Cereal Rye as a Cover Crop

Benefits

• Weed control 
• Nitrogen capture 
• Erosion control  
• Soil moisture management  
• Improved soil organic matter, aggregation  
• Compaction alleviation 
• Potential alternative uses (forage, straw, grain) 

Adaptation

Cereal rye is a winter annual grass, surviving the winter in all of Pennsylvania and north into USDA Hardiness Zone 3 (Table 1). It fits into many crop rotations and has been the most widely used cover crop species in Pennsylvania, as it can be established in the fall after corn or soybean harvest. It germinates at temperatures as low as 34°F and does best on well-drained soil with a pH between 6.0 and 7.0. When seeded early in the recommended September to November 1 window for Pennsylvania, rye establishes quickly and tillers in the fall, but stays vegetative until spring. When paired with ample fertility, it has explosive spring growth and can reach 6 feet tall and up to 10,000 lb/A of dry matter, especially when termination is delayed later in the spring.  Dry matter production can still reach greater than 2,000 lb/A if seeded late in the window, or even after the recommended window in southeastern PA, but only if termination is delayed in the spring (Figure 1).

Rye aggressively competes with weeds and can provide effective weed suppression, especially of winter annual species. Rye produces secondary chemicals that can be toxic to other plant species, but recent research shows this effect is short-lived (subsides within about a week of rye termination), and likely plays only a small role in weed suppression. Rye mulch protects soil from erosion while reducing water evaporation, especially when roll-crimped at termination. Additionally, rye takes up free nitrogen (N) from the soil, which reduces nitrate leaching to groundwater. Its large root system helps resist and alleviate soil compaction that can result from heavy field equipment. Its abundance of above- and below-ground biomass makes it an effective tool to help increase soil organic matter, too.

One disadvantage of rye is its high C:N ratio when mature (rye straw C:N ratio = 80), which may cause N deficiency in the next crop. Further, its aggressive growth in the spring can make it difficult to control at a farmer’s preferred growth stage during rainy weather when fields are inaccessible. It also consumes large quantities of soil moisture in the spring, which can cause moisture stress during establishment of the following crop in a drought year.  

Cereal rye is the earliest maturing of all the small grain species (Table 1), though there is variability among varieties of cereal rye. "Variety not stated," or "VNS" rye is the most affordable option and is widely planted, but lacks predictable characteristics of specific varieties. Depending on the wholesaler, some seed dealers can access information about specific lots of VNS seed that they purchase, including growth habit. Besides VNS seed, the most used cultivars in the mid-Atlantic region include Aroostook and Wheeler. Farmers specifically using cereal rye as potential forage or for straw may seek out hybrid rye cultivars for maximum yield, while growers relying on rye for weed control and roll-crimping for termination, such as within organic production systems, may choose varieties that are earlier-maturing, if available. Reach out to your seed dealer directly for information on the cultivars they have in stock.

Establishment

Optimum seeding date is from September to late October, but cereal rye can be planted as late as December or January in southern parts of the state. In the latter case, only spring benefits will be seen, as there will be no cover in the fall. Regardless of the preceding crop, it is important that the remaining residue is uniformly distributed to ensure good rye seed-to-soil contact and avoid concentrated swaths of residue, especially in no-till fields.

Seeding depth should be 1-1.5 inches. If soil conditions are dry, increase seeding depth to 2 inches. If broadcasting after main crop harvest, stand establishment can be improved by following up with a cultipacker or shallow cultivation pass.

Rye can be planted into standing corn or soybeans to increase days of cover crop growth in the fall. Cereal rye is not recommended for early-season (V4-V5) drill-interseeding into corn, but broadcasting into corn or soybeans around late September is an option. Broadcasting into corn is commonly done in the Midwest, and Penn State and University of Delaware research has shown successful establishment by broadcasting into standing soybeans at the R6 (full seed or "green bean") stage, though this method works best when timed directly before rainfall (Image 2). Broadcasting can be done with a wide range of equipment, from ground rigs with spinner or boom spreaders, to aerial application (plane, helicopter, or drone).

The recommended rye seeding rate is usually expressed as 30-120 lb/A. Because there are, on average, 18,000 rye seeds per pound, this means you plant between 0.5 to 2 million seeds per acre. However, due to extreme variability in seed size between lots, following the same pounds per acre instead of seeds per acre recommendations with two different seed lots can lead to up to 2x differences in actual number of seeds planted. Most existing research and Extension publications use the pounds per acre recommendations, but the industry is collectively moving towards the seeds per acre method.

Half a million to 2 million seeds per acre is a large range; your seeding rate should be based on your goal for the cover crop. If accumulating maximum biomass is the goal, aim to plant early and delay termination, since fall growing degree days have the largest impact on biomass production. Our research shows that spring biomass only increases with seeding rate after first frost in more northern locations, like Massachusetts and Vermont. In Pennsylvania, a late-planted rye cover crop will not produce as much biomass as an early planted rye cover crop, regardless of seeding rate. However, if the goal is maximum groundcover, increasing seeding rates can be beneficial, in addition to early planting. Therefore, weed suppression can be improved, even if biomass does not increase.  

Use the higher end of the seeding rate range when broadcasting compared to drill-seeding; consider further increasing the rate to 150 lb/A when seeding into standing soybeans. Be sure to calibrate your drill or broadcast spreader regularly to account for the drastic variation in seed size from lot to lot.

Cereal rye performs well in a cover crop mix but tends to dominate other species. Therefore, rye seeding rate should be reduced to 25-50% monoculture rate to give companion species a chance to establish. Use species that complement rye instead of competing with it. For example, mixing rye and hairy vetch is a good combination because rye will cover the soil in the fall when hairy vetch grows very slowly. Hairy vetch does not compete with rye for N because it fixes atmospheric N. In the spring, rye will provide a structure for hairy vetch to climb. On the other hand, mixing rye with oats is not recommended because early established oats will compete excessively with rye in fall, possibly resulting in a thin rye stand in the spring when the oats have winterkilled. On the other hand, if the mixture is established late, the oats will not provide an additional benefit to having only rye there.

Management

Fertility

Fertilizing cover crop rye is usually not economical, but it may be beneficial to apply 15 to 30 lbs/A N to rye at planting when it is established late after a heavy N-feeder such as corn. This will give the rye an early boost in growth and improve stand. In most cases, however, manure is applied to rye or the soil has a manure or legume history that provides adequate to excessive fertility for the cover crop. Over-fertilization will lead to lodging. 

Fortunately, cereal rye is an excellent N scavenger and can be used in high-N environments to prevent N from leaching into groundwater. Nitrogen uptake is determined by dry matter accumulation--rye is about 3% N on a dry matter basis, so if rye accumulates 6,000 lb/A DM by time of termination, it has taken up approximately 180 lb/A N. This also poses a crop management challenge, as the N in the cover crop tissues is not immediately available to the cash crop planted afterwards. Nitrogen from rye becomes available more slowly in no-till compared to tilled systems.

This occurs because soil microbes tend to release excess N into the soil if their cover crop residue food source has a C:N <20, while the soil microbes use free soil N to help break down cover crop residue with a C:N >20, temporarily "tying it up" or immobilizing it. The C:N ratio of rye progresses from 14:1 for young rye plants to 20:1 at mid-boot stage, to 40:1 at anthesis (Figure 2). Therefore, once reaching the reproductive stage, and a C:N  greater than 40, free soil N is not available quickly to the next crop, which often leads to N deficiency in crops such as corn that need a lot of N. To combat this, it is recommended to apply a high starter N-rate or apply N early in the season.  With legumes such as soybeans, this is not a problem.

Termination

Chemical

Glyphosate is the most effective burndown herbicide for cereal rye. A rate of 0.75 to 1.5 lb ae per acre and appropriate adjuvants should be used.  For effective control, rye must be actively growing. Use caution when burning down very early in the season; glyphosate activity increases when days are warm (55°F) and sunny, and night temperatures exceed 40°F for several days in a row. Inclusion of triazine herbicides (atrazine, metribuzin) can antagonize glyphosate activity, which prolongs rye kill. Planting conditions are optimized when rye is completely dead or when planting green, so consider herbicide application factors that affect glyphosate activity (product rate, weather, tank-mix antagonism) when targeting a rye termination date in your system.

Paraquat (Gramoxone SL) can be used as an alternative to glyphosate, but is generally less consistent, and works better when rye is small; however, it works much faster than glyphosate, and can be sprayed just a few days before the intended planting date. To optimize control with paraquat, use a high carrier volume (20 GPA) and flat fan nozzle tips to improve coverage. Inclusion of a non-ionic surfactant, a triazine herbicide (atrazine, metribuzin), and UAN as a partial carrier can each improve paraquat activity.

While well-established rye stands can provide high levels of winter annual weed suppression, it is important to consider the need for herbicide tank-mix partners to broaden the weed control spectrum when terminating rye. In many cases, inclusion of a synthetic auxin (2,4-D or dicamba) in glyphosate- or paraquat-based burndown programs will improve control of broadleaf winter annual weed species that glyphosate does not control, such as glyphosate-resistant horseweed. Pay particular attention to pre-plant restrictions associated with these tank-mix partners.

Mechanical

Another method of rye termination is incorporation with a heavy disk or moldboard plow as a green manure before further seedbed preparation. Over 18 inches tall, chop the cover crop with a flail mower or other mower that uniformly distributes the residue prior to plowing. It is not recommended to incorporate mature rye because soil microbes require a lot of N to break down high C:N rye, causing N deficiency in the next crop.

Rye can also be harvested as forage or grazed, targeting the boot stage; however, this typically does not kill the rye, and regrowth is expected. Thus, regrowth should be tilled under or sprayed as previously noted for complete kill.

Roll-crimping is used to terminate rye in reduced-tillage organic systems, or in conjunction with planting green (planting soybeans or corn directly into the living cover crop) to help manage residue. Roll-crimping can be done in a separate operation ahead of planting (Image 3), or the device can be mounted on the front of the tractor that pulls the planter or integrated on the planter itself. Roll-crimping is recommended to help manage rye taller than about 18 inches to minimize residue impacts on crop growth. Roll-crimping alone will not kill the cover crop until after anthesis (pollen shed), and if rolled-crimped before anthesis, it should be followed by a burndown spray.

There are benefits and drawbacks to different rye residue management approaches when planting green, especially for corn. For example, planting into standing rye can reduce the number of weeds in the corn row, but it can cause etiolation, or “legginess,” and reduced corn yield (Wallace et al.,2023). Roll-crimping rye with row cleaners allows more weeds to grow in the corn row, but decreases weeds between rows, and can increase yield compared to corn planted green into unrolled rye. However, whether rolled or unrolled, planting green into high rye biomass (+9,000 lb/A DM) reliably reduces weeds compared to rye killed two weeks before cash crop planting.

For more details, including planting green best practices, read our detailed research summary.

Any amount of rye cover crop residue calls for special attention to the planting operation of the next crop, but the importance increases as rye grows taller and has more biomass. Row cleaners should be such that they do not wrap in tall rye, and adjustment of gauge wheels to account for mulch depth is needed to make sure seeds reach the proper depth. Seed firmers are valuable to push the seeds to the bottom of the V-slot created by the opener(s). Closing wheels should break up the soil and firm the soil on top of the seeds.

Management Summary

• Seeding rate: Plant rye at 30-120 lb/A.
• Seeding depth: 1-2” or broadcast (and incorporate) and sequester up to 180 lb/A N
• Biomass accumulation: Up to 10,000 lb/A
• Burndown herbicide: Glyphosate, paraquat
• Soil preparation and main crop planting: Apply larger balance of N up front before corn to compensate for N tie-up by rye.