Cover crops for integrated weed management

Cover crops can suppress weeds and volunteers by competing for light, water and nutrients. Some species also release chemicals that inhibit weed development.

Carefully managed cover crops can suppress weeds through various means. The effect varies depending on the cover crop and the weed species:

• They add diversity to the rotation and reduce opportunities for weeds to adapt to a cropping pattern
• Several cover crop types can out-compete weeds and help provide a cleaner seedbed
• Management practices associated with growing cover crops (e.g. mowing and grazing) can suppress weeds
• Long-term leys, with a lack of soil disturbance, can reduce viable seed numbers
• Some brassicas contain high levels of chemicals that can sterilise soil

Note: Make sure cover crops do not seed and become weeds. For example, phacelia can self-seed prolifically and become a weed.

Weed competition

Cover crops can compete with weeds for light, water, and nutrients.

• Increased competitive ability is linked to early emergence, seedling vigour, rapid growth, and canopy closure
• When establishing the following crop, ensure cover is uniform and minimise soil disturbance
• Some cover crops work by allowing weeds to become established and then destroyed before they produce viable seed. In this situation, cover crop canopies need to be open enough for weed germination

A note on black-grass

Cover crops only have a small impact on black-grass. Agronomic factors, such as cultivation timing and type, use and timing of glyphosate, date of crop establishment and diversity of rotation, have a bigger effect on black-grass populations. A change in the timing of crop establishment has the greatest impact.

Allelopathy

Allelopathy is where chemicals produced by one plant (or plant-associated microorganisms) affect the growth and development of another plant. The release of allelochemicals can be affected by plant age and vigour, environmental factors and the presence of other plants.

The impact of these chemicals is affected by soil texture, organic matter, temperature, light and microbial breakdown. Some plant species secrete chemicals into the soil (both during their life and after incorporation) that inhibit weed seed germination. Sometimes, these can also inhibit germination in subsequent crops, especially directly sown (i.e. not transplanted) small-seeded crops; the effect can last for several weeks.

Cover crops reported to have in-field allelopathic effects include rye, oats, barley, wheat, triticale, brassicas (oilseed rape, mustard species, radishes), buckwheat, clovers, sorghum, hairy vetch, sunflowers and fescues. However, it is not easy to separate physical competition and allelopathic effects.

Cover crops can disrupt pest life cycles and reduce their populations. Brassicas are also used as a biofumigant to manage some soilborne pests. Certain crop species can also be used as trap crops and to encourage beneficial organisms.

Cover crops contribute to integrated pest management (IPM) through a variety of mechanisms.

Biofumigation

When certain cover crop material is chopped up and incorporated into the ground, it releases toxic compounds that help sterilise the soil. For example, brassica cover crops release glucosinolates – and products of their degradation, such as isothiocyanates – as well as volatile sulphur compounds that are toxic to many soilborne pests. Biofumigant cover crops have been demonstrated to be useful for managing beet cyst nematodes and rhizoctonia root rot in sugar beet and potato cyst nematodes in potatoes.

How cover crops are produced, destroyed and incorporated will affect the efficacy of biofumigation.

Biofumigation for PCN management

Trap crops and host disruption

Some cover crops can act as a trap crop by promoting pest egg hatch, including some nematode species.

Cabbage root fly and other brassica pests can be disrupted by diverse planting, for example, with intercropped cover crops (understory or strips). However, the approach requires experimentation in each system.

Predator habitat

Cover crops provide habitats for general predators, which is especially important over the winter.

Summer-flowering plants also encourage beneficial predators such as hoverflies, lacewings and parasitic wasps.

Cover crops can encourage some pest species, but they can also help control pests in grass leys. However, long grass-clover leys can harbour soilborne pests, such as leatherjackets and frit flies, which can reduce dry matter production significantly and even destroy a grass ley reseed.

Good management can reduce the impact of soil pests:

• Autumn ploughing and disking can reduce wireworm numbers
• Close mowing between July and September reduces egg-laying by crane flies (the adults of leatherjackets)
• A quick mustard crop (high-glucosinolate variety) can be effective at reducing soilborne pests through biofumigation, providing it achieves a large biomass with rapid incorporation into sufficiently damp soil

Some cover crop scenarios can increase slug populations. However, ryegrass or lucerne are unlikely to cause problems. Some cover crop species host soilborne pests, providing a ‘green bridge’ between main crops. Ploughing in leafy crop residues may also support egg laying by bean seed flies.