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Not Always a Weed? Annual ‘Italian’ Ryegrass

Wheat producers have legitimate concerns about ryegrass, a notoriously invasive weed in cereal crop rotations. It seems crazy to wheat growers for anyone to plant Italian ryegrass anywhere for any reason.  It is a matter of perspective, however, whether Italian ryegrass is a weed or an excellent forage crop with benefits to soil health.  While some in Idaho are using it, experts caution that it requires meticulous management and is never recommended as part of a cereal rotation.  

Dr. Albert Adjesiwor, weed management specialist at the University of Idaho (UI) Research and Extension Center in Kimberly, agrees with them, “Personally, I don’t think is ever a good idea to plant Italian ryegrass in cropping systems containing cereals.”  This article will explore why ryegrass is planted in Idaho, the risk Italian ryegrass poses to cereal crops, and best management practices when ryegrass is planted in your region.

The ryegrass species, Lolium, has many sub-species that vary widely in characteristics and hybridize within the genus, resulting in many intermediate forms.  Ryegrass species are usually diploids (containing two sets of chromosomes) but easily form tetraploids (containing four sets of chromosomes). This genetic plasticity has allowed some ryegrass species to quickly develop herbicide resistance across most known herbicide sites of action. 

However, besides an invasive weed, ryegrass is a high-quality forage for cattle, dairy cows, and sheep.  Together, cattle and dairy operations accounted for 4.5 billion of the 8.5 billion farm cash receipts in Idaho.  Planting ryegrass for use in forage and grazing operations is an established practice in Idaho. 

Perennial ryegrass (Lolium perenne L.) flowers and sets seed in the second year after vernalization i.e. experiencing temperatures of 45 degrees or lower for 8 weeks. It is one of the highest quality forage grasses for pasturing cattle and sheep. Not surprisingly, it is grown on 250,000 acres in the northeast United States and in the states of Washington and Oregon. Perennial ryegrass can be found throughout Idaho in golf courses, shady lawns, pastures, and forage crops. 

Annual ryegrass (Lolium multiflorum Lam.) commonly called “Italian” ryegrass, is grown on 1.2 million acres mostly in the southeast United States. It has little cold tolerance and behaves like an annual crop in the Midwest.  Annual Italian ryegrass is a high-quality annual grazing grass for dairy cattle. Its high yields and ability to maintain productivity through the midsummer slump make it useful in pasture rehabilitation and revitalization of 5th-year alfalfa fields.

Annual Italian ryegrass is a frequent cool-season grass component of cover crop mixes.  Multi-species mixes are used to create diversity in the crop and do so by having at least one representative from these crop classes: cool-season grass, cool-season broadleaf, legumes, warm-season broadleaf, and warm-season grass.  Annual Italian Ryegrass establishes quickly, developing an extensive fine root system and lots of biomass. Its exceptional grazing qualities make it a desirable component of cover crops intended for grazing. The massive fine root system improves soil structure and supports soil health. 

Annual Italian ryegrass is not dependent on cold temperatures for flowering. It will prolifically set seed the first season.  The crop is killed by frost, but it leaves behind a plentiful bank of seeds in the soil and that is what concerns Dr. Adjesiwor, “Wheat producers use burn-down herbicide applications before planting wheat in the spring or fall to control weeds and disease harbored in the ‘green-bridge’ between crops,” he explained. “Glyphosate is the most commonly used herbicide, and is very effective on annual Italian ryegrass, but overuse in northern Idaho has encouraged the development of herbicide-resistant populations. We want to avoid a similar situation in this region.”  

In southwestern and southcentral Idaho, many broadleaf crops are grown in the rotation including glyphosate-resistant sugar beets and alfalfa.  Glyphosate-resistant crops make it easy to spray out annual Italian ryegrass before it goes to seed, thus preventing seeds from being added to the soil seed bank. In this rotation, cereals are often out of the field for 5 years. Multiple broadleaf crops allow more options for using herbicide chemistries with different modes of action.  In these cropping systems, the soil health benefits outweigh the risk of establishing an herbicide-resistant population of annual ryegrass. 

Crop rotations in southeastern Idaho are more limited, being centered on potato, sugar beet, and cereals. “In these cropping systems, wheat is more frequent in the rotation. Thus, the repetitive use of glyphosate in glyphosate-resistant sugar beets and chemical fallow, places a huge selection pressure for selecting Italian ryegrass with resistance to glyphosate,” explains Dr. Adjesiwor.

Cover crops have been promoted by the NRCS for improving soil health, reducing soil erosion, and improving infiltration of water to reduce nitrate and phosphorus runoff while protecting water resources. Derek Tilly, manager of the NRCS Aberdeen Plant Materials Center in Aberdeen, weighed in on Italian ryegrass, saying, “Occasionally, in pasture or forage applications and cover crops being grazed NRCS recommends incorporating Italian ryegrass.  But NRCS has issued strong warnings about using Italian ryegrass in wheat or cereal rotations.” Tilly referenced a section of the Plant Materials Technical Note No. 67, Cover Crops for the Intermountain West, January 2017, page 7, quoted below.

“It [annual ryegrass] usually winterkills, but if it overwinters, it will grow quickly and produce seed in late spring. It can be a serious problem in oat and wheat crops if allowed to set seed and has been shown to develop herbicide resistance. “ 

AgriTerra, located in Rupert, is one of the largest sellers of mixed-species cover crops in southern Idaho, and markets some specific-use cover crop mixes containing ryegrass. Luke Adams, AgriTerra representative, said this, “We use Tetilia Italian Ryegrass in mixes for forage and cover crops because it doesn’t go to seed in the first year. Mixes are planted late August following small grain, or flown in over standing corn silage, to get fast growth for late grazing in October.  The mixes are followed in the spring with a broadleaf crop, often sugar beet or potato. I don’t ever recommend using a grass species to a customer doing a small grain to small grain crop rotation.”  Adams explained cover crops are intended to break the crop cycle to manage disease, pests, and weeds. Cover crop mixes for use in small grain to small grain rotations would likely have brassica and other species not grown on the farm in the cash crop rotation.

Dr. Adjesiwor warns, “Weeds (or crops) like Italian ryegrass can easily get out of hand no matter how well they are managed because of two main reasons: 1) they easily develop resistance to herbicides, and 2) the seeds can remain dormant in the soil , leading to infestations for multiple years after just one seeding.  If annual Italian ryegrass is the only feasible choice, it must be ensured that it never goes to seed (zero tolerance for escapes), but Italian ryegrass should be avoided if at all possible. Avoid planting small grains for at least two years after planting annual Italian ryegrass to ensure that any volunteer annual ryegrass is identified and controlled.” 

Dr. Jared Spackman, Barley Agronomist, at UI Aberdeen Research and Extension Center, added his perspective to risks associated with planting annual Italian ryegrass. “Because annual (Italian) ryegrass is a quick-growing and an aggressively competitive bunch grass, it is recommended that you only utilize it in your cropping system if you plan to closely manage it through mowing or grazing to prevent it from going to seed. While it generally winterkills, some years it may overwinter, growing quickly in the spring and setting seed. Because annual Italian ryegrass is so aggressive, it will outperform other small grains, especially under irrigation, and other species when used in a cover crop mix. It should not be planted before wheat or other small grain rotations.” 

Ryegrass plays many roles in cropping systems, including herbicide-resistant weed, annual grassy species for crop rotation, a cool-season grass component in cover-crop mixes, nutritious high fiber biomass for forage mixes, or a perennial turfgrass. Perennial ryegrass is not cause for concern, but the consensus about annual Italian ryegrass is clear: don’t plant it.  However, if you must, follow common-sense practices to reduce the risk of establishing herbicide-resistant populations of annual Italian ryegrass (see side bar).  

Understanding the uses and characteristics of ryegrass species will help farming, cattle, and dairy operators make decisions that can prevent annual ryegrass from becoming a serious weed problem in southeastern Idaho’s cereal crop rotations.

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