Crops Oilseed rape (Brassica napus ssp. oleifera)

Young plant

Both annual (spring-sown) and biennial (winter-sown) types of Brassica napus ssp. oleifera are cultivated. The plant has a strong taproot and an erect habit; some kinds may attain a height of 2 m but most modern varieties are shorter at maturity. The leaves are lobed, with prominent midribs, and are arranged alternately.

The plant in flower

Flowers, usually bright yellow, are produced in loose spikes at the ends of the main stem and branches, the lowest buds of each group opening first.

Crop begining to ripen

Ripening seedpods

Pod split to show ripe seeds

The seeds are formed in thin pointed pods, and ripen from green to shiny black; seeds from lower pods may be shed before those at the top are ripe. A fine, firm seedbed is necessary for the small seeds to germinate evenly; the objective is to establish up to 100 plants per square metre for the winter crop and rather more for the spring types, which branch less freely. Seed rates of 6-8 kg per ha are normal, winter varieties being sown early enough to be well-established before the onset of cold weather. The crop is very responsive to spring applications of nitrogen fertiliser: maintenance dressings of phosphate, potash and sulphur are likely to be required. A range of herbicides, fungicides and insecticides is available for use as necessary Winter crops can be harvested from late July, spring ones usually ripening during September; yields average some 3 t per ha for winter crops and 2.1 t per ha for spring varieties, but may be considerably higher. Biodiesel production from oilseed rape has been shown to be energetically efficient at all but low yield levels but is not commercially viable at present without tax concessions: where these apply, for example in France, areas of oilseed rape are grown specifically for this purpose. It may be possible for vegetable oils to replace mineral oils completely in some environmentally-sensitive applications, but the total area of oilseed rape which can be grown for all purposes is limited by rotational considerations. It is now possible to alter the fatty-acid profile of the oil by advanced breeding methods; several varieties are under development, with high levels of specific fatty acids for industrial processes, but considerable care will be necessary to ensure that cross-contamination between all types is minimised.