The energy value of canola meal is determined by the relative proportion and availability of carbohydrates and residual oil. Available carbohydrate found in the cotyledons consists of sucrose, stachyose, D-fructose and D-glucose, with only trace amounts of starch (Blair and Reichert 1984 {1903}). During processing, 'gums' that contain glycolipids, phospholipids and variable amounts of triglycerides, sterols and fatty acids from the canola crush are added back to canola meal, which contribute to the overall fat level (Thacker and Aherne 1984 {1902}). A high level of pectins, cellulose, arabinan and arabinogalactan (collectively referred to as non-starch polysaccharides or fibre) is concentrated in the hull, which constitutes 165-187g kg-1 the canola seed (Mitaru et al. 1984 {1904}).
In comparison with SBM, Schone et al. (1992{1713})
found organic matter digestibility lower for RSM and the excreted
organic matter to be two-thirds lignin. In this trial, NE of
RSM was 8.3MJ/kg DM (dry matter) as compared to 9.8MJ/kg DM for
SBM. Energy digestibility values are found in Table 3.
| Table 3 Apparent ileal digestibilities of energy of canola meal | ||
| Screw pressed RSM | Screw pressed RSM NH3 | |
| Methodology | Ileal Caecal Anastomosis | Ileal Caecal Anastomosis |
| 40 kg | 40 kg | |
| Energy | 46.3 | 53.3 |
| Reference | {1694} | {1694} |
| Table | 4 | 4 |
Fibre has repeatedly been implicated as the factor responsible for the lower digestibility of canola meal as it is known to affect ingestion, secretion, absorption and transit time of nutrients. Imbeah and Sauer (1991 {1804}) reported that CM diets passed faster through the small intestine than SBM diets (P<0.05). Gdala et al. (1997 {1267}) found fat, raffinose oligosaccharides and low molecular weight sugars (glucose, fructose, sucrose, raffinose, stachyose) to be equally digested from a cereal based diet containing 120g kg-1 rapeseed cake or SBM. However, the non-starch polysaccharides (NSP) were almost quantitatively recovered in the ileal digesta with digestibility values ranging only from 1 - 8.2%. Pectic polysaccharides were degraded at the caecum and the proximal colon presumably because of the ability of pectin to enhance the attachment of microbes, enabling high microbial activity. Moreover, the high water holding capacity of these types of cell walls causes swelling, which increases in the surface area available to the microflora (Gdala et al. 1997 {1267}) Although there appears to be significant fermentation of CM fibre to volatile fatty acids in the hindgut, the contribution to basal metabolism or net energy of maintenance is only 15%. The contribution may be more in breeding boars and gestating sows, as these pigs have greater intestinal volume and slower passage with a lower feeding level (Dierick et al. 1989 {2056}).
The potential contribution of fibre to the energy balance of pigs depends upon many factors, including type and source of fibre, treatment, age, stage, liveweight, feeding level, physiological state and environment of the animal. For canola meal, fibre composition rather than quantity may be limiting digestion. Treatment to improve canola quality has involved dehulling, ammoniation, hydrothermal processing and enzymatic supplementation. Success has been variable. Bourdon and Aumaitre (1990 {1819}) found dehulled, solvent extracted 00 RSM provided the highest DE and apparent N digestibility as compared with expeller-processed 0-RSM. Dehulling removed 40% of the crude fibre or 50% of the ADF, however, hemicellulose was not affected, reflecting its location in non-hull constituents. Mitaru et al. 1984 {1904}) reported yellow hulled canola (B. rapa) contains only 40% of the fibre of brown hulled canola (B. napus). Yet despite lower lignin, the degree of lignification appears similar as each successive increase in the percentage of canola hulls (brown or yellow) added to a grower hog diet significantly depressed energy, dry matter and ether extract digestibility (Bell and Shires 1982 {1929}).
Yin et al. 1993 ({1694}) found higher digestibilities
of energy, crude protein and amino acids in ammoniated Chinese
rapeseed meal than in traditional screw pressed meal due to changes
in the physical or chemical structure of the cell wall constituents
rather than the removal of toxic factors. Rapeseed meal subjected
to either a hydrothermal process (60oC, 30% moisture,
30 minutes) or treated with an enzyme cocktail (cellulase, protease,
xylanase and cellobiase) resulted in only slightly higher (P>0.05)
ADF and NDF digestibilities. Unfortunately these treatments also
slightly reduced lysine content and availability (Nasi 1991 {1786}).
In another trial, an enzyme mixture containing xylanase, -galatosidase
and protease significantly increased the digestibility of xylose,
mannose and arabinose. No effect of enzyme supplementation on
the ileal digestibility of the other NSP constituents was found
(Gdala et al. 1997 {1267}).