Cover cropping

Background

The Soil Science Society of America (2008) defines a cover crop as a close growing crop that provides soil protection, seed protection and soil improvement between periods of normal crop production. Further, planting a variety of cover crop species with different rooting depths and root structures increases soil organic matter throughout the entire profile and improves any soil physical issues when the plants die. The term and practice is now being adopted in North America (mainly by no-till practitioners) as a management system to further enhance the benefits of conservation cropping.

In Australia, winter cover crops can include plant species such as forage oats, tillage radish, field peas, purple vetch, albus lupins, arrowleaf clover and biserrula. Summer crop species include species such as forage sorghum, Japanese millet, white French millet, Sudan grass, tef grass, lablab, cowpea, red clover and forage rape.

Cover crops are terminated with herbicides or by rolling when actively growing, prior to seeding the subsequent crop. The timing of termination can vary for different cover crop species and the pre-emergent requirements of the subsequent crop. The residues are not ploughed in as green manure. Timing of termination in Australian conditions is under investigation.

A North American example of cover crop type and management is planting a forage crop (oats, peas, radish, turnip, red clover and hairy vetch) which is then green chopped along with weeds. Then, if seasons are favourable, the vetch, red clover and turnips regenerate to provide grazing in autumn.

The advocated benefits of cover cropping are:

• Improved soil quality, principally through the active root growth of cover crops and the decomposition of cover crop residues by soil microbial activity. This adds to the soil organic matter reservoir ,thereby improving SOM, which has the following benefits:
  • Improved soil structure – less erosion.
  • Better gas exchange.
  • Improved water infiltration (through soil macropores) and water retention, particularly to depth.
  • Increased nutrient exchange and increased soil nitrogen (N) resulting from a higher return of vegetative residues. Legumes and grass species scavenge and recycle mineral N which would be lost by leaching. Legume cover crops also fix atmospheric N.
  • Greater soil biodiversity (Soil Biology and Biochemistry 2020).
  • Greenhouse mitigation due to significant soil C sequestration, reduction in nitrous oxide emissions and reduced energy use for fertiliser production.
  • Provide habitat for beneficial insects and other soil fauna.
  • Continuity of ground cover and lower sheet erosion.
• Soil physical conditioning, particularly through gramineous plants with fibrous root systems, to improve porosity and modify compaction.
• Weed management via:
  • Weed competition in the fallow period (living mulch).
  • Providing a weed suppressive mulch.
  • Allelopathy effect when living and after termination.
• Soil fertility (nutrient) enrichment from mineralization of decaying biomass. This releases a wide range of plant nutrients and is particularly the case with leguminous plants.
• Reduced nitrate leaching and nitrous oxide emissions from cereal cover crops.
• Saving of fertiliser costs by building soil fertility from green manuring by rolling or desiccation with knockdown herbicides.
• Saving of herbicide and fungicides from increased plant competition.
• Reduced runoff and erosion by providing surface cover and below-ground biomass between the growing seasons of the main crops. The runoff water has improved quality.
• Diminished need for insecticide application by attracting beneficial insects -predators of damaging insects.
• Providing a buffer to protect the soil from temperature extremes.

At this stage cover crops appear suited to semi-humid and humid regions. Adoption in semi-arid or water limited regions may be limited as cover crops reduce critical plant available water for the main crops. This is an area of research required in the cropping zones of Australia, along with research into management, including early termination and species selection. This research is being carried out at Wagga Wagga NSW and is described below. Some claim that cover crops increase soil water by reducing runoff and improving infiltration and soil water storage, particularly to depth. Cover crops can decrease evaporation due to a mulching effect, but there must be sufficient rainfall when the cover crop is actively transpiring. On the other hand, cover crops in excessively wet summers and autumns could result in increased water use and a drier soil at sowing. This would be a rare situation in most Australian cropping regions.

As farmers want to reduce the use of chemicals to terminate cover crops and fallow weeds, research has been reported on non-chemical methods in the American Journal Of Alternative Agriculture (2002). Basically, their results showed that in Missouri mowing a rye/vetch cover crop killed 100%, and 90% was killed by rolling. In northern Carolina, undercutting provided a 95% kill for 5 of the 6 broadleafs and 2 of the 5 grasses present. Mowing effectively killed the 6 broadleafs, but regrowth occurred with 3 of the 5 grasses. In general, rolling did not kill broadleaf or grass cover crops, with the exception of nearly mature millets and buckwheat. This work shows that there will need to be Australian research to investigate termination times and types of termination, particularly the impacts on subsequent grain crops.

It is interesting to note that the current GRDC recommendation for weed control is via the Weedsmart BIG SIX Tactics. These are:

• Rotate crops and pasture.
• Double knock to preserve glyphosate.
• Mix and rotate herbicides.
• Stop weed set.
• Crop competition.
• Harvest weed seed control.

There is no mention of cover crops as a weed control technique, despite the claims made in North America. Obviously, there need to be Australian research results available before the technique can be recommended to growers by the GRDC.

Issues/ Disadvantages

• N depletion with high carbon residues, which cause microbes to use soil N.
• Much of the positive impacts of cover cropping in America are from winter cover crops in the moist regions.
• Management of cover crops should be carefully monitored for the synchronisation of N release with subsequent crop requirements.
• Additional costs of planting cover crops and knockdown, either mechanically or with herbicides.
• Reduction of soil moisture for the main or subsequent crops.
• Cover crops are not compatible with tillage if it is required for management operations, such a lime incorporation and seedbed levelling.
• Occasionally, cover crops can increase pests and diseases.
• Care needs to be taken with possible cover crop allelopathy on the subsequent crops.
• In semi-arid regions where water is most limiting cover crops may not be a viable management option.

Decisions to be made with cover crops

If a farmer wants to use cover crops to improve soil health and tilth some objectives need to be considered:

• Is the main purpose to add available nutrients and N to the soil?
• Is it to provide large amounts of carbon?
• Is erosion control the primary goal?
• Are cover crops being used to alleviate a soil compaction problem?
• Will the cover crop, climate and water-holding capacity of the soil result in excessive water depletion for the primary crop?

With these objectives considered, decisions are needed on:

• Plant species required to meet the primary objectives.
• The optimum date for planting.
• When to eliminate/terminate the cover crop for the primary crop.

Current Eastern Australian Research

Dr Saliya Gurusinghe is studying the use of cover crops in rotations to suppress weeds and reduce reliance on herbicides. He is located at the Charles Sturt University Graham Centre, Wagga Wagga, NSW. This work is being funded by GRDC and involves Charles Sturt University, the University of Sydney, and the Queensland Department of Agriculture and Fisheries.

Despite the large amount of literature on cover crops adoption has been slow, particularly in southern New South Wales. This is due to a lack of information about:

• Suitable species.
• Planting windows.
• Appropriate termination times.
• Soil water use, particularly the impact on the subsequent grain crop.
• Cost of establishment.
• Suppression of weeds (which may be in and competition for resources such as sunlight, nutrients and water which may impact the following grain crop).
• Possible production of phytotoxic secondary metabolites exuded by the plant roots of the living cover crop or leached from decomposing cover crop residues.

Dr Gurusinghe’s experiment has been running since 2017 at Wagga Wagga, Narrabri and Kingaroy, using selected winter and summer cover crops. Winter cover crops include forage oats, tillage radish, field peas, purple vetch, albus lupins, arrowleaf clover and biserrula. Summer crop species include forage sorghum, Japanese millet, white French millet, sudangrass, tef grass, lablab, cowpea, red clover and forage rape.

The trials measure establishment, canopy light interception, crop biomass and weed biomass. Trials during 2019 investigated monoculture and several binary multispecies mixes and various sowing rates. It is hoped to predict what cover crops grow best in particular environments and the appropriate termination times for optimal weed suppression.

Results to Date

• Cover crops with rapid growth producing groundcover and biomass drastically reduce typical agronomic weeds.
• Winter cover crops of oats and radish performed well.
• Summer cover crops of sorghum, Sudan grass, tef grass, Japanese millet, white French millet and forage rape performed well.
• Legume cover crops performed poorly due to the drought. They performed better in a binary cover crop mixture.
• Legumes appear to be necessary in the mix as laboratory experiments have shown certain legume residues, such as biserrula, yellow serradella, cowpea, and lab lab, suppressed early weed germination and the growth of weed seedlings. They also released high levels of certain flavonoids suggesting residues may be phytotoxic when incorporated into the soil.

The Future

Questions to be answered are:

• Impact of termination time on weed suppression and soil water conservation?
• The cost of establishment?
• The impact of selected cover crops on the C:N ratio?
• The impact of soil water use on the subsequent grain crops?
• The true economic impact of cover crops in reducing fallow herbicides and N applications and the predicted conservation of soil water for subsequent crops, compared to traditional no- or zero-tillage practices of fallow weed control with herbicides and moisture conservation?

• Acknowledges the positive soil and economic results from farmer and research investigations in North America.
• Due to the often vastly different climates, soil types and crop types in North America, cover cropping needs to be assessed with various Australian climates and soil types over a number of years before any definitive recommendations can be made.
• SKN seeks to establish a close link with current GRDC research to assess the impact of cover cropping on overall farm management, economics and soil improvement, in order to provide general extension advice.
• SKN will communicate anecdotal farmer experience and results from on-farm trials. Factors which need to be recorded are soil type, climatic conditions, types of cover crop species used, interactions with grazing or forage harvesting, termination types and timing, and any impacts on subsequent crops.
• Any assessment or measurement of Soil Carbon Sequestration in any experimental or farmer trials must follow Australian standards for Carbon Trading purposes. Indicative soil carbon sequestration rates (i.e. is my soil carbon increasing or decreasing?) can be made with techniques such as the Weil Soil Carbon Test and LECO.
• The adoption of cover cropping for improved soil health, particularly for carbon sequestration, needs to be combined with zero tillage and conservation grazing techniques. This implies the use of a disc seeder or a seeder modified to handle high residue levels, such as seeder attachments to remove residue in the planting row.

Appendix: Dave Brown ‘Gambarra’ Greenethorpe

Case study: Gabe Brown – Cover Cropping Advocate In North America