Glossary > Feed Ingredients > RAPESEED

The cultivated varieties of rape are mainly varieties of B. napus and B. campestris. Both may be grown as annuals or biennials, depending on the variety and time of sowing. The plants are from 0.6 to 1 m high and have thick succulent stems and leaves, although the leaves of B. campestris are thinner, less succulent and greener or less glaucous. The seed contains about 40% semi-drying oil. The oil is used as salad and cooking oil or in the manufacture of margarine.

Rapeseed oilcake contains compounds that are potentially toxic to certain age groups or classes of livestock and poultry, but experiments in recent years have indicated numerous ways in which the meal can be used without detriment to animals. The toxic factors derive in part from glucosides, which yield toxic substances when hydrolysed enzymatically. One of these substances inhibits the uptake of iodine by the thyroid and thus causes goitre. Rapeseed may also be contaminated with mustard seeds so that the press cake will contain residual mustard oil that may be harmful. B. campestris meals contain a considerably smaller amount of the glucoside than B. napus meals.

One detoxification method involves moistening the ground seed and allowing the enzyme to hydrolyse the glucosides. After the oil has been extracted, the cake is steamed, This treatment removes most toxic factors, but the substance causing goitre remains in the cake. Other methods involve extraction of the toxic factors from the cake with hot water or ethyl alcohol. The safest method of avoiding the total effect of the toxic factors is to limit use of the oilcake. An almost glucoside-free variety has been developed (Brassica napus L. cv. Bronowski); the growth responses to the oilcake of this variety are reported to be equal to the results from soybean oilcake. The content of toxic constituents differs greatly between varieties and may also depend on the processing method.

Feeding trials with cattle and sheep have shown that ruminants are less susceptible than other classes of livestock to the toxic effects of rapeseed meal. Adult cattle can be given from 1 to 1.5 kg a day without detrimental effects on feed consumption, growth or milk flavour. Young or pregnant animals should be given less.

Rapeseed meal is not very palatable to sheep and therefore should not exceed 20% of their total ration.

Solvent-extracted meal can constitute up to 4% of the total ration for young pigs (up to 20 kg) and up to 10% of the total ration for market pigs (20-90 kg). It should not be used in rations for breeding stock during reproduction and lactation.

Rapeseed meal should not be used in starter rations for poultry, but it may be used to fatten birds or layers, for which the maximum desirable level is about 10% of the total ration. It should be noted that oilcakes produced by the expeller process may be low in lysine. Poultry fed rapeseed oil meal are likely to have enlarged thyroid glands; however, this condition will usually be of no economic importance if a stabilized iodine is incorporated in the diet. Some strains of layers may produce eggs with a fishy taste when they are fed rapeseed meal.