Intermediate crops are cover crops planted between main crops. They meet agronomic, economic, and regulatory objectives. They can be harvested or not. If they are not harvested, they are destroyed and sometimes mixed into the soil. The specific name for an intermediate crop depends on the farmer’s reason for planting it. Definitions of 4 types of intermediate crops.
Catch crops are harvested and used
Catch crops are a type of cover crop generally sown by livestock farmers. The harvested biomass is used to feed the herd (forage). It is used for animal grazing, green feed, or silage. The whole plant can be harvested or just the seeds. A sub-group of intermediate crops is grown for energy purposes. The harvested biomass is used to supply a methanization unit to produce gas (methane), electricity, or heat by co-generation. The gas is produced by plant fermentation.
It takes three to four months of intercropping to grow a sufficient amount of biomass.
Catch crops are not adapted to certain main crops because:
- They are harvested after July 30: grain or silage maize, beet, potatoes.
- They are sown before October 1: rape, barley.
Nitrate-fixing intermediate crops
The aim of nitrate-fixing intermediate crops is to trap soluble nitrogen and avoid leaching. As cover crops develop, they absorb nitrates, preventing losses and pollution to surface water and groundwater. They are planted in the autumn after the cash crop has been harvested. They are generally destroyed and left in place to serve other agronomic purposes (soil structure, green manure, etc.).
For a nitrate-fixing intermediate crop to perform well, it's important to choose the right variety, alone or mixed, and to take care during planting. The more biomass produced, the greater the crop's capacity to fix nitrogen. The main nitrate-fixing intermediate crops used today, alone or in mixed, are mustard, oats, rapeseed regrowth, mixtures of legumes, phacelia, sainfoin, etc.
Green fertilisers destroyed to increase nitrogen in the soil
The purpose of using cover crops for green manure is to return fertilising elements to the soil for the next crop. They are not intended for harvest. After being destroyed, and the residues possibly buried, the organic matter decomposes to provide minerals, particularly nitrogen, for the following crop to uptake. Cover crops often include legumes.
The following green-fertiliser crops can be sown in autumn to be ready for the following spring: vetch, trefoil, rye, mustard, phacelia, broad bean, buckwheat, ryegrass.
The following crops can be planted in early spring, before the May crops: vetch, pea, clover, lupin, fenugreek, trefoil, phacelia, alfalfa. Mixtures are also possible, such as vetch, peas and oats.
From April to September, if the fields are empty, possible species are: fenugreek, mustard, vulpine, vetch, buckwheat, clover, sweet clover, or mixes such as vetch, peas and oats.
In organic farming, perennial cover crops such as alfalfa, ryegrass, sainfoin or red or white clover are used for 2 or 3 consecutive years. They are included in the crop rotation and provide additional agronomic benefits. These crops should be destroyed before flowering, and before they reach a mature, woody stage, which could complicate decomposition and the release of nitrogen.
Cover crops to protect the soil
Intermediate crops used as cover crops are sown to protect the soil. They are planted to avoid leaving the soil bare between two crops or when the cash crop has not yet developed. They are sown either after harvest, or in the existing cover crop. They are destroyed before they start competing with the annual crop. The aim is to avoid erosion. That’s why it is one of the key practices of soil conservation agriculture. Cover crops improve the quality of the soil and biodiversity; two essential factors in the farming system. Rainwater also infiltrates more easily with a cover crop.
Plants are chosen for their root systems. A mix of plants is often used to get the benefit from several rooting systems which grow to different depths. Root exudates allow bio-organisms in the soil to multiply. The roots also penetrate the more compact or dense layers so that water can seep deep down. Crucifers such as radish are a good example.
Intermediate crops can have several purposes
Intermediate crops can fulfil various objectives. It is not unusual for a nitrate-fixing intermediate crop to be used as green manure. It just has to be destroyed and left on the field. A catch crop can also be a nitrate-fixing crop while it is growing. It can’t be used as green manure, however, as it is harvested. In all cases, intermediate crops are used as cover crops, because they naturally cover the soil when they grow, providing all of the advantages that that entails.