Understanding The Role Of Cover Crops In Farming
Loss of topsoil due to conventional commercial farming practices is a worrying trend worldwide. Soil is vital for food production but it is being lost from agricultural areas faster than it can be formed. This is precisely why change is needed in our current conventional farming practices.
Cover cropping and conservation agriculture (CA) are practices contributing to regenerative agriculture which is being hailed as a major tool in the fight to halt this and reverse the harmful, unsustainable impact of commercial farming. It is gaining momentum worldwide and here, in Zimbabwe, it is also gathering a large following of farmers, determined to find out more on how to implement these measures into their operations.
With this in mind, Dutch pasture seed specialists Barenbrug offer a wide range of cover cropping solutions for the pressing problems of topsoil loss, soil compaction, nematode infestations and carbon- and nutrient depletion in our soil. They also offer advice on the inclusion of cover crops into cropping rotations, and their benefits.
What Is Cover Cropping?
The definition used in and, for the purposes of this story, taken from the company’s fifth product guide, says “Cover crops are planted primarily to manage soil fertility, soil quality, water, weeds, pests, diseases and biodiversity in an agro ecosystem (Lu et al. 2000), to improve qualities of neighbouring natural ecosystems. Farmers choose to grow and manage specific cover crops types based on their own needs and goals.” (Snap et al, 2005)
To explain how cover cropping fitted in with regenerative agriculture, the following basic principles apply:
- Avoidance - as much as practical - of aggressive tillage. In a five-year rotation, turning the soil would be needed at some stage, but it should be done as little as possible.
- Soil cover. To avoid extremes of temperature, minimise soil moisture loss, and to reduce weeds. Green is good and all-year-round green is better.
- Importance of living roots in that soil. There is a synergy between living roots and the soil microbe population. Living roots excrete sugars and that’s food to support more microbes which decompose soil organic matter (SOM) releasing nutrients and also making some minerals in the soil more readily available to those plants.
- The more diverse the range of plants, the more robust the soil system.
- The use of chemicals would still be needed but that overuse was both dangerous to soil health and an expensive input.
Cover crops would also release SOM, providing food for the soil microbes and also increasing the water holding capacity (WHC) of the soil.
In regenerative agriculture, all of the above principles need to be in place. Learn them, develop practices yourself, adapting them, season to season. Barenbrug has positioned itself to offer cover cropping solutions for poor soil health, that can be implemented into current farming practices. However, there is no one-fits-all solution. Farmers need to identify their own reasons for growing cover crops. When looking at implementing them into farming, farmers should realise that cover cropping is not just a one-season solution for poor soils and that farmers should look at long-term solutions.
Considerations For Farmers
Farmers are urged to treat their cover crops exactly the same as they would their cash crops.
Planting time, planting pattern, fertilisation, weed control, animal management and the species and variety of cover crops to suit the purpose and environment all need to be considered when planning a cover crop.
Livestock can be used as a tool to control weeds in the cover crop, by ultra high stocking rates, but when the crop is grazed down to hoof height, then it is time to move them off to allow for good recovery/regrowth. Tall species may need to be slashed to decrease shading of shorter species.
Soil compaction is often a big challenge faced by farmers embarking on including cover crop into their rotations and plant species such as sugar beets, turnips and daikon radishes play an important role in breaking up compacted layers of soil with their dense, fibrous roots.
At some point before planting the following cash crop, the cover crop species will need terminating. There are a number of options to achieve this such as the use of moldboard plough, flail mower, roller crimper, animals and chemicals. The timing needed to be specific to a particular farming operation. Trade-offs of benefits have to be considered and this includes biomass (mulch), nitrogen fixing, soil moisture and “volunteer’ seeding. Later/more mature crops provides more biomass but a higher C:N (lower N%), and at maturity/allowing seeding can result in weed potential; with legumes the flowering/early green pod stage maximises N% in the biomass.
Farmers need to time the breakdown of cover crops and nutrient release to match the nutrient needs of the following cash crop. They need to understand that the optimum biomass C:N ratio is about 24:1 (about 1.7% N, about 10% crude protein) to supply microbe needs to enable fast organic matter decomposition for release of nitrogen for the next crop. But the challenge is to keep soil cover (mulch), some material with a lower % N to degrade more slowly. With grass biomass addition of some fertiliser N, eg 8-10kg/ha, will counteract slow N release.
Planting row crops into cover crop residue also presents challenges. Specialised no-till equipment is needed, as it can be difficult to slice through residue if it is too dry and there may be poor seed to soil contact, reducing plant populations. There may be a lack of seed furrow closure if soil is too wet.
A soil health Haney test to measure nutrient availability and microbe activity should be done to let farmers know they are on the right path. This system should assist farmers in cutting down input costs - fertiliser, weed control, etc, but it has to be done in a responsible manner. Carbon helps the soil’s ability to store water. Soil needs to absorb and maintain as much moisture as possible: high infiltration, less runoff and high water holding capacity (WHC). It is not how much rain received but how much can be used.
Farmers should understand the relationship between microbes and plants. Microbes are recyclers (degrade OM), refiners (fix N), and miners (transport nutrients). Their secretions are referred to as glues and gums, aggregating soil particles, and this is a visual indication of a healthy soil. How do we achieve a high population of microbes in our soil? Diversify! The greater the plant diversity, the greater the diversity of microbes and the more robust the soil system. It is recommended planting a core set of 3-5 species of cover crops. It is essential to select species with complementary growth periods, growth forms and nutrient acquisition strategies. Identify advantages (eg nematode control) and disadvantages (eg allelopathic effects on the next crop ** see below, harbouring pests and diseases of the next crop), work out drawbacks (eg weed potential) and missing ‘ingredients’ (eg deep strong roots to break through a plough pan). Determine purpose of the cover crop. Work out the mix needed, taking into consideration growing conditions, growing window (time available for the cover crop) and budget. Determine row spacing, and seeding rates. Plant, observe and adjust.
Concerning the issue always at the forefront of farmers’ minds, being that of returns versus input costs, nitrogen fixing cover crops could reduce the farmer’s reliance on adding fertiliser to the following crop in the rotation. Livestock grazing is both important an important complement to the biodiversity and also another source of income. Barenbrug call it “covergraze”. Provision of grazing and fodder impacts decisions on timing and species mix.
In addition to soil conservation and nutrient management, the nutritional values of most cover crops will meet the needs of grazing livestock.
Soils are a farmer’s biggest asset. Regenerative farming models nature through diversity. It features an ecosystem of diverse and rotating crop varieties, beneficial animal pasturing, and a reliance on natural pollinators. Everything plays a part in returning life to the soil, which then yields nutrient-rich produce in season. Other tenants of sustainable agriculture include minimal soil disturbance, cover cropping, and other systems intentionally designed to build topsoil. Barenbrug supports and practises, through its sales ethos, those very same principles.
** Plant allelopathy is the ‘chemical warfare’ among the plants imposed by one plant on another to suppress the latter and take advantage from that suppression. In the phenomenon of plant allelopathy, allelopathic plants create adverse conditions to other neighboring plants by reducing their seed germination and seedling growth. The allelopathic plants are very effective in weed killing and known as Nature’s Weed Killers.
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