Purdue research studies effects of cereal rye on soybean yields 

By Shaun Casteel
Purdue University Extension Soybean Specialist 

Cereal rye is the most commonly grown cover crop in the Upper Midwest because it overwinters and produces considerable biomass in the spring. Cereal rye can decrease nitrogen (N) losses in tile-drained landscapes, increase soil carbon (C) sequestration, improve N cycling and reduce soil erosion. 

Soybeans are often planted into cereal rye with no problems, but some situations have been noted where yield was reduced. These reductions often are associated with stand establishment and/or plant survival (e.g., slugs, vols). I have observed other situations where plant development and nutrient supply and uptake seem compromised. 

One field caught my eye a few years ago when a farmer was exploring the potential for ammonium sulfate (AMS) on his farm. The field happened to have cereal rye across the whole field followed by strips with or without AMS applied after planting. 

Soybeans without AMS were shorter, had fewer pods, fewer nodes and lighter in color (Figure 1). This translated into 11 bushels less per acre than the AMS treatment (84 bushels vs. 73 bushels). This begged the question if it was a sulfur (S) responsive field or if there was an interaction with the presence of cereal rye preceding the soybean crop. Furthermore, is the positive response of soybeans to AMS due to N, S or both nutrients. 

Evaluating cereal rye’s influence 

With the support of the Indiana Soybean Alliance, the state’s soybean checkoff program, we began our evaluation of cereal rye’s influence on the availability of N and S for soybean development and maximum production in 2023 and 2024. The experimental design was 2 x 4 factorial with cereal rye (yes/no) and four fertility regimes: none, 40 pounds of N/acre (urea), 20 pounds S/ac (pelletized gypsum), and 40 pounds N/acre plus 20 pounds S/acre. 

The eight treatments were established in field-scale trials near West Lafayette in 2023 and 2024 as well as Columbia City in 2023. Preliminary results will be discussed from West Lafayette in 2023 and 2024. Fields near Columbia City, Farmland and Butlerville have been added for evaluation in 2025. 

Target termination for cereal rye was 12-16 inches tall with approximately 1,000 pounds of dry biomass per acre to allow for timely planting of soybeans and fertilization. The cereal rye was terminated on April 18, 2023, and accumulated 553 pounds of acre of biomass with 228 pounds C, 11.4 pounds N, and 0.9 of a pound S per acre. This biomass resulted in C:N ratio of 20.2 and C:S ratio of 257:1. 

Slight differences between no cover and cereal rye were noted in soil nitrate, but no differences in soil fertility and sulfur were noted in 2023. Fertilizers were applied May 4-5 followed by soybean planting on May 6. Plant stands were not influenced by cereal rye or fertilizers (average of 118,400 plants per acre). 

Soybean concentrations of N and S were influenced as early as V4 with the primary sources from the fertilizers and the interaction of cereal rye and fertilizer. Cereal rye suppressed S concentrations of soybean at R2 (full bloom) and R4 (full pod). 

The early suppression of S translated into a shortfall in N at R4. Nitrogen concentrations increased with the addition of gypsum (alone or in combination with urea) at R2 and R4. Gypsum (calcium sulfate) provided S, which is needed as a co-factor of nodulation and N fixation (i.e., N supply for soybean). Gypsum additions improved S concentrations at V4, R2, and R4 as well as improving the N:S ratio at the same time. Cereal rye also created an imbalance of N to S in soybeans at V4 and R2. 

Results on soybean yield 

Soybean yield was suppressed approximately 7 bushels per acre with cereal rye compared to the untreated control in 2023 (61.2 vs. 54.7 bushels/acre) and in 2024 (81.5 vs. 73.8 bushels/acre). The addition of urea did not increase or decrease soybean yield regardless of cereal rye. 

The addition of S overcame the yield suppression by cereal rye to the point that yields were equal between cereal rye and no cover in 2023 at approximately 71 bushels per acre and in 2024 at about 86 bushels per acre (Table 1). The vast majority of this yield increase was through better leaf retention that translated into larger seeds. Protein increased with S applications while oil decreased. 

Cereal rye immobilized S primarily, which reduced soybean growth, N supply via nodulation and N fixation, pod development, leaf retention and yield. Sulfur applications overcome these negative effects via pelletized gypsum, but AMS should be another S fertilizer to consider. 

This adaptive management could provide an opportunity to increase soybean yield and protein, while providing soil and environmental benefits that are often noted with cereal rye inclusion. We will continue this evaluation in 2025 across multiple soil types and locations to determine repeatability.