Dr. Dinesh Panday, Dr. Reza K. Afshar, Dr. Arash Ghalehgolabbehbahani
Rodale Institute, Kutztown, PA, United States
Abstract
In regenerative organic farming, weed management is crucial to protect crop quality. This study explored roller-crimping as a method to terminate cereal rye cover crops as a weed management strategy for CBD-hemp production. Two-year field research was conducted at Rodale Institute- Pocono Organic Center to evaluate the effectiveness of rye cover crop biomass in suppressing weeds during CBD hemp cultivation. The study compared different approaches—cover crop termination method (incorporation/ disked vs roller-crimping), row to row spacing (30 vs 60 inches), and plant to plant spacing (24 vs 30 inches. Although the highest CBD yield was found in plots with disked cover crops, 30 inches row spacing, and 24 inches plant spacing, roller-crimping significantly reduced broadleaf weed pressure in the later stages of growth. This method shows promise in reducing the need for labor-intensive weed control and supports soil health by minimizing soil disruption in regenerative organic CBD hemp production.
Introduction
Hemp (Cannabis sativa L.) is a re-emerging crop in the United States since 2014, having been banned for over half a century due to cheaper alternatives and legal constraints related to psychoactive compound i.e., delta-9-tetrahydrocannabinol (THC). The flower hemp sector, occupying 26% of total acres, contributed $179 million in the United States. Hemp-derived cannabidiol (CBD) retail sales are on a rapid rise, forecasted to reach $4.5 billion in 2024.
Despite these promising prospects, there are significant knowledge gaps regarding the best agronomic practices for industrial hemp production, particularly in weed management (Dhakal et al., 2023). Currently, no herbicides are registered for hemp, making weed control a major challenge that can significantly impact yields. Weed management methods such as plastic mulch, mechanical weeding, and manual hand weeding are used, but each has drawbacks. Plastic mulch raises environmental concerns, mechanical weeding can damage soil structure, and hand weeding is costly.
Cover crops are becoming a viable tool to improve soil health and water quality in the United States (Adeli et al., 2019), and they can also suppress weeds (Bozzolo et al., 2024). The method used to terminate cover crops significantly influences their ability to suppress weeds in subsequent crops. Rodale Institute developed a roller-crimper, a tool that rolls over cover crops and crimps them at the surface. The crimped stem can no longer uptake moisture and nutrients, causing the cover crop to die in place and form a natural barrier over the soil (Davis, 2010). As a result, the field remains weed-free and healthier for crop growth and development.
This project aims to explore the application of a roller crimper for improving weed management in certified organic fields. Additionally, the project aims to explore optimal row spacing and plant spacing for better crop production, including the effect on changing concentrations of secondary metabolites in CBD hemp.
Materials and methods
The experiment was conducted in a research field at Pocono Organics, Long Pond, PA in 2022 and 2023. The soil texture is a Clymer loam (6.8 pH, 4.1% OM, 2.4% total C, and 0.2% total N) and research site had 3 to 8% slope. The region’s climate is characterized by hot and humid summers and cold winters.
For this research, an experimental trial was arranged in a three factorial complete block design with four replications. Those three factors were cover crop termination method (A), row-row spacing (B), and plant to plant spacing (C). Each factor included two levels- cover crop termination: incorporation/ disked (A1) and roller-crimping (A2), row-row spacing: 30 inches (B1) and 60 inches (B2), and plant-plant spacing: 24 inches (C1) and 30 inches (C2). The individual plot size was 10 ft wide and 20 ft long.
Each year, the field was prepared after termination of cereal rye cover crop using roller-crimper or disk tillage in early June. Hemp seedlings, raised inside greenhouse in potting mix, were manually transplanted at 3 weeks of age in early to mid-June. In 2023, due to a scarcity of certified organic seeds, we employed different cultivars— Kirsche in 2022 and Lindorea in 2023. The OMRI-listed organic fertilizer (bloodmeal, 12:0:0 NPK) was spread uniformly at rate of 150 lb N ac-1 into two splits: 1/3rd during transplantation (basal dose) and 2/3rd after 2 weeks from transplantation on the soil surface. A drip irrigation system was installed in each plant-to-plant row after transplanting the seedlings.
Following a systematic sampling approach (quadrant size of 0.25 m2), we collected weed sampling data from each plot two times: 4 weeks after transplantation (Event 1) and 4 weeks before CBD-hemp harvest (Event 2). Weed samples were categorized into three groups: (i) broadleaf, (ii) grass, and (iii) sedge and expressed in terms of % coverage. Plant height was also recorded during the growing season.
CBD hemp was harvested in late September each year. Data on total hemp biomass yield, secondary metabolite profile (CBD and THC), CBD yield, and bud weight were collected. Secondary metabolite profiles were analyzed in a certified commercial lab (Lancaster DHIA, Manheim, PA). The effects of treatment on observed parameters were tested using the PROC MIXED procedure in SAS, with statistical significance evaluated at P < 0.05 unless otherwise stated.
Results
Plant Height and Yield. Plant height showed a significant increase with interaction effects of cover crop termination method and row-row spacing in 2022 (Table 1). The tallest plants were observed in plots where cover crops were disked, and hemp plants were planted with a row spacing of 60 inches (Figure 1a). In 2023, we only observed a significant effect of cover crop termination method on plant height, with the tallest plants in the cover crops disked treatment (Table 2).
Similarly, dry bud weight was significantly affected by interaction effects of cover crop termination method and row-row spacing in 2022 (Table 1). The highest dry bud weight (17.9 g) was observed in treatment with cover crops disked and a row spacing of 60 inches (Figure 1b). Dry bud weights were comparatively lower in 2023 than in 2022 (Table 2).
CBD hemp biomass yield was significantly affected by interactions of cover crop termination method, row-row spacing, and plant-plant spacing in 2022 (Table 1). The highest biomass yield was observed in treatment with cover crops roller-crimped, a row spacing of 30 inches, and plant spacing of 24 inches, followed by plant spacing of 30 inches (Figure 1c). Any treatment combinations with row spacing of 30 inches showed better yield than others. Consistent results were observed in 2023 as well (Table 2 and Figure 2c).
CBD yield was significantly affected by interactions of cover crop termination method, row-row spacing, and plant-plant spacing in 2022 (Table 1). The highest CBD yield (587 t ac-1) was observed in treatment with cover crops disked and planted by maintaining row spacing of 30 inches and plant spacing of 24 inches, whereas lower yields were observed in any treatment combinations including cover crops roller-crimped (Figure 1d). CBD yield was affected only by row-row spacing in 2023 (Table 2).
Secondary Metabolites Yield. In this study, CBD concentrations ranged from 2.5-7.0% in 2022 and 6.0-11.4% in 2023. We observed significant effects of cover crop termination on the concentrations of CBD and THC in 2022 (Table 1), but not in 2023 (Table 2). The CBD concentration was higher in cover crops disked plots compared to roller-crimped. This trend was also observed for THC concentration in 2022. A significant positive relationship was observed between the concentrations of CBD and THC: r2= 0.72 and P < 0.001 in 2022 (Figure 3a) and r2= 0.77 and P < 0.001 in 2023 (Figure 3b). This indicates that as CBD concentrations increase, THC concentrations also increase.
Weed Pressure. During the growing season in both years, weed pressure was relatively higher during the second measuring date (Event 2) compared to the first measuring date (Event 1). In 2022, we observed a significant effect of cover crop termination method on broadleaf weed pressure and grass type weed pressure in Event 2 (Table 1). Notably, cover crops roller-crimped was effective in reducing broadleaf weed pressure in Event 2. Additionally, we observed the main effects of row-row spacing and plant-plant spacing on weed pressure in Event 2.
In 2023, there was no effect of cover crop termination method on weed pressure. However, we observed interaction effects of cover crop termination method and row-row spacing on weed pressure in Event 2 (Table 2). Plots that were cover crops roller-crimped and maintained row spacing of 30 inches experienced the highest broadleaf weed pressure in Event 2, whereas those with a roller-crimped and row spacing of 60 inches had the lowest weed pressure (Figure 2a). Conversely, plots that were roller-crimped and maintained at row spacing of 30 inches received the lowest grass weed pressure in Event 2 (Figure 2b). These results illustrated that application of roller-crimper can decrease the pressure of broadleaf weeds.
Summary
While our exploration of the effectiveness of cover crops roller-crimped did not lead to a general recommendation, this study provides valuable insights:
Biomass yield, CBD yield, and CBD and THC:
2022:
- CBD hemp biomass yield was highest with roller-crimped cover crops, 30 inches row spacing, and 24 inches plant spacing.
- CBD yield was highest (587 lb ac-1) with disked cover crops, 30 inches row spacing, and 24 inches plant spacing.
- Higher CBD and THC concentrations in disked plots.
- Positive relationship between CBD and THC concentrations (r² = 0.72, P < 0.001).
2023:
- No significant effects of cover crop termination on metabolite concentrations.
- Positive relationship between CBD and THC concentrations (r² = 0.77, P < 0.001).
Weed Pressure:
2022:
- Higher weed pressure at the second measuring date (Event 2).
- Significant effect of cover crop termination on broadleaf and grass weed pressure.
- Roller-crimped cover crops reduced broadleaf weed pressure.
- Row and plant spacing affected weed pressure in Event 2.
2023:
- No effect of cover crop termination on weed pressure.
- Interaction effects of cover crop termination and row spacing on weed pressure in Event 2.
- Roller-crimped plots with 60 inches row spacing had the lowest broadleaf weed pressure.
- Roller-crimped plots with 30 inches row spacing had the lowest grass weed pressure.
The findings from our study on the effectiveness of roller-crimped cover crops suggest several future research directions: varietal testing, additional trials on spacing and cover crop interaction, understanding weed species dynamics, evaluating the impact of cover crop termination methods on soil health and microbial activities, and conducting farm economic analyses. Additionally, developing methodologies to improve weed management using a roller crimper in certified organic fields will enhance the sustainability of hemp production.
Funding Information
This work was made possible by funding from the Pennsylvania Department of Agriculture’s Commonwealth Specialty Crop Block Grant Program (grant number C940000924) and support from Pocono Organics and the Rodale Institute.