Co-authored by Mike Cowbrough, OMAFRA Weed Management Specialist – Field Crops
An annual ryegrass cover crop is excellent at scavenging nitrogen, improving soil structure, and preventing erosion. It tolerates low light levels well enough to be inter-seeded into an established corn crop and grows quickly enough with adequate moisture to help suppress weeds. For ruminant producers, ryegrass is exceptionally palatable to livestock and can provide high-quality grazing, baleage, or haylage.
In recent years, many grain farmers have moved away from using annual ryegrass as a cover crop due to termination issues. While it is not the right fit for every operation, a better understanding of ryegrass genetics and management may keep this shade-tolerant cover crop on the options list and make it easier to kill ahead of the next crop.
Variety Selection
When using annual ryegrass as a cover crop, the variety grown can make a big difference in how likely it is to winterkill. The release date of the variety, whether it is an Italian or Westerwold ryegrass, and the ploidy all are important factors to consider when trying to reduce annual ryegrass survival. Although it is typical for cover croppers to grow the least expensive common seed they can find, with annual ryegrasses this approach can make termination more difficult.
Choose older varieties
Most of the breeding efforts on ryegrasses are happening in Europe and the British Isles. Plant breeders there are working to expand the range of ryegrass so more areas can use it as a forage crop. Poor overwintering success is one of the reasons that adoption of ryegrass for forage has lagged outside of mild temperate climates. This means that newer varieties tend to be more winter hardy than old ones and may partially explain why in Ontario we are seeing more annual ryegrass survive winter than in the past. Unless your seed supplier has winter hardiness data to the contrary, when choosing between new and old varieties for a cover crop, pick the old one.
Consider Westerwold instead of Italian
Another consideration is the type of annual ryegrass. Italian ryegrass has historically been preferred as a cover crop because it does not set seed the year it is established. Like a winter cereal, Italian ryegrass has a vernalization requirement to set seed. The flip side of this biology is that when Italian ryegrass overwinters it grows aggressively in the spring to try and reproduce. Like winter cereals, it is more difficult to terminate in the spring. Producers could choose to use a Westerwold ryegrass instead. Like spring cereals, Westerwolds do not require cold weather to set seed, but they are also less winter hardy than Italian ryegrasses. Westerwold ryegrass also has a higher yield potential than Italian, which means more biomass from the cover crop.
If cover crop growers are prepared to manage seed production in a Westerwold, it may be an easier option to terminate than Italian ryegrass. Forage seed suppliers estimate a spring-seeded Westerwold takes about 12 weeks of growth before it is ready for first-cut silage harvest, and 6-8 weeks for each cut after that. Ideal silage harvest timing is between flag leaf and boot stage, so this gives an idea of how long it takes for a Westerwold to start flowering.
Select a tetraploid variety
Ploidy is a description of how many sets of chromosomes a ryegrass plant has. Humans only have two sets of chromosomes: one set from each parent. Ryegrasses may have either two (diploid) or four (tetraploid) sets of chromosomes. Diploids tend to be more winter hardy and handle stressful conditions – like wet soils and moderate fertility – better than tetraploids. Tetraploids are higher yielding and more palatable as forage, but they don’t tiller as aggressively as diploids. Choosing a tetraploid variety for a cover crop makes it more likely to winterkill.
Cover Crop Management
Apply fall nitrogen to encourage winterkill
Shorter days and cooler temperatures are the signals most grasses rely on to tell them it’s time to harden off for winter. Ryegrasses have their priorities mixed up, and the cover cropper could use this to their advantage. Like other grass crops – think corn and cereals – ryegrasses have a huge demand for nitrogen. If they are given a nitrogen application in the fall, ryegrasses will keep growing leaf material instead of hardening off for winter. About 50 lbs N/acre is enough to fuel more leaves. This makes a ryegrass cover crop a great place for a fall manure application. Applying nitrogen in the fall does not guarantee winterkill, but it does increase the odds of ryegrass winterkilling.
Cut annual ryegrass a month before the first killing frost
The other stressor typically missing from a cover crop is cutting. Overwintering crops need time in the fall to store energy reserves that will fuel regrowth the next spring. Anyone who has grown alfalfa knows OMAFRA has published dates for a fall rest period. Most perennial forage grasses grown in Ontario tend to handle winter better than alfalfa, so there aren’t published “don’t cut” dates, but the biological preparation process is similar. Cutting annual ryegrass about a month before the first killing frost limits how much time it has to prepare to overwinter. Taking a fall cut doesn’t guarantee that ryegrass will winterkill, but it can help increase the odds.
Details matter for successful termination
When annual ryegrass overwinters, cover croppers are dealing with a plant that can be difficult to terminate. In the United States, populations that are resistant to herbicide groups 1, 2, 9, 10 and 15 have been documented. This presents several challenges for successful control. The success of herbicide application depends on getting the details right.
Growth stage will make-or-break a burndown
- Annual ryegrass should be actively growing, which means it broke dormancy at least 5-7 days before spraying.
- Herbicide is most effective when ryegrass is 10-20 cm (4-8 in.) tall and still tillering.
- After the first node has developed for stem elongation, ryegrass becomes much more difficult to kill.
- Burndown becomes easier again once ryegrass flowers, but there is an increased risk of seed set which creates further weed problems. Waiting until flowering usually interferes with other cropping plans for the field.
Weather conditions are important
- Nighttime temperatures should be above 4˚C for at least three consecutive nights to make sure the ryegrass is actively growing.
- Daytime temperatures should be above 13˚C, although above 15.5˚C is optimum.
- Ryegrass is challenging to control in cool, cloudy, or wet weather.
Herbicide application must be done right
- Experience has shown that the rate of glyphosate needed to control ryegrass ranges from 1.5-1.87 L/acre (Figure 1).
- Because the ryegrass was established the previous fall, it is not considered a new seeding or seedling-stage cover crop.
- Low herbicide rates often stress the plant rather than kill it and make termination more difficult later.
- Ensure thorough spray coverage (medium droplet size and moderate pressure).
- Do not mix glyphosate with atrazine, as this can reduce efficacy by up to 40%.
Although one pass should control ryegrass if growth stage, weather, and application details are managed well, growers should plan to do two passes in case the first spray had to occur during cool, cloudy weather to meet the ideal growth stage timing.
The Oregon Ryegrass Growers Seed Commission offers additional suggestions to successfully terminate annual ryegrass with herbicide. The “Take Action” resource for managing pesticide resistance also offers best practices for managing ryegrass. Note that many products available in the U.S. are not registered for use in Canada or may have different label restrictions. The Ontario Crop Protection Hub outlines which products are labelled for use in Ontario. Always read and follow the label – the label is a legal document.
Summary
Successful annual ryegrass termination starts with the seed purchase. Choosing an older variety, a tetraploid, or a Westerwold – or some combination of these – may increase the likelihood of the cover crop winterkilling. Fall management, like applying nitrogen and mowing, can stress the ryegrass going into winter to increase the odds of winterkill. Finally, paying close attention to the growth stage of the ryegrass, weather conditions, and getting the product and sprayer set up correctly will make termination much easier.
If you see ryegrass in your cereal stubble, it would be worthwhile to spot spray plants with a 2% glyphosate solution to see if they will die. If they survive, contact either mike.cowbrough@ontario.ca or ftardif@uoguelph.ca so that resistance testing can be pursued.