To plant green or to plant brown? Rye cover crop termination timing before soybeans

cereal rye,cover crop

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Cereal rye 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 also competes with weeds, as Mike Cowbrough has written here (see Figure 1). However, in most cases, rye doesn’t put on much growth until the month of May.

cereal rye, canada fleabane
Figure 1. An overhead look at the level of weed control that cereal rye provided when it was seeded the previous fall (mid-November) at 60 lbs/acre (left) compared to no cereal rye planted (right). Photo: Mike Cowbrough.

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 rye negatively impact yield? This was the motivation for a series of on-farm trials with Brant County Soil and Crop Improvement Association, in which soybeans grown after early-terminated rye were compared to those “planted green” into rye.

Four on-farm trials were conducted in 2017 and 2018 (Table 1). Sites had between 2 to 4 field-length replicates of each treatment (early vs. late rye termination). One site outside the county, Lambton 2018, was added to the study.

Table 1. Site descriptions and cereal rye seeding details of trial locations

Site Soil type Previous crop Rye seeding date Rye seeding rate Seeding method
St. George 2017 Beverly silt loam Grain corn Nov. 4, 2016 35 lbs/acre Broadcast + incorporated with minimum tillage
Brantford 2018 Brantford  silty clay loam Silage corn Oct. 20, 2017 90 lbs/acre Drilled
St. George 2018 Brantford silty clay loam Soybean Oct. 23, 2017 37 lbs/acre Drilled
Lambton 2018 Perth clay Soybean Late Aug., 2017 20 lbs/acre Interseeded in standing soybeans

Rye growth accelerates in May

Across 4 sites, rye biomass increased on average by 4.3-times when terminated at time of soybean planting compared to 2 weeks prior (Table 2).

Table 2. Rye biomass (dry) at early versus late termination timings across all sites.

Site Replicates Early Termination Plant Green
Date Rye Biomass (lbs/acre) Date Rye biomass (lbs/acre)
St. George 2017 3 May 12 429 May 23 1,264
Brantford 2018 4 May 9 302 May 25 2,524
St. George 2018 2 May 11 138 May 23 1,228
Lambton 2018 3 May 8 670 May 24 1,601

At the site with the greatest amount of rye biomass, Brantford 2018 (Figure 2), allowing an extra two weeks of growth resulted in over 8 times more plant material. Extra biomass contributes to building soil organic matter faster and provides a long-lasting mulch. At this site, late termination also resulted in an extra 48 lbs/acre of nitrogen scavenged by the cover crop – from 12 to 60 lbs/acre. Nitrogen taken up by rye is released slowly over the season and is less likely to be lost to the environment. Since soybeans fix their own nitrogen, this uptake doesn’t negatively affect the crop.

cereal rye,cover crop
Figure 2a. Early (left) and late-terminated (right) rye strips at the Brantford 2018 site before planting. May 25, 2018.
cereal rye, plant green
Figure 2b. Late-terminated strips at the Brantford 2018 site just after planting. May 25, 2018.

Planting green can impact soybean stand and crop development

Delaying termination of rye does not come without some risk. Soybeans stands were reduced at some sites (Table 3).

Table 3. Soybean population, seeding rate and method

Site Plants per acre Seeding rate Seeding method
Early Termination Late Termination
St. George 2017 110,000 120,000 160,000 Drilled, 15″
Brantford 2018 101,000 89,000 140,000 Planted with planter-mounted roller crimpers, 30″
St. George 2018 123,000 118,000 160,000 Drilled, 15″
Lambton 2018* 117,000 87,000 140,000 Planted, twin rows on 30″ centres

*Soybean seeding depth was accidentally not adjusted to account for depleted soil moisture in late termination plot.

It’s particularly important to plant into moisture and ensure that the seed trench is closed. Also, if conditions are very dry leading up to planting, terminate rye early to avoid planting into even drier conditions.

Soybeans in later-terminated rye also tended to have delayed development. Across all sites in 2018, plants were consistently one growth stage behind in the “plant green” plots (Figure 3). Soybeans are adaptable, to a point, to reduced stands and moderate delays in development. At the Brantford 2018 site, for example, despite delayed early season growth, soybeans planted green into rye had an equal or greater number of pods per plant and seeds per pod compared to those in the early-terminated rye.

Figure 3a. Soybeans at the Brantford 2018 site. Soybeans planted green into rye were one growth stage behind and shorter than those planted into early terminated rye. June 26, 2018.
Plant green, soybeans, cereal rye
Figure 3b. At the Lambton 2018 site on July 31, soybeans planted into early terminated rye (right-hand side of photo) were taller and one growth stage ahead.

Weed effects

Effects on weeds were not evaluated in depth. However, a greater density of Canada fleabane was observed in early-terminated strips at the Brantford 2018 site (Figure 4). At this site, the herbicide program did not include a product with good efficacy on glyphosate-resistant Canada fleabane.

cereal rye, soybeans, canada fleabane
Figure 4. Soybeans in early-terminated rye (left) had a much greater density of Canada fleabane than soybeans planted green into rye (right) at the Brantford 2018 site.

Yield results

Soybean yield varied from a low of 50 to a high of 69.5 bushels/acre across all sites (Table 4). There was no statistically significant difference in yield between early and late termination at any site. There was, however, an overall trend of reduced yield with planting green into rye (average loss of 1.4 bushels/acre).

Table 4. Summarized yield results from all sites. Statistically significant differences in yield between early vs. late termination at each site are shown by different letters. Yield is based on 13% moisture.

Site Yield (bushels/acre)
Early termination Late termination Difference
St. George 2017 57.0 A 60.2 A +3.2
Brantford 2018 51.3 A 50.0 A -1.3
St. George 2018 69.5 A 66.7 A* -2.8
Lambton 2018 63.1 A 58.6 A -4.5
Average -1.4

*Farmer cooperator forgot to include a herbicide with good efficacy on glyphosate-resistant Canada fleabane in late termination strips, which resulted in greater weed pressure.

Making sense of the numbers

With a trend toward yield loss at 3 out of 4 sites, it’s clear that delaying termination of rye until soybean planting carries some risk. Overall, however, the loss was low. At the Lambton 2018 site, very dry conditions in June and July likely made matters worse following planting that was too shallow in the plant green strips. And at the Brantford 2018 site, a reduced plant stand was likely the reason for the slightly lower yield, since pod and seed number didn’t differ.

Based on these observations, the following lessons were learned:

  1. When planting green into rye, consider a higher seeding rate (e.g. 160,000 seeds/acre and above) to minimize yield lag due to a thin soybean stand.
  2. Pay close attention to seeding depth, since rye can create drier conditions.
  3. Don’t be too concerned about delayed early season growth, but consider the impact of delayed maturity on harvest timing, especially if you plan to seed wheat after soybeans.

Putting it together

There’s no doubt that allowing an extra couple weeks of growth maximizes the soil benefits of a rye cover crop: more biomass, more time with active roots and a lasting mulch. It’s a new practice, however, and still needs some fine-tuning.

If you’re new to rye as a cover crop, gain confidence by terminating it a couple of weeks before planting soybeans in the spring. If you’re more experienced, leave a couple strips to be sprayed after soybean planting this coming spring and see how it works on your farm. Have a goal in mind – is it weed management, building organic matter or overall soil health? And finally, be flexible and adapt your plans according to weather conditions.

“Planting soybeans green into rye is an opportunity to have a green cover from fall harvest to spring planting. It helps with erosion control, builds organic matter and may help with weed suppression.”

Owen McIntyre, farmer cooperator, Brant County

More on-farm research to come

Starting this fall, an OSCIA Tier Two project by Heartland and Eastern Valley regions will evaluate the effects of cover crop rye on weeds, crop yield and more. The two-season project will look deeper at the question of termination timing in the spring and evaluate the use of a roller crimper.


This project was conducted by Brant Soil and Crop Improvement Association with OSCIA Tier One funding. Thank you to the farmer cooperators who made it possible.