Cover cropping

What is cover cropping?

A cover crop is a crop sown to establish groundcover. 

The crop is then usually sprayed out before seed set and the increased groundcover protects soil and increases soil water storage. 

Cover crops can:

• suppress weeds during the growth of the cover crop and after it has been killed
• make short term contributions to soil carbon compared to bare fallows
• increase soil microbial biomass. 

These benefits need to be weighed against the costs of planting a cover crop and the risk of reducing soil moisture for the following crop.

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Benefits of cover crops

• Improvements to the yield of grain and cotton crops following cover crops.
• Higher populations of beneficial arbuscular mycorrhizal fungi (AMF).
• Greater breakdown of disease carrying crop residues due to higher moisture retention under groundcover.
• Using a legume crop such as field peas, vetch or faba beans, will provide nitrogen (N) fixation of around 25kg N fixed per tonne of dry matter (DM) grown. Sown in late winter a crop like this could be expected to grow around 3t/ha of DM, fixing around 75kg of N per ha.
• Increases available N and weed control for the following crop adds value economically that is hard to quantify.

Planning a multispecies cover crop

This video covers the science behind cover crop selection and species diversification.

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Importance of soil water in cover cropping

The additional water captured by the cover crop, should be equal to or greater than if the soil was left in a bare fallow. This can cause an improvement in water storage during fallow and in crop and can also result in greater crop emergence because of better surface moisture under groundcover.

However, in some cases cover crops have a negative impact on starting soil water for the next crop. It is important to consider this when making the decision to use a cover crop.

When to plant a cover crop

• It is recommended to plant cover crops as early as possible in the cropping cycle and terminate the cover crop once sufficient biomass has been achieved.
• If there is a longer period between spraying out the cover crop and planting the following crop, there is more likely to be a positive influence on soil water.
• When starting levels of groundcover are very low (15% or less) cover crops are more likely to have a positive impact on soil water as the increase in groundcover will be greater.
• Paddocks with very low levels of cover represent the best case for using cover crops.
• It should be remembered that the positive effect of the cover crop will continue after planting the following crop as greater cover improves infiltration and slows evaporation of early in-crop rain.

How to choose a cover crop

Cover crop choice may be influenced by the crop chosen to follow it because of the possibility of cover crops harbouring diseases which could affect the main crop. Common summer cover crops include:

• millet
• retained sorghum
• hybrid sorghum
• forage sorghum
• sudan grass
• summer legumes (cowpeas, vetch, lab lab etc.).

Limited information indicates that millet is not a good host of crown rot (Fusarium pseudograminearum) and is unlikely to significantly add to crown rot infection in a following wheat crop.

Legumes will not produce biomass as quickly and may well produce less total biomass compared to grass and cereal crops. Legume residues will breakdown faster due to the higher Nitrogen (N) content in the residues. Both these factors can result in less groundcover.

Legumes cover crops have been used to increase soil N but as with soil water this does not always occur. If legumes are to fix N for following crops initial soil N should be low and/or legumes must be retained until soil nitrogen is depleted sufficiently forcing them to fix atmospheric N.

Grazing and harvesting cover crops

Cover crops can be used for grain or grazing after a favourable start to the season, but sufficient biomass should be left behind (more than 50%) when grazing to provide enough groundcover otherwise the original purpose of the cover crop is compromised.

Before deciding to keep a cover crop for grain, thought needs to be given to adequacy of nutrition and stored water to achieve an economic yield.