DIETARY-FIBERS IN OILSEED RAPE - PHYSIOLOGICAL AND ANTINUTRITIONAL EFFECTS IN RATS OF ISOLATED IDF AND SDF ADDED TO A STANDARD DIET

Insoluble dietary fibres (IDF) and soluble dietary fibres (SDF) have been isolated from two double low rapeseed varieties, a partly yellow seeded and a normal brown seeded spring variety. Both types of fibres were also isolated from hulls of the brown seeded variety as well as from the extracted meal (NRM) produced by an aqueous enzymatic processing technique. The isolated fibre fractions were given to rats in balance experiments of two different levels each: 2 and 6% IDF vs 0.8 and 2.4% SDF in DM. The fibres replaced dietary maize starch. Feed efficiency, true protein digestibility (TD), biological value (BV), net protein utilization (NPU) and digestibility of energy (DE) were used as biological criteria. The total amounts of dietary fibres (DF) were at the same level for the partly yellow seeded and the normal brown seeded varieties. However, the ratio between IDF and SDF for the two varieties were different. As expected the DF level, especially IDF, was much higher in hulls compared to dehulled rapeseed meal, while the level herein was close to the level in the seeds. DF from the different sources exhibited differences in their properties. The main reason to the negative influence of DF on protein (N x 6.25) digestibility is obviously due to a low availability of fibre associated nitrogen. Furthermore, the fermentable part (SDF) of DF will stimulate microbial growth and thus increase microbial N excretion. The present study on rats could verify investigations on piglets with a negative relationship between IDF from NRM and true protein digestibility. SDF had also effect on the intestine and the consistency of the digesta was much more slimy with SDF in the diet. Increased flatus caused by SDF was observed which could not be seen when IDF was fed. The results of the present study demonstrate that the DF level in rapeseed meal is high and of low digestibility. This is the main reason for the low energy digestibility and a relatively low protein digestibility, as a relatively big proportion of N in rapeseed meal is associated with the fibre fraction.