Oilseed Rape Crops

Introduction

Oilseed rape (Brassica napus) is probably the most eye-catching and commonly identified crop grown in the UK today. This is due to a combination of its height (1-2m) and its “flashy” yellow flower colour.  It is grown specifically for the oil contained within the seed (mainly used in commercial food processing, cooking oils and margarines), however the meal left after oil extraction is utilised as a high protein animal feed. Both parts of the seed contain some unwanted compounds; erucic acid and glucosinolates.  These compounds were found to be harmful (immunotoxic) to both humans and animals. However, by 1974 plant breeders had largely succeeded in reducing them to low levels, illustrated by the ‘double low’ varieties available today that contain low levels of erucic acid (less than 2% of measured fatty acid) and glucosinolate (less than 35mmol/kg in the meal.

With the exception of the HEAR (high erucic acid rape) and specialist oil varieties, all varieties cultivated in the UK are double low.

Throughout most of Western Europe, milder winters allow the growth of winter varieties of rape, which are sown in late summer to early autumn. These winter varieties require a period of cold (‘vernalisation’) to flower without delay the following year. Plants sown before the winter can take advantage of a longer growing season, and so usually produce a hgher yield than spring sown crops. Thus seed sown in late August-September develops to produce plants which flower in April/May of the following year and are harvested in July. Spring varieties predominate in more northern latitudes. In the UK, spring oilseed rape is sown in March/April and harvested in August/September.

The average yield of oilseed rape depends on climate, soil fertility, intensity of production, fertiliser input, species and variety of rape crop. The average yields thus show a wide range from country to country. The average world yield has increased over 50% from nearly 0.9 t/ha to 1.5 t/ha during the last 20 years.  With the exception of the African continent and the former USSR in all other countries a significant yield increase has been observed particularly over the last decade. The high yields found in European countries (for example 3.5 t/ha in Germany) are generally based on high production standards together with favorable climatic conditions allowing the use of high-yielding winter types of oilseed rape together with a high fertiliser input. In Canada and parts of Asia, yields are lower because of low fertiliser inputs and of hard winters permitting cultivation of only spring type varieties. In other countries, yields might be increased using improved varieties, irrigation and advanced farming technology combined with optimum fertiliser use.

Whereas in Europe and the United States the winter form of Brassica napus (oilseed rape) predominates, in Canada it is mainly the spring forms of Brassica napus and Brassica rapa that are grown because of the insufficient resistance of the winter types to the very low temperatures of the Canadian winter. Oilseed rape on the Indian subcontinent is mostly cultivated as Brassica juncea (mustard) and Brassica rapa where it is grown as different tree types called toria, yellow and brown sarson. In China, Brassica rapa and Brassica juncea are often replaced by special varieties of Brassica napus.

In nearly all species, the seeds are more or less spherical with a diameter between 1.2-2.8mm and a weight of 1.5-7 mg. The color of the seeds is mostly black, but sometimes red-brown or yellow.

The Brassica oilseeds are harvested by combine harvester in the UK.  In many cases the crop is directly combined after it has been left to senesce naturally or after a desiccant has been applied. These methods are used on about 65% of the UK crop. An alternative method is to cut the crop and to leave the cut stems and pods in swaths to ripen in the field before combining.

The following is a brief description of current oilseed rape varieties cultivated in the UK: -

Winter oilseed rape varieties

Various varieties available (refer to list available from SAC)

A very high yielding variety with an exceptionally high oil content and low glucosinolate content. Late maturing with very good stem stiffness.

Winter high erucic acid rape(HEAR) varieties

Various varieties available (refer to list available from SAC)

There are a number of contracts available for HEAR varieties. The oil of such varieties contains a high content of erucic acid (around 50-55%) which has several industrial uses including use a biofuel (biodiesel). As HEAR varieties yield a non-food product, they may be grown on set-aside, providing a contract is obtained.

Other winter specialist oil types

Various varieties available (refer to list available from SAC)

A high oleic/low linolenic variety. Relatively low yielding, but the oil has a high value. Good light leaf spot resistance.

Spring swede rape

Various varieties available (refer to list available from SAC)

A very high yielding variety with stiff straw and early maturity.

Insufficient data is available on spring rape to support a UK Recommended List for the crop. Northern growers should be aware that later maturing varieties may present unacceptable harvest risks in late seasons.

Spring swede high erucic acid rapeseed

Various varieties available (refer to list available from SAC)

A high erucic, low glucosinolate variety. Very short strawed with moderate maturity.

Other spring swede specialist oil types

Various varieties available (refer to list available from SAC)

A high oleic/low linolenic variety. Early maturing with average height and good stem stiffness.

Spring turnip rape

Various varieties available (refer to list available from SAC)

A high yielding variety of average height with very high oil content and extremely good stem stiffness.

Spring turnip rape is a small seeded, vigorously growing spring sown rape, which matures at least two weeks earlier than most of the more widely grown spring swede rape types. This can be a major advantage in late seasons. Turnip rape is less sensitive to sowing date than swede rape. It is very resilient to adverse conditions at harvest and has been successfully combined direct without swathing or the application of a desiccant. It appears to be particularly susceptible to pollen beetle. Spring turnip rape varieties generally yield less than spring swede varieties but because of low growing costs the margin may be equivalent.

Author - Armitage; copyright 2007

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