Oilseed Rape (Canola)

A brassica oilseed crop

 Quick Index 

Canola is a trade name for the refined oil but is sometimes used to refer to the whole plant.

Oilseed rape is a very useful crop as the seed is typically 42% oil and the meal left after removing the oil is about 42% crude protein. Furthermore the oil is particularly high quality and high in monounsaturates and should logically be a premium product (as it is as "Canola", the trade mark of rape oil produced in parts of North America).

The meal is a useful animal feed.

There are two quality types

  1. Edible oils, used as a food grade oil.
    It is most often encountered as an odourless frying oil low in unsaturated fats but it is also an ingredient in many food products, especially where a "healthy" oil is required.
  2. Industrial (although some edible types are also used industrially), which are NOT edible but have been bred to contain high levels of compounds critical for some industrial processes.
    Even the raw oil, with some adjustment, will run diesel engines (it can be run on its own but apparently doesn't do the engine much good.)
    One of the contributors to UBA has, for the last couple of years used it in the oil tank of his chainsaw. This is a total loss system, on a busy day about 2litres gets thrown off the chain between two operatives. He buys it direct from the local supermarket. (This is not recommended for a saw not in regular use as if left it gums up the system and after a while the saw acquires a green tinge.

The plant

Rape seed is a tiny round black seed and is sown at 3 to 8 kg/Ha (ie at under 1gm/sq m) which germinates rapidly to produce a typical brassica (cabbage family) plant. After a period of establishment it runs to flower producing the characteristic brassy yellow flowers and a faint odour of honey.


It is responible for much of the honey produced in lowland UK, although the honey is so strong it tends to crystallise and so is often blended. The pollen is designed for pollination by bees and is heavy and sticky (as anyone who walks through a field in flower will discover).


The seed pods are about 5mm in diameter, pointed at both ends, and 30 to 80mm long. Harvesting may be preceeded by dessication (spraying to kill the plant evenly) or swathing/windrowing (cutting the plant and leaving it on the stubble to dry) or direct cutting with a combine harvester. Attachments to the front of the combine are common for both picking up the windrow or direct cutting (which often comprises a long extension between the knife and the combine intake). The pods are easily shattered and the seeds lost and the small ballbearing-like seeds will run like water through any tiny holes.

Several species of bird, including the Linnet (a farmland bird in decline), are able to take advantage of rape crops for food. Others, including Sedge Warbler and Reed Bunting, regularly use it for nesting. Their breeding attempts often last beyond the desication or cutting point and rsearch has shown that the bird's nests almost always survive the spraying process but most are destroyed by cutting.


After drying and cleaning, and often some storage, the seed is shipped to a crushing mill. Uniquely the seed is sold subject to standard bonuses and allowancies for oil, admixture and moisture and so the quality (or lack of it) is passed to the producer. The straw (known as haulm) is usually chopped and ploughed in.

To avoid the oil being green in colour when crushed there are strict limits on the amount of green in the seed (perversely indicated by a reddish tinge to the seedcoat) and loads of seed for crushing will be rejected if they exceed these limits.


Oilseed rape comes in two drilling date types:

  1. Winter: Sown in Aug/Sep the plant overwinters and flowers in the spring and comes to harvest in July/Aug (drilled earlier and harvested later in Scotland). This is the commonest form and is the highest yielding, producing between 1.8 and 5T/Ha (typically 3.7T/Ha) of seed.
  2. Spring: Sown in March/April and flowering later than winter types it comes to harvest in Aug/Sept. It is notoriously variable in yield with yields from under 1T/Ha to 4T/Ha (typically under 2.5T/Ha).

There are also some other strains used more-or-less identically, such as Turnip Rapes.


Historically rape oils were poor quality, containing high levels of eructic acid and the meal contained high levels of glucosinolates. After WW2 breeders reduced the level of eructic acid to very low levels so the oil became a quality oil for human consumption and in the 80's the glucosinolates were also bred out so the meal became an excellent protein source for animal feed. Glucosinolates break down into mustard oil which gives mustard and other brassicas (including leaf brassicas) their characteristic bitter/hot flavour. It is very toxic and is one reason why brassicas have very few specialised pests.


Pests of rape are largely specialists. Flea beetle can devastate seedlings and cabbage stem beetle will eat the leaves and lay eggs in the stem which the maggots consume, killing the plant. They are easily controlled by an autumn spray of insecticide. Pollen beetle can cause damage by destroying flowers before they are open but are easily controlled by a late winter insecticide, which is often not required. Cabbage seed weevil will bore into pods and lay eggs, but the damage is small and usually confined to the headlands, however the holes it leaves are used by pod midge to lay their eggs in the pod where their maggots can consume the entire pod. These are controlled by specially approved insecticides at early flower, but again are often not required or just the headland needs treatment. There are a number of serious fungal disease of rape that can be controlled by fungicides to a useful extent although many have to be applied before symptoms are visible.


Storage of rape requires more care than cereals. Firstly the small ballbearing-like seeds make it unsafe to walk on and people have been drowned trying to walk on deep bins (linseed is even worse) as in some circumstances it will flow almost like water. It is hard to blow air through a deep heap (or even a shallow layer in a drier) to cool it for long-term storage so heaps are often shallow or require high pressure blowing fans.


Flowering rape is frequently blamed for inducing hayfever and asthma, for example:

"The Secretariat commented that DH received a regular round of complaints each year about effects attributed to pollen from oil seed rape, ie, eye and upper respiratory tract irritation in some people who live near rape fields. (It was noted later in the discussion that proven allergy to oil seed rape pollen was relatively rare - the symptoms might also be due to direct irritation)."
From: http://www.doh.gov.uk/comeap/nt18feb00.htm

However, as rape pollen is large and sticky it doesn't blow far in any quantity. Much of the time it is probably blamed in error because it is showy. Less obvious pollen producing hedgerow plants and trees that flower at the same time may be the real culprits.

With contributions from: Chris Mead
Mary Fisher, Simon Avery,
Andrew Heggie & Derek Moody