Cover crops offer numerous benefits, such as improving soil health, recycling nutrients, and suppressing weeds. The occurrence of herbicide-resistant waterhemp (Amaranthus tuberculatus) and the prevalence of biotypes with multiple resistance has increased interest from farmers of integrated solutions. Allowing cover crops to grow longer increases their biomass, enhancing these positive environmental effects and weed suppression. However, keeping cover crops in the field for extended periods can change the light environment for subsequent cash crops, potentially affecting their growth and yield. This project aims to enhance soybean production by examining how the timing of cover crop termination affects the light environment and triggers soybean shade avoidance response. An additional emphasis will be placed on weed suppression, particularly concerning the growing challenge of waterhemp in Iowa. Early biomass accumulation of cereal rye can provide a physical barrier and modify the light spectrum at the soil surface, potentially reducing waterhemp germination and emergence. By studying the interactions between termination timing, row spacing, and light environment, we aim to optimize cover crop management for improved weed control, including suppression of herbicide-resistant waterhemp populations.
We will study cereal rye as a cover crop, focusing on its impact on the Red:Far-Red ratio (R:FR), with soybean grown in 15- and 30-inch row spacing and four termination timings (7 days before planting, at planting, 7 days after planting, and 14 days after planting). A treatment without cover crops will serve as a control comparison. This variation will help us understand how canopy architecture interacts with light dynamics and shade avoidance responses under different cover crop termination timings. We will measure light reflectance, R:FR, plant height, early season biomass, early season phenology, weed density, grain yield, and yield components to understand the physiological basis of shade avoidance responses and the weed suppression potential provided by cereal rye in different management scenarios.

We anticipate four learning outcomes: (1) comprehensive characterization of the influence of cereal rye cover crop management on the R:FR ratio in cropping systems that include soybean. (2) Identification of critical windows during which herbicide applications can effectively terminate cereal rye without triggering excessive shade avoidance response in soybean. (3) Insights into how row spacing interacts with light environment and weed suppression potential, including effects on waterhemp dynamics. (4) Development of practical, evidence-based recommendations for terminating cover crops in soybean production systems, enhancing crop stability, weed control, and productivity.

Soybean production is particularly sensitive to environmental changes, especially those related to light competition. When soybean plants detect nearby vegetation, such as a living cover crop, they may respond by growing taller and reducing branches. These shade avoidance responses can lead to weaker plants, poor canopy structure, and a higher risk of lodging, all of which can reduce grain yield and harvest efficiency. The suppression of problematic weeds like waterhemp, which is increasingly herbicide-resistant, depends on optimizing early-season cover crop biomass. This project investigates how cereal rye management, including row spacing, influences early-season light competition and weed suppression, with the goal of identifying how long the cover crop can remain in the field to maximize soil and ecological benefits without compromising soybean productivity.

Updated August 20, 2026:
This trial was initiated with cover crop planting in a field with known waterhemp infestation in Fall of 2025. A graduate student has been hired and will start in May of 2026 and assist in planting, data collection, and will lead the project for the duration. Everything is in line to establish the soybean and conduct the trial as planned in 2026. We have two row spacings (15 and 30 inches with five termination timings (no cover crop, 7 days before planting, at planting, 7 days after planting, and 14 days after planting) and established as weedy or weed-free for a total of 20 treatments with 4 replications. We will measure light reflectance, R:FR, plant height, early season biomass, early season phenology, weed density, grain yield, and yield components to understand the physiological basis of shade avoidance responses and the weed suppression potential provided by cereal rye in different management scenarios.

This research will provide clear guidance on how long cereal rye can remain in the field to maximize its soil health and weed suppression benefits without compromising soybean productivity. The integration of narrow and wide row spacing data will allow recommendations to be tailored to different production systems. Growers need integrated weed management recommendations which are economically viable and do not incur losses to their system.