Although canola meal would not be expected to support growth without supplementation it does contribute significantly to the mineral requirement of pigs. Central to mineral availability is the presence of phytic acid, the main storage form of phosphorus in oilseeds. Canola meal contains about 1.22% total phosphorus of which 50 - 80% is phytate bound, nevertheless it provides about twice as much non-phytate P as SBM (Bell 1993 {1714}). Absorption of phytate from RSM prior to the colon results from phosphatase degradation (Sandberg et al. 1993{1721}) as inherent phytase activity is destroyed by heating during oil extraction (Nasi et al. 1995 {1620}). Phosphorous digestibility is significantly higher in full fat rapeseed than RSM [42 vs.(versus) 24%], which was attributed to complexing in the extracted meal (Rodehutscord et al. 1997 {1595}).
Phytic acid in CM affects not only the availability of P and supplemental P (digestibility was not improved by the addition of 20% inorganic P) but appears to interfere with the absorption of other minerals (Nasi et al. 1995 {1620}). Serum, liver and pancreatic zinc were lower for CM-fed pigs than SBM-fed pigs even with zinc supplementation, indicating poorer availability in CM than SBM diets (Keith and Bell 1987 {1872}; Bell et al. 1988 {1857}). Larsen and Sandstrom (1993 {1703}) found Ca and zinc to be the most poorly absorbed minerals in CM. However, over a one-month trial period Zinc from RSM alone was sufficient to maintain growth in growing pigs without dietary addition. Ileal and fecal absorption of Ca, P, Mg, Fe, Zinc, Cu and Mn from rapeseed meal are indicated in Table 4.
Improvement of phosphorus availability has been attempted
in several ways. Calcium addition did not affect P absorption
in pigs, with the possible exception of the colon where absorption
of P is insignificant. (Sandberg et al. 1993{1721}).
Dietary phytase addition significantly increased P digestibility
from 36 - 45% (Nasi et al. 1995 {1620}), while Rodehutscord
et al. (1997 {1595}) reported a 42-66% improvement in digestibility
for full fat seeds and from 24-73% for RSM. Phytase supplementation
lead to significant improvements in ash and ether extract digestibilities,
along with Ca:digestible P ratio in CM diets to near optimal (3.52:1).
This improvement was lower than that observed with SBM and did
not translate into improved pig gains. In another study, phytase
treatment improved absorption of Ca and Mg and slightly enhanced
crude protein digestibility. Although the role of the other enzymes
in the preparation cannot be discounted completely, there is documentation
that phytic acid binds digestive enzymes. No response was seen
in dietary antinutritional factors to phytase treatment (Nasi
et al. 1995 {1620}).
| Table 4 Apparent ileal and fecal digestibilities of minerals of canola meal | ||
| Ileal | Fecal | |
| RSM | RSM | |
| Methodology | T-C | |
| Pig weight | 30 kg | |
| Ca | 36.5 | 13.1 |
| P | 44.2 | 34.1 |
| Mg | 36.7 | 21.8 |
| Fe | 45.2 | 22.3 |
| Zinc | 39.7 | 14.2 |
| Cu | 55.8 | 34.9 |
| Mn | 37.5 | 11.1 |
| Reference | {1703} | {1703} |
| Table | 4 | 5 |