Cover crops play an increasingly important role on farms worldwide. In addition to slowing erosion, improving soil structure, and providing fertility, there is a growing evidence that cover crops can help farmers to manage pests.[i]
Cover crops can reduce insect infestations, diseases, nematodes, and weeds. Applying cover crops help to minimize reliance on pesticides, cutting costs and reducing chemical exposure while protecting the environment and increasing consumer confidence of your products.
New strategies that preserve farm’s natural resources while improving profitability emphasizes a farm as an “agro-ecosystem” – a dynamic relationship of the mineral, biological, weather, and human resources involved in producing crops or livestock.
Environmentally friendly pest management starts with building healthy soils. Crops grown in biologically active soils resist pest pressures better those grown in poor soils. Reducing or eliminating pesticides favours diverse, healthy populations of beneficial soils flora and fauna. Eliminating or reducing tillage that causes losses of soil structure, biological life or organic matter make crops less vulnerable to pest attacks.
In natural ecosystems, insect populations are controlled by their natural enemies. Conserving and encouraging beneficial organisms that control pest insects is a key to achieving sustainable pest management.
By including cover crops in your rotations and not spraying insecticides, beneficials often are already in place when you plant spring or summer crops. However, if you fully incorporate cover crops into the soil, you destroy or disperse most of the beneficials that were present. Conservation tillage is a better option because it leaves more of the cover crop residue on the surface. The goal is to provide year-round food and habitat for beneficials to ensure their presence within or near primary crops.
When crops are attacked by pests, they send chemical signals that attract beneficial insects. The beneficials move in to find their prey. Maximising natural predator-pest interaction is the primary goal of biologically based Integrated Pest Management (IPM), and cover crops can play a leading role.[ii]
Beneficial insects need nectar and pollen to survive and reproduce. For example, adult parasitoid wasps feed on flowers, while the parasitoid larvae prey on pests such as aphids, caterpillars, and stink bugs. Lady beetle, aka ladybugs, feed on flowers, especially during times of prey scarcity. In addition to flowers, cover crops, such as vetch and fava beans, have extra-floral nectarines or spurs at the base of the leaves. These secrete a sugary syrup that attracts beneficial insects, such as syrphid flies to control aphids.
In the field, although plants are exposed to a wide diversity of micro-organisms, plant infection by micro-organisms is rare. A pathogen must cross many plant barriers before it can cause disease to root, stem, or leaves. You can use cover crops to reinforce these barriers.[iii]
Plant cuticle layers which protect the plants against pathogens can be physically damaged by cultivation, spraying or even by the impact of soil splashing raindrops or irrigation. In well-developed minimum till or no-till crop systems with cover crops you may not need cultivation for weed control and you can minimise spraying.
Many benign organisms are present on the leaf and on the stem surface. These biofilms play an important role in plant disease. Cover crops can help this natural protection process work by reducing the need for application of synthetic crop protection materials and by supporting beneficial micro-organism populations.
Soil-borne pathogenic fungi can also negatively impact production of many crops. Implementing non-hosting cover crops on your production, you can reduce plant disease incidence and severity by enhancing natural disease suppression.[iv]
However, in soils with high levels of disease inoculum, it takes time to reduce population levels of soil pathogens using only cover crops. Pathogen suppression with cover crops should be viewed as a long-term strategy which combines disease management with weed suppression, reducing erosion and nitrate leaching, and improving soil structure.[v]
Most nematodes are not plant parasites, but feed on and interact with many soil-borne micro-organisms, including fungi, bacteria, and protozoa. Damage to the crop from plant-parasitic nematodes results in a breakdown of plant tissue, such as lesions or yellow foliage; retarded growth of cells, seen as stunted growth or shoots; or excessive growth such as root galls, swollen root tips or unnatural root branching.
You can gradually reduce a field’s nematode pest population or limit nematode impact on crops by using specific cover crops. Nematode control tactics involving covers include:
- Manipulating soil structure or soil humus
- Rotating with non-host crops
- Using crops with nematocidal effects, such as brassicas
A study in sugar beets production found that malt barley, radishes and mustard worked as the standard nematicide. Increased production more than offset the cover crop production and lamb grazing increased profit even further. [vi]
Cover crops can be used to smother crops to shade and outcompete weeds. Cover crops can also serve as a “living mulch” to manage weeds in vegetable production.
Cover crops are left to grow between rows of the cash crop to suppress weeds by blocking light and outcompeting weeds for nutrients and water. They may also provide organic matter, nitrogen and other nutrients mined from underneath the soil surface, beneficial insect habitat, erosion prevention and wind protection.
To avoid competition with the cash crop, living mulches can be chemically or mechanically suppressed. Some cool season cover crops such as crimson clover may die out naturally during summer crop growth in warm climates and do not compete for water or nutrients. However, cover crops that regrow during spring and summer can compete strongly for water with spring planted crop.
A test conducted in Ohio, US, demonstrates how cover crops can successfully replace herbicides in some horticultural production systems. The study demonstrated that a killed mulch of a cover crop mix of rye, hairy vetch, crimson clover, and barley kept processing tomatoes nearly weed-free for six weeks in an Ohio test. Tomato fields kept weed-free for 36 days yield as much as fields kept weed-free all season. [vii]
Cover crops often suppress weeds early, then prevent erosion or supply fertility later in the season. For example, shade-tolerant legumes such as red clover or sweet clover that are planted with spring grains grow rapidly after grain harvest to prevent weeds from dominating fields in late summer.
[i] Snapp, S. et al. 2005. Evaluating cover crops for benefits, costs, and performance within cropping system niches. Agron. J. 97:1-11.
[ii] 420 Tumlinson, J.H., W.J. Lewis and L.E.M. Vet. 1993. How parasitic wasps find their hosts. Sci. American 26:145-154.
[iii] 314 Paxton, J.D. and J. Groth. 1994. Constraints on pathogens attacking plants. Critical Rev. Plant Sci. 13:77-95.
[iv] Magdoff, F. and H. van Es. 2001. Building Soils for Better Crops, 2nd Edition. Sustainable Agriculture Network. Beltsville, MD. www.sare.org/learning-center/bsbc
[v] Vincent V. Michel, Karolis Urba & John Clarkson, Mini-paper – Green Manures and cover crops to reduce the pressure of soil-borne diseases in annual crop, 2020
[vi] Koch, D.W. 1995. Brassica utilization in sugarbeet rotations for biological control of cyst nematode. SARE Project Report #LW91-022. Western Region SARE. Logan, Utah. www.uwyo.edu/Agexpstn/research.pdf
[vii] Creamer, N.G. et al. 1996. A comparison of four processing tomato production systems differing in cover crop and chemical inputs. HortSci. 121:559-568.