To plant green or to plant brown? Rye cover crop termination timing before soybeans (summary of 2017-2020 data)

Apr 01, 2021

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Cereal rye (Secale cereal) has several strengths as a cover crop option. It’s relatively inexpensive and easy to kill. Rye improves soil structure, builds organic matter and helps protect against water and wind erosion. It can suppress weeds as discussed here. It’s also very winter hardy and often the only cover crop option to seed after grain corn or soybeans. However, in most cases, rye doesn’t put on much growth until the month of May.
Spring termination decisions
Delaying termination up until soybean planting provides an opportunity to enhance rye’s benefits by giving it more time to grow in the spring. The question is, does planting soybeans “green” into living rye negatively impact yield? This was the motivation for a series of on-farm trials from 2017 to 2020, in which soybeans grown after early-terminated rye were compared to those “planted green” into rye.
The trials had two main treatments:
  • Early rye termination (two weeks before soybean planting)
  • Late termination (day of planting, also referred to as “Plant Green”).
Trials in 2019 and 2020 also had a no-rye control treatment. Of the 14 sites over four years, soil texture ranged from loam to silt loam and clay loam. Rye was seeded as early as August, to as late as November 9th following grain corn. Seeding rates were as low as 20 lbs/acre to a high of 100 lbs/acre. Seeding methods ranged from drilling to late season inter-seeding to post-harvest broadcasting. Sites had between two to six randomized replicates of each treatment.
Rye growth accelerates in May
Across 15 sites, rye biomass increased on average by 2.7-times – from 500 lbs/acre to 1,300 lbs/acre – when terminated at time of soybean planting compared to ~2 weeks prior. At the Kenilworth 2019 site, biomass increased by nearly 10-times when termination was delayed until planting. Extra biomass contributes to soil organic matter, soil structure and provides a longer-lasting mulch.
Planting green can impact soybean stand and crop development
Delayed termination of rye does not come without risk. Soybeans stands were reduced at some sites. On average, soybean stands across 7 sites were reduced by 7% when planted green. Two sites in 2018 had stands less than 100,000 plants/acre due to planting into thick rye at a relatively low seeding rate (140,000 seeds/acre).
When planting green into a moderate or thick stand of rye, a minimum soybean seeding rate of 160,000 seeds/acre is recommended. It’s also particularly important to plant into moisture and ensure that the seed trench is closed.
Soybeans in late-terminated, higher biomass rye tended to have delayed development. At the Brantford and Lambton sites in 2018, plants were consistently one growth stage behind in the “plant green” plots relative to the early termination strips 
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