Cover crops for integrated weed management

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.

Further information