Rye Cover Crop Termination Timing and Nitrogen Application Effect on Corn Yield

• Delaying cover crop termination timing close to planting can significantly reduce corn yield potential.
• Well-placed nitrogen (N), applied with a planter, may minimize yield loss caused by cereal rye.
• Terminating cereal rye early reduces total biomass and the carbon to nitrogen ratio, lessening the amount of N immobilization and associated yield effect.

Cover crops have grown in popularity in recent years. One of the most common cover crop choices is cereal rye as it can be easily established in the fall, overwinters well and grows vigorously in early spring. Benefits of planting cover crops can vary with geography and farming practices. The fibrous roots of rye can reduce soil erosion, improve soil structure and increase soil permeability. These combined benefits can help reduce field runoff of pesticides and nutrients, such as phosphorous that is tightly bound to soil particles, as well as increase in-season soil moisture availability. Another benefit to using cover crops is their ability to scavenge nitrogen and other nutrients at a time when a crop is normally not present. Nutrients taken up by the cover crop are protected from loss while later decomposing into organic matter for uptake by the following crop.

Challenges to Cover Crop Establishment

Despite the many benefits of cover crops, there are challenges that come with adapting cover crops into a crop rotation. 

• Narrow establishment window: Cover crop seeding date can present a challenge, especially when trying to establish after crops such as corn and soybeans that may not be harvested until November. Seeding with an airplane or modified high-clearance sprayer may allow for earlier cover crop establishment before cash crops are harvested, giving them a better chance to have significant growth before a fall frost. 
• Reduced soil moisture: Drought-prone soils could be dried out further by rye moisture uptake.
• Increased pest pressure: Delaying termination of rye until after planting can result in a “green bridge” for disease and insects to relocate from cover crop to newly seeded crop after die-back begins. Pythium, wheat stem maggot, black cutworm and armyworm have been observed to cause worse crop damage after cover crop termination.
• Reduced N availability: Corn planted directly into green rye can often have a yellow, stunted appearance when emerging. It is believed that previously applied N may be tied up in the rye cover crop and not available until the rye dies and begins to be mineralized back into the soil.
• Allelopathy: Allelopathic chemicals are released by rye and are known to inhibit germination and early growth of many smaller-seeded plants. Terminated rye begins to degrade and release allelopathic chemicals into the upper soil surface. Field effects of allelopathic chemicals on large-seeded crops like corn and the impact on germination and early growth are not well understood. Lab studies have indicated a possibility of reduced early growth or germination; therefore, it is often recommended to terminate rye at least 2 weeks before planting.
• Hybrid sensitivity: There is not a great understanding as to why corn planted into rye will sometimes have a negative yield impact. The effect of rye mulch on soil warming and drying, in combination with other factors such as N tie-up and soil pathogens, likely all interact to potentially create a negative response. Due to this, understanding the reason for hybrid differences is even more challenging.

2022 Cover Crop Management Trial

Cereal rye cover crop trials were initiated at Clinton, IL, to further investigate the effects of timing rye termination on the following corn crop. Rye was broadcast seeded following soybean harvest in the fall of 2021. Areas with no cover crop were maintained to compare the effect of various cover crop termination timings to no cover crop. The following spring, cereal rye blocks were either terminated 2 weeks prior to planting or on the day of planting (green planting) (Figure 1). Each of the trial treatments (no cover, early termination, termination at planting) was planted with 6 corn hybrids to better understand individual hybrid response to rye cover crop. At planting, 30 lbs. of N were applied as UAN in a 2x2x2 placement to half of the 3 trial treatments to better understand how cereal rye residue affected N availability. Early growth differences and stand counts were taken for hybrids across all treatments. The trial was harvested with a research combine, and grain weight and moisture were recorded for all plots.

Trial Results

Stand establishment was more challenging when delaying termination until the day of planting (4.8% fewer plants). When rye was terminated 2 weeks prior to planting, establishment was similar to when no cover crop was present. Corn growth at the V5 stage appeared identical between the no cover crop and early termination timing but was significantly reduced when the termination timing was delayed until the day of planting (Figure 2).

Cereal rye cover crops generally reduced yield potential, although they were more penalizing when termination was delayed until time of corn planting. Terminating cereal rye 2 weeks prior or on the day of planting reduced yields by 13 and 24 bushels per acre (bu/ac) respectively within treatments receiving an extra 30 lbs. N/ac (Graph 1). Yield loss was most likely the result of a combination of slightly reduced plant stands, reduced N availability and corn plant competition with rye biomass. N was likely a limiting factor independent of cover crop presence as indicated by the small response to nitrogen when no cover crop was present (Graph 1). The additional 30 lbs. N/ac applied with the planter also increased early plant growth when termination was delayed until time of planting but resulted in a smaller overall yield increase and did not recover the full yield potential (Graph 1). Delaying termination allowed for more cereal rye biomass to develop and resulted in higher cereal rye carbon to nitrogen (C:N) ratios. Increased C:N ratios are known to also increase N immobilization and is likely one of the primary reasons for yield loss in this trial with delayed termination timings.

Golden Harvest^® corn hybrids G10D21 and G12S75 incurred larger yield penalties with the earlier termination timing than other hybrids. All hybrids were negatively affected by the delayed termination timing (Table 1). The 2021 rye cover crop trials evaluating the same hybrids showed G12S75 was one of the most sensitive to cereal rye cover crops whereas G10D21 was slightly less susceptible (data not shown). The 2022 trial again implies that both hybrids may be more sensitive to rye cover crop systems with both early and late termination timings. The other 4 hybrids tested had smaller yield penalties when cereal rye was terminated early but all hybrids experienced large statistical yield losses when termination was delayed (Table 1). Hybrid management decisions based on this single trial should be limited, but also indicate that hybrid differences may exist. This trial reinforces the overall importance of terminating cereal rye as far in advance of corn planting as possible. It also suggests that well-timed and placed nitrogen applications can help minimize some potential yield loss.

Tips for Planting into Green Cover Crops

Terminating cereal rye at least 14 days before the anticipated planting date greatly reduces many of the risks associated with rye cover crops. When this is not an option, consider the following for green planting:

• Allow corn or soybeans to germinate prior to terminating cover crop.
• Total N should not need to change, but timing 30-60 lbs./ac of highly available N at or around planting close to the seed may reduce tie-up by rye.
• Apply in-furrow N containing starter or higher rates of N in 2x2, 2x2x2 or dribbled to the side of row at planting, and consider additional early sidedress, if needed.
• Scout for insects and be prepared to manage pests that could become an issue in the emerging crop, such as armyworm and black cutworm.
• Pay extra attention at time of planting to:
  • Adjust down pressure and depth. Seed depth changes due to cover crop residue.
  • Use opening wheels, coulters, trash whippers or other planter attachments to clear cover crop debris.
• Consider planting rye cover crop into wider, 30-in. rows to allow for clean corn/soybean planting strips the following spring.
• Always make sure soil conditions are optimum for good seed germination and growth. 
• Larger, maturing cover crops can be hard to control with herbicides. Ensure adequate application rates are used while plants are actively growing.
• Consider the planting restrictions of herbicides that could be used to terminate a cover crop.
• Maximize spray volume and ensure application weather conditions are good to get the best control of cover crops.
• Tillage can be an option for certain species, but multiple passes may be needed which negate the benefits the cover crop is providing.

If you want to learn more about cover crop considerations, please reach out to your local Golden Harvest Seed Advisor or agronomist.