Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu
Rapeseed (Brassica napus) has been planted on many acres in Delmarva as on overwintered cover crop, particularly for vegetable rotations. It has a deep root system, is good at reducing surface compaction, scavenges significant soil nitrogen, and suppresses weeds.
In addition, rapeseed is in the mustard family and produce chemicals called glucosinolates in plant tissue (roots and foliage). These glucosinolates are released from plant tissue when it is cut or chopped and then are further broken down by enzymes to form chemicals that behave like fumigants. The most common of these breakdown products are isothiocyanates. These are the same chemicals that are released from metam-sodium and metam-potassium, commonly used as chemical fumigants.
While rapeseed has shown some promise as a biofumigant, results in Delaware have been inconsistent, often with minimal benefits. It is important to note that success with biofumigant crops depends on a number of factors. The following are some suggestions to achieve the best results:
- Produce as much biomass of the biofumigant rapeseed crop as possible. This requires that you have a good stand, fertility, and sufficient growing time. The more biomass that is produced and that is incorporated, the more chemical is released. However, as plants mature, the will reach a point where levels of these active chemicals will decline and you should not let the plants go past full flower.
- Plant material must be thoroughly damaged so that enzymes can convert glucosinolates into isothiocynates. This means that you need to chop the material as much as possible and work it into the soil as quickly as possible so as to not lose the active compounds to the air. A delay of several hours can cause significant reductions in biofumigant activity. The finer the chop, the more biofumigant is released. A flail mower is the best tool for achieving this.
- The chopped material should be incorporated immediately after chopping. Tillage operations should be performed immediately behind the flail mower.
- The chopped material should be incorporated as thoroughly as practical to release the biofumigant chemical throughout the root zone of the area that is to be later planted to vegetables. Poor distribution of the biofumigant crop pieces in the soil will lead to reduced effectiveness. A tractor mounted rotary tiller or power spader is the best tool for this.
- Sealing with water (irrigating) after incorporation will improve the efficacy by reducing gas loss from the decomposing rapeseed (the active fumigant released). Soil conditions should not be overly dry or excessively wet. Packing the soil will also help this sealing process.
A major limitation to the use of rapeseed as a biofumigant crop use is the fact that you cannot deliver high enough levels of the active chemicals to do a complete fumigation job and the biofumigation effectiveness is also limited by the depth of incorporation. However, you are adding organic matter and do get the benefits associated with that addition.
The bottom line: the use of rapeseed as a biofumigant crop can suppress soil borne pests but should not be considered replacement for chemical fumigants, nor is it a substitute for adequate rotations. However, using rapeseed in combination with disease tolerant vegetable varieties can allow for a greater chance of success in tight rotation situations.