder’s tines out of the row.  A weeder with stiff, straight or 45° bent tines will work best to break and bury young ryegrass at the post emergence weeding.  A relatively high crop seeding rate also will increase wheat yield and decrease dockage, especially when wheat is planted in a narrow row spacing (3" = 7.5 cm vs. 8” = 20 cm) (Justice et al. 1994).

At harvest, use a chaff collection system on the combine or set the combine to collect as many of the ryegrass seeds as possible and then clean them out of the grain afterward.  Although some seeds will shatter to the ground during combining, Italian ryegrass tends to mature later than winter wheat (Justice et al. 1994).  Most seeds will still be on the plants at the time of wheat harvest, particularly if wheat harvest is not delayed (Maity et al. 2022).  Collecting seed with the combine can tip the balance toward long term control of the weed.  Tiller numbers were reduced by 29-69% in the spring following Italian ryegrass seed removal during harvest of the preceding wheat crop (Beam et al. 2019).  The cleanings can be fed to cattle, but only after thorough grinding since a large proportion of undamaged seeds will pass through the animals’ gut.  If the field is cut higher than 1.5" (4 cm), graze the stubble to prevent regrowth of the ryegrass.  Alternately, Italian ryegrass seeds can be killed by heat produced from narrow-windrow burning of crop residues after harvest (Maity et al. 2022, Norsworthy et al. 2020, Spoth et al. 2022).  Narrow-windrow burning of ryegrass infested wheat residue reduced plant and seedbank densities of Italian ryegrass substantially after four years of treatment (Maity et al. 2022).  When all other measures fail, some growers harvest small grains infested with Italian ryegrass as a high quality forage to prevent ryegrass seed production (Trusler et al. 2007).  

Many of the tillage, cultivation and crop rotation strategies discussed above apply to vegetable crops as well as grains.  Flush seeds out of the soil with a short fallow before planting cool season vegetable crops.  Plastic mulches effectively suppress this weed.  Italian ryegrass has been used as a cover crop, but this should be avoided in areas where it is a weed problem.  It also can be a problem when using other winter annual cover crops such as rye or hairy vetch because it continues to grow and sets seeds after these cover crops are mechanically terminated (J. Teasdale, personal observation).

Ecology

Origin and distribution: Italian ryegrass is native to southern Europe, southwest Asia and northwest Africa, but has been widely introduced throughout the temperate zones and upland tropics (Beddows 1973).  It now occurs in North America, Australia, New Zealand, and South America (Lacefield et al. 2003, Hannaway et al. 1999).  In North America, it occurs throughout the U.S.A. and Canada (USDA Plants) and in Mexico (Hannaway et al. 1999).

Seed weight: 1.3 to 2.6 mg, with a mean of 2.0 mg (EFBI)

Dormancy and germination:  Most seeds of Italian ryegrass lack primary dormancy although some biotypes produce up to 55% dormant seeds.  These seeds lose dormancy after 6 months of after-ripening (Hides et al. 1993).  Usually, seeds germinate soon after entering the soil if moisture is sufficient (Rapp 2006).  Seed viability can be as high as 90-95% although germination rates in the field are often lower than this (USGS 1996).  Seeds will germinate at temperatures as low as 36 °F (2 °C), with the greatest germination occurring at day/night temperature from 50/41 to 77/41 °F (10/5 to 25/5 °C).  When after-ripened, seeds can germinate at day temperatures as high as 86 to 104 °F (30 to 40 °C) (Young et al. 1975).  Light promotes germination of previously buried seeds, but substantial germination can still occur in the dark (Schafer and Chilcote 1970).  Cold (40-50 °F = 5-10 °C), saturated soil conditions tend to preserve seeds in a viable, but non-germinating state, and induce secondary dormancy (Schafer and Chilcote 1970).  Germination and establishment proceed more rapidly than that of other grasses under cool conditions (Hannaway et al. 1999, UC SAREP).

Seed longevity:  In undisturbed soil in Oregon, seeds of Italian ryegrass died off at a rate of about 58% per year and depletion of the seed bank occurred more rapidly in well drained than in poorly drained soil (Rampton & Ching 1970).  In Wales, an average loss rate of 58% per year was observed (Lewis 1973).  In another experiment in Oregon, an average of only 36% of seeds survived burial for 180 days from October to April, indicating a more rapid rate of seed bank decline (Schafer and Chilcote 1970).

Season of emergence:  Spring seedlings can emerge anytime from March to May, depending upon local conditions (USDA Plants).  In California and Oregon, emergence usually occurs with the onset of the fall rainy season (Fischer et al. 1985, Holloran et al. 2004, Young et al. 1975).  Fall emerging plants will function as winter annuals in climates where they survive winter (UC SAREP, USDA Plants). 

Emergence depth:  Italian ryegrass emerges best from depths of 0.25-0.5” (0.6-1.3 cm) (Lacefield et al. 2003). 

Photosynthetic pathway:  C3 (Hannaway 1999). 

Sensitivity to frost:  Italian ryegrass is sensitive to severe cold (EFBI), but grows as a winter annual in Plant Hardiness Zone 6 and south (SAN 1998). 

Drought tolerance:  Italian ryegrass grows poorly during drought (Lacefield et al. 2003).  Growth is also reduced when temperatures exceed 77 °F (25 °C), even when moisture is available (Hannaway 1999).  On non-irrigated sites, the fibrous root system can grow to over 3 feet deep and well-established plants have the potential to tap deep soil water reserves to persist through drought periods.  Unlike many other grass species, roots will not regrow after drying out (Young et al. 1975). 

Mycorrhiza:  Annual ryegrass is a mycorrhizal species (Dhillion and Friese 1994, Omacini et al. 2006).

Response to fertility:  Nitrogen is the most critical nutrient for growth of Italian ryegrass, and the species responds favorably to very high N application rates (Hannaway et al. 1999).  Annual ryegrass grows well at pH 5.0-7.8 (Hannaway et al. 1999) but does best from pH 5.5-7.0 (Valenzuela and Smith 2002). 

Soil physical requirements:  Italian ryegrass is well adapted to a wide range of soil types from fine to coarse textured (USDA Plants).  It tolerates saline soil conditions (USDA Plants).  It tolerates wet soil conditions and can survive flooding for 15-20 days provided temperatures are below 81 °F (27 °C) (Hannaway 1999). 

Response to shade:  Although some sources list Italian ryegrass as being shade intolerant (Valenzuela and Smith 2002, Holloran et al. 2006), the species also has been described as moderately shade tolerant (USDA Plants).  We have found that it can establish and grow within a cover crop canopy as well as under a closed corn canopy.

Sensitivity to disturbance:  Trampling by livestock can kill the plant when the foliage is frosted.  Italian ryegrass regrows quickly after mowing if the weather is cool and the plant is cut above 3” (7.5 cm).  Cutting stems below the growing point, for example at 1” (2.5 cm), causes the plant to weaken or die (Hannaway et al. 1999, UC SAREP).  Plants should be mowed or killed before early bloom to prevent seed set.

Time from emergence to reproduction:  Fall emerging plants flower in late spring and early summer in Colorado and Montana, but flower in early spring in many other areas (USDA Plants).  More than 11 hours of daylight are required to initiate flowering (Hannaway et al. 1999).  The embryo becomes viable 10 days after pollination and seeds mature 20 days later (Beddows 1973, Hides et al. 1993).

Pollination:  Cross fertilization by wind-borne pollen is normal, but self-pollinated plants often produce a few seeds (Beddows 1973).

Reproduction: Drilled, fertilized stands of Italian ryegrass produced 113-121 seeds per inflorescence and roughly 300 seeds per plant (Hides et al. 1993). Plants in the Pacific Northwest produced 400-1100 seeds per plant (San Martin et al. 2021). Lower seed production would be expected from plants growing with a competitive crop .At six site-years in central Oregon and Washington, seed retention at the beginning of wheat harvest averaged 41%, with little variability (San Martin et al. 2021). However, in the Southcentral U.S.A., seed retention at wheat harvest varied considerably by year and location (Maity et al.2022), a result partly explained by higher retention under cooler, humid conditions and lower retention under hot, dry conditions. Plants do not spread vegetatively (USGS 1996).

Dispersal:  The chaff has short bristles (USDA Plants) that probably aid in dispersal on animal fur.  About one-third of seeds pass through cattle unharmed and could be dispersed with manure; however, holding the manure in a tank for 3 months kills the seeds (Takabayashi et al. 1979). The species is a contaminant of grass seed unless the seed is produced under strict standards (Rampton and Ching 1970).

Common natural enemies:  Mites sometimes damage Italian ryegrass, particularly during dry weather (Valenzuela and Smith 2002).  Crown rust (Puccinia coronata) and gray leaf spot (Pyricularia sp.) are common disease of the species.  Stem rust (Puccinia graminis ssp. graminicola) is common in Oregon but rare in the Southeast (Blount et al. 2005).  Ryegrass blast (Magnaporthe grisea) can cause substantial damage in the Southeast (Trevathan et al 1994).

Palatability:  Humans do not eat any part of Italian ryegrass.  The species makes nutritious forage for livestock, and several million acres are planted with the species, primarily for winter pasture in the Southeast and Northwest.  Italian ryegrass can cause annual ryegrass toxicosis, but the disease is rare.  The toxic compounds are only produced when the species is infected by the nematode Anguina agrostis and only when this nematode is infected by a bacteria, Clavibacter toxicus, which is itself infected with a virus (Hannaway et al. 1999).  

References:

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• UC SAREP Cover Crop Database.  Annual ryegrass.  https://ucanr.edu/sites/asi/db/covercrops.cfm?crop_id=2
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