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

Jake Munroe, Mike Cowbrough and Sebastian Belliard (OMAFRA)

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 (see Figure 1). 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:

1. Early rye termination (two weeks before soybean planting)
2. 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 9^th 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.

For a condensed summary of results, click here.

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 (Figures 2 and 3). 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 (Figures 4 and 5).

Weed effects

Effects of rye and rye termination were evaluated at five trial sites in 2019 and several sites in 2020. Visual differences in weed abundance were minimal at the on-farm sites in 2019. At the Elora Research Station, however, where there was high weed abundance, visual differences amongst treatments were more obvious, with later termination of rye having the least amount of weeds (see Figure 6). The same trend was observed at the Wroxeter, Johnston site in 2020 (Figure 7).

Overall, there was a trend towards lower weed abundance at the beginning of the critical weed-free period in soybean (V1) planted into the cereal rye treatments (five sites, 2019). Weed abundance was the lowest in the late termination treatments (Figure 8), which is predictable given that rye termination with glyphosate was applied later than the other treatments.

It should be noted that weed abundance was high enough in all treatments to negatively affect yield if not controlled, so cereal rye is not going to replace other methods of weed control; however, it can be a tool to improve the performance of other weed control methods simply because they don’t have to control as many weeds.

Yield results

Soybean yield varied from a low of 33.4 to a high of 69.5 bushels/acre across all sites (Table 2). There was a statistically significant difference in yield between early and late termination at only one out of ten sites. At the Elora 2019 site, the yield difference was almost certainly due to only a single herbicide application in the Late Termination treatment, as well as plant stand damage caused by roller crimper activity in wet conditions. Otherwise, yields between early and late terminated rye treatments did not differ.

Table 1. Summarized yield results (based on 13% moisture). Statistically significant differences in yield amongst treatments at each site are shown by different letters.

Site	Yield (bu/acre)
No Rye	Early Term.	Late Term.
St. George 2017	–	57.0 A	60.2 A
Brantford 2018	–	51.3 A	50.0 A
St. George 2018	–	69.5 A	66.7 A
Lambton 2018	–	63.1 A	58.6 A
Elora 2019 AB*	46.8 A	42.9 A	33.4 B
Kenilworth 2019	54.7 A	54.9 A	54.7 A
Wroxeter 2019	52.6 A	51.4 A	51.8 A
Winchester 2019	51.2 A	48.4 A	49.7 A
Morrisburg 2019	48.7 A	48.4 A	48.1 A
Newington 2019	–	44.5 A	43.2 A
Wroxeter, Adams 2020	56.9 A	59.3 A	57.0 A
Wroxeter, Johnston 2020	66.7 A	68.2 A	67.7 A
Kenilworth 2020	53.0 A	52.8 A	52.8 A
Winchester 2020	63.2 A	63.3 A	64.1 A
Average (excluding Elora 2019 AB)	–	56.3	55.7

^{* Yield data combined for Elora 2019 A and B due to limited plot length}

Delaying termination post-planting

Delaying termination of cover crop rye one or two weeks after soybean planting is a practice followed by some growers. At the Winchester site, post-planting termination was evaluated in both 2019 and 2020 and different yield results were observed in the two seasons.

In the wet spring of 2019, delaying termination one-week post-planting did not hurt soybean yields. In the dry spring of 2020, however, soybean yields were significantly reduced when rye was terminated one (-8.2 bu/ac) and two weeks (-10.6 bu/ac) post-planting (where rye was seeded in late September). A similar trend was observed at the Morrisburg site in 2020. It is likely that excess moisture uptake by rye was the largest factor that contributed to yield reductions at both Eastern Ontario sites in 2020.

Delaying rye termination beyond soybean seeding is not a recommended practice at this time, especially under dry spring conditions.

Summary

• Planting soybeans green into rye did not negatively impact yield relative to early termination
• Delaying termination by ~2 weeks and planting green increased rye biomass by over 2.5 times
• Delaying termination of rye resulted in lower weed abundance in both 2019 and 2020. Late-terminated rye can be a tool to assist other forms of weed control.

Making sense of the numbers

Despite the benefits, delaying termination of rye until soybean planting carries some risk. The following strategies can be used to minimize it:

1. When planting green into rye, use a minimum seeding rate of 160,000 seeds/acre minimum to ensure an adequate stand
2. Pay close attention to moisture at seeding depth, since rye can create drier soil conditions
3. When seeding into thick rye:
   1. Ensure good slot closure
   2. Consider the impact of delayed soybean maturity on harvest timing, especially if you plan to seed winter wheat after soybeans
4. Be flexible – if conditions turn dry in May, consider early termination

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. There are also weed management benefits.

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 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

Acknowledgements

This project was supported by OSCIA Tier 1 and Tier 2 project funding. Donations of cereal rye seed were generously made by Cribit Seeds and Centre de Criblage de Marc Bercier. Thank you to all the farmer cooperators who participated.

For the full project report, visit OSCIA’s Applied Research website.