2.2 Impact of Canola Meal on Feed Intake
It appears that the main advantage of SBM over CM, even with lysine supplementation, is that SBM-fed pigs are able to maintain higher feed intakes, especially in younger animals. Recent findings suggest that with CM diets hypothyroidism is less important than FI. Removal of glucosinolates by hot water extraction increased feed intake by 18kg pigs furthermore addition of this GL extract to a SBM based diet reduced intake. Sinapine removal with calcium hydroxide had little effect on intake, nor did treatment by cation exchange (Lee et al. 1984 {1913}). Pelleting seemed to improve T3 levels (Bell et al. 1991 {1798}) and result in higher feed intakes. Pelleting may be sufficient to increase the DE content since no effect of added wheat was seen (Bell and Keith 1988 {1849}). Because of indications of lower FI and ADG during early growth, it is generally recommended that canola meal be restricted 90-100g kg-1 in the grower diet (Baidoo and Aherne 1987 {1869}; Baidoo et al. 1984 {1901}; Thomke et al. 1983 {1922}).
One method of assessing feed preference is by allowing animals to choose between alternative feeds. When given a choice between diets containing 225g kg-1 high GL, low GL, SBM or fish meal, pigs consistently chose lower proportions of the diets containing RSM. Total feed intake was lower by 11%, which was not significant in this test. With the other choices, pigs selected feed to meet their protein requirements, decreasing protein intake with increasing body weight. With the RSM diets there was a compromise between minimizing GL intake and achieving a protein intake sufficient for maximum performance. This was especially marked in the early stages, when protein requirements were greatest. Individual pigs did display varying tolerance for GL (Kyriazakis and Emmans 1992 {1910}). Similarly, Gill et al. (1995 {1637}) found free choice intake was lowest for RSM and highest for a 50:50 RSM/SBM mix. Pigs chose approximately 28% of their diet as SBM and only 22% as RSM in the grower stage (30-50 kg). In the finisher stage (50-90 kg) the proportions were equivalent (approximately 20%). Taken over the entire feeding period, proportions showed no significant differences. Although proportions were similar, reduced feed intake over both the grower and finisher stages resulted in reduced growth rate and feed conversion for pigs offered RSM (P<0.05). There was some compensatory gain when RSM fed pigs at 50kg were switched to SBM. Protein intake at this point increased to 37% of the diet. Over the entire period, growth rates were lowest for the RSM pigs due to lower DE and protein intakes, apparently due to aversion of RSM. Mixtures produced better results. Since RSM pigs were lighter there were corresponding differences in carcass weights and P2 fat levels. Pigs apparently discriminate against RSM due to GL, thus avoiding toxic factors at the expense of protein requirements. Initially pigs avoided RSM, then intake approached normal followed by a second aversion to RSM. Initial palatability may have been a factor but later avoidance reflected metabolic disturbance rather than flavour. The best performance was obtained with a diet containing 150g kg-1 of each SBM and RSM. Delaying the introduction of canola meal resulted in higher feed intakes and ADG not unlike those seen on a SBM based diet (Imbeah and Sauer 1991 {1804}).
Response to the addition of flavouring agents varied among flavours. Pigs weighing 7.4-18kg chose a SBM-based diet 2.5 times more than a diet containing 50g kg-1 CM and 7 times more when the CM inclusion level reached 200g kg-1 . Addition of 0.150g kg-1 monosodium glutamate or 100g kg-1 dextrose or 40-50g kg-1 corn oil to CM diets did not improve performance by pigs. However addition of 100g kg-1 dextrose to a 50:50 blend of CM and SBM increased FI and ADG equal to a 100% SBM diet. Commercial feed flavour addition to diets containing CM improved FI, ADG (P<0.01) and FC (P>0.05), but performance was still lower than SBM based diets and declined as CM level increased (Baidoo et al. 1986 {1888}).
Other explanations for decreased palatability invoke
sinapines and tannins or even dolichols found in the lipid fraction
(Danielsen et al. 1993 {1652}). In this latter trial,
rapeseed was dehulled using enzymes and fed to 4-8 week-old (8.6kg)
pigs. As insoluble dietary fibre was increased at the expense
of skim milk powder and fish meal, both true N digestibility (-1.6%)
and DE (-1%) decreased for each percent increase in dehulled RS.
However there was no significant decrease in deposited N, indicating
high protein quality and no significant antinutritional effects.
Feed intake was low but improved considerably with the addition
of a flavouring agent. Daily weight gain was not significantly
affected. They attributed the depressed protein and energy digestibilities
to insoluble dietary fibre. Since the reduced digestibility of
protein and energy in RS was not exclusively a function of the
hulls, insoluble dietary fibre in the dehulled seeds needs to
be examined as well.