Cover crops suppress fallow weeds and support profitable grain crops
Before the introduction of herbicides, cover cropping was a cornerstone practice to suppress weeds in cropping systems. The relative ease and soil moisture conservation benefits of chemical fallow and no-till cropping saw a strong swing away from the practice.
With increasing herbicide resistance in weeds, is it feasible to reinstate cover cropping as a fallow management practice in dryland broadacre cropping systems?
This research investigated the multifunctionality of cover crops, including their ability to suppress weeds in fallow, in a three-year dryland crop rotation in the northern grains region. Compared to a bare fallow, the researchers found that cover crops enhanced 8 of the 14 measured ecosystem functions, including weed suppression.
Practical tips
- All well-grown cover crops can provide weed suppression services in the fallow phase of a crop rotation.
- Growers can expect 50 to 75 per cent lower weed biomass growth in a cover crop compared to a nil-cover crop bare fallow (with no herbicide applied).
- Cover crops provide various other ecosystem functions that may be particularly valuable in different farming system scenarios.
- Cereals, such as forage oats, barley, millet, and rye, are consistently good cover crop options that provide multifunctional ecosystem services.
- Grain yield in crops following a cereal cover crop and profitability across the rotation are comparable to those achieved by implementing a bare fallow.
- Adding a legume component to the cover crop mix provided additional nitrogen cycling benefits but reduced some other benefits.
- Forage brassica was the least beneficial species to include in a monoculture or multi-species cover crop. Its high water demand restricted fallow efficiency and negatively impacted cash crop yield and profitability.
The WeedSmart Big 6 integrated weed management program includes herbicide and non-herbicide tactics to manage the weed seed bank. Cover crops, also known as brown or green manure crops, compete against weeds while building soil health to support highly competitive crops across the rotation. Although most cover crops are terminated with herbicide application at peak biomass, they are a non-herbicide weed control tactic during the growing period.
Experimental design features
These field experiments were conducted between March 2020 and August 2022 on two sites at The University of Queensland’s research facility at Gatton (see Figure 1). The soil is a self-mulching, deep-cracking vertosol, and the site received below-average rainfall (455 mm) in the first year and above-average rainfall (>1000 mm) in the second year.
The randomised complete block design included four replicates of 10 cover crop treatments grown in 8 m by 5 m plots. The treatments included various combinations of three contrasting functional types – legume (vetch in year 1 and faba bean in year 2), brassica (forage rape) and cereal (forage oat).
The treatments tested single species and mixtures involving two species (50:50) and three species (70:15:15) with different dominant species. All cover crop treatments were assessed against the performance of a bare fallow treatment where weeds were allowed to germinate and grow during the cover crop growth periods. All stubble was maintained, and weeds were controlled using herbicides from the termination of the cover crops to the planting of the next crop.
The cover crops were sown on 35 cm row spacing and terminated with herbicides (brown manured) at 86 and 80 days in years 1 and 2, respectively. Three months after termination, a summer cash crop was grown – maize in Year 1 and mungbean, followed by winter wheat in Year 2.
For this experiment, weeds were allowed to germinate and grow in all the cover crops and bare fallow (nil-cover crop) control treatments. All plots (cover crops and bare fallow) were treated with herbicide at cover crop termination and were kept weed-free until the next cover crop and bare fallow phase was established.
Fourteen ecosystem functions related to N and C cycling, soil water conservation, improved soil health and biological function, weed suppression, and overall system productivity and profitability were measured. Weed suppression was calculated as the reduction in weed biomass in the cover crop compared to the background weed burden represented by the bare fallow.
Figure 1. Schematic representation of the three main categories of the fallow management system tested across the 3-site years showing the different phases of the crop–fallow rotation. The red circles show the timing of soil water and mineral N samplings at cover crop sowing (1, 5), cover crop termination (2, 6), cash crop sowing (3, 8), and cash crop harvesting (4, 7). Cumulative precipitation (mm) during the different phases is provided in the bottom bar. * Cover crop treatments varied with the choice of functional types and mixture composition (Poaceae vs. Fabaceae vs. Brassicaceae). ** Only 41 mm of precipitation was received during the cover crop phase in Year 1 and was supplemented with another 44 mm to ensure crop growth, i.e., this specific cover crop phase received 85 mm of total water input, comprising rainfall and irrigation.
Detailed findings
This research adds to the body of evidence that including a cover crop phase in a broadacre cropping rotation increases agro-ecosystem functionality of the farming system.
Compared to the control (bare fallow), various cover cropping options improved 11 of the 14 measured ecosystem functions (see Figure 2), most noticeably crop biomass production, nitrogen supply and retention across the rotation, nitrogen accumulation during fallow, soil biological activity, promotion of free-living nematodes, and weed biomass suppression. In these experiments, these benefits were achieved while producing the same cash crop yield and system profitability as the nil-cover crop fallow treatment.
Figure 2. Relative performance of a cover crop system compared to a bare fallow regarding a range of measured agro-ecosystem functions. Values closer to the outside show higher performance for that function.
Weed control function
Cover crops suppress weeds mainly through physical interference (i.e. reduced access to sunlight, soil water, and N resources) and chemical interference (allelopathy).
All ten cover cropping treatments in this trial provided a similar level of weed control (75–50% suppression) (see Figure 3). The more competitive forage oat-dominated cover crops had higher weed biomass suppression (75%) compared to forage brassica-dominated (64%) and legume-dominated (51%) cover crops.
Figure 3. Distribution of % weed biomass suppression due to cover crop relative to a bare fallow.
Considerations when choosing cover crop species
The cover crop combinations that included forage oats as the dominant species provided the most improvement in ecosystem functions relative to a bare fallow.
These cereal-dominated cover crops generated higher cash crop yields, system profitability, and improved soil health by generating additional crop residue, recruiting free-living soil nematodes, and improving soil aggregation.
The moderate biomass and water use of the oat monoculture and the oat:legume mixtures provided greater ecosystem function multifunctionality than monocultures of forage brassica. They also produced more standing residue to reduce erosion and conserve soil moisture after termination than the legume monoculture.
The most diverse treatment, the three-way species mixture, did not result in higher ecosystem benefits. This may be due to the competitiveness of forage brassica and its tendency to dominate other species in a mixture. The forage brassica also negatively affected the benefits associated with the other species in the mix.
The forage brassica-dominated treatments were less water efficient than the oat treatments, consequently restricting crop yield and system profitability. Although the forage brassica treatments provided high biomass and potential suppression of plant-parasitic nematodes, they are not a preferred cover crop choice for water-limited environments.
The forage oat monoculture and oat-legume mixed cover crop treatments contributed to the highest system gross margins (on par with a bare fallow system). Many studies have reported that mixtures of oat and legume cover crops enhance ecosystem functions and mitigate negative trade-offs.
Yield penalties, low profitability, and increased management costs are major barriers to cover crop adoption in water-limited environments. Monoculture oat cover crops generally have lower seed and management costs and are less risky than multi-species mixes.
This study demonstrates that integrating cover crops into crop-fallow rotations increased agroecosystem function multifunctionality, leading to improved agronomic and environmental outcomes compared to a bare fallow.
Where specific ecosystem functions are desired, growers can select a species mix to suit. They can be confident that any well-established and grown cover crop will achieve at least 50 to 75 per cent weed suppression.
Does cover cropping pay in a northern dryland cropping system
After a forage oat cover crop, the maize yield was 19 per cent higher and the winter wheat yield was 23 per cent higher than after the bare fallow. This translated into +96 $/ha for the maize crop and +318 $/ha for the winter wheat, compared to the same crops grown after a bare (nil-cover crop) fallow. Forage rape monoculture reduced maize yield by 21 per cent compared to the bare fallow, mainly due to high biomass production and high water use that was not compensated by additional ground cover following cover crop termination or enhancement of soil ecosystem functions.
Research papers
The research outlined in this article was conducted by Ismail Garbaa,b, Lindsay Bellc, Bhagirath Chauhana,d and Alwyn Williamsa.
a School of Agriculture and Food Sustainability, The University of Queensland, Gatton
b Centre for Dryland Agriculture, Bayero University Kano, Nigeria
c CSIRO Agriculture and Food, Toowoomba
d The Centre for Crop Science, Queensland Alliance for Agriculture and Food Innovation (QAAFI), The University of Queensland, Gatton
The research work did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors. Open Access funding enabled and organised by the Council of Australian University Librarians (CAUL) and its Member Institutions.
Read the full papers:
- Ismail I. Garba, Lindsay W. Bell, Bhagirath S. Chauhan, Alwyn Williams, Optimizing ecosystem function multifunctionality with cover crops for improved agronomic and environmental outcomes in dryland cropping systems, Agricultural Systems, Volume 214, 2024, 103821.
DOI: https://doi.org/10.1016/j.agsy.2023.103821 - Garba, I.I., Williams, A., Integrating Diverse Cover Crops for Fallow Replacement in a Subtropical Dryland: Implications on Subsequent Cash Crop Yield, Grain Quality, and Gross Margins. Agronomy 2023, 13, 271.
DOI: https://doi.org/10.3390/agronomy13010271
