2.0 Feeding Canola Meal to Pigs
Canola meal substitution for soybean meal in pig diets is a very effective way to lower diet costs, while maintaining the nutrient density desired for today's pig. Studies have shown that complete replacement of CM for SBM yields unsatisfactory results in younger animals, although, full substitution is possible in finisher pigs. As yet, no single component or fraction of CM has been conclusively identified as responsible for these observations, rather it is considered the result of many minor factors working in concert.
a. Effects on Market Pig Performance
Impaired performance of pigs fed rapeseed has often been attributed to glucosinolate induced goitrogenic and hepatoxic responses, including increased thyroid, liver and kidney weights, interference with iodine uptake and decreased levels of T4 and T3. Although these effects have been well documented they may not account entirely for poorer pig performance. There is mounting evidence to suggest that in spite of significantly increased thyroid (up to 55% higher) and liver (16%) weights and a compromised iodine uptake no negative effects on metabolism or performance are noted (Chabiera et al. 1994 {1668}; Thomke 1984 {1908}). Recent studies taking a detailed look at morphological, functional and histological changes within the liver, thyroid and blood have indicated that even in high GL rations performance did not differ between pigs fed 0 or 00 rapeseed presscake meal (RPM) or a SBM control provided normal circulating T3 concentrations were maintained. In diets containing up to 150g kg-1 RPM pigs were able to compensate with enhanced T3 secretion and extrathyroidal conversion of T4 to T3. However compensation seemed to reach its limit at a 300g kg-1 inclusion level of rapeseed presscake meal (0-RPM).
If T3 levels are not compromised the limiting factor to growth becomes feed intake (Spiegel et al. 1993 {1729}). This is supported by the findings of Busato et al. (1991 {1788}) where T3 and free T3 levels remained the same in pigs fed diets containing either 0, 00 RSM or SBM. Insignificant differences were found in ADG or F/G for pigs with identical (restricted) feed intakes. Surprisingly, performance tended to be better for the 0-RSM fed pigs than the 00-fed pigs, suggesting the presence of growth inhibiting substances other than GL and goitrin. Comparing diets containing 250g kg-1 RSM of either 7 or 24 mol/g, Fandrejewski et al. (1994 {1672}) found no significant differences in performance or carcass composition of pigs (30-60kg) with equivalent ME and lysine intakes. They suggested that significant differences in pig performance should not be expected with GL intakes of <10.9mol/d provided that diets were balanced for E, CP and AA. In comparison to a SBM control, Speigel et al. (1993 {1718}) reported length of fattening period increased above a goitrin intake of 4.6mmol/d. Since feed conversions did not change it is assumed that lower average daily gain was the result of decreased feed intake.
Thyroid gland weight in pigs (25-100kg) did not differ between SBM (6.66g) and RSM (7.0g) diets (GL level 2.9mol/g). Intake of GL was < 1.3 mol/g. In contrast to similar studies, FI (feed intake) was significantly higher for the RSM diet and overall ADG was not significantly different during either the grower or finisher periods. In this experiment feed intake was restricted to 90% of ad libitum, in accordance with standard Finnish feeding practices. This restriction may mitigate the effects of fibre in RSM which is often blamed for poor performance or alternatively, the low level of GL was insufficient to impair feed intake (Siljander-Rasi et al.1996 {1612}).
These findings may explain the non-existent or marginal
advantage seen when feeding very low glucosinolate (VLGL) CM.
Performance of grower-finisher pigs fed very low glucosinolate
canola meal (<2.0mol g-1) with diets equal in CP
and lysine was intermediate between those fed SBM and traditional
CM, indicating marginal improvement of VLGL over regular CM (10-15mol
g-1). Average daily gains, feed intakes and feed conversions
were consistently highest for the SBM fed pigs in the grower phase
(23-57) kg, whereas there were no significant differences noted
in the finishing phase (57-100kg). During the grower stage the
plasma T3 levels for pigs given VLGL were lower than
for SBM (P<0.05), but not during the finishing stage. T4
levels were equal suggesting that DE and fibre levels were still
the major limitations (Bell et al. 1991 {1798}). Similarly,
Chabiera et al. (1994 {1668}) found that energy digestibility
was 12-15% lower for RSM diets as compared to a SBM diet in finishing
pigs (60-110 kg). However, when RSM was supplemented with energy
and lysine, no effect was seen on ADG, body composition or E content.
In this study an insignificant decline in T3 and T4
levels was noted. The fact that finisher pigs are unaffected
has been observed for some time and in some instances compensatory
gain during the finisher period was sufficient to reverse the
poorer performance during the grower period resulting in no significant
differences between CM and SBM fed pigs.
b. Effects on Pig Reproductive and Prenatal Performance
Response of gestating and lactating sows to glucosinolate intake is minimal and similar to that of market pigs. However, fetuses are particularly sensitive to GL metabolites. Pregnant gilts counteracted the antithyroid effects of GL byproducts from a diet containing up to 180g kg-1 LG RSM through thyroid hypertrophy and increased thyroid activity. Sexual development of gilts was unaffected (Thomke 1984 {1908}) although weaning-to-estrus increased when gilts were given 180g kg-1 LGL or 120g kg-1 HGL CM. Farrowing interval and longevity in sows were not affected by 200g kg-1 VLGL CM. Embryonic survival was generally unaffected, although results indicated variation in sensitivity among gilts and their fetuses. This may reflect reactivity to specific GL byproducts since the aliphatic GL are quantitatively lower in LGL than HGL rapeseed but indole GL are similar (Etienne and Dourmad 1994 {1666}).
The uptake of goitrogenic products by fetuses results from GL that cross the placental barrier causing hypertrophy of the thyroid and iodine deficiency symptoms in fetuses. Plasma T3 and T4 of fetal piglets and liver monodeiodination, were lowered when HGL RSM was fed to gestating sows. Varying degrees of edema, goitre and sparser hair covering were other indications of neonatal hypothyroidism. Since T3 and T4 hormones are critical components of piglet thermoregulation after birth, a concern was that the RSM piglets were more susceptible to the cold stress (Berthon et al. 1993 {1728}).
Piglet thyroid function is affected by GL in the
sow's diet, as plasma T4 was significantly reduced
in 3 week old piglets when the lactation diet contained 200g kg-1
LGL or HGL RSM. Thyroid hypertrophy in these piglets persisted
even when RSM was removed from the diet of the lactating sow.
Plasma iodine levels declined, possibly due to GL byproducts
competiting with iodine during transfer of iodine to the milk.
However upon RSM removal from the lactation diet, piglet T3
levels surged to that of the control pigs (Berthon et
al.1993 {1728}) resulting in weaning weights being unaffected
by RSM inclusion, provided N and energy in milk remained the same.
Postnatal mortality was somewhat higher in diets with RSM, to
be expected with a malfunctioning thyroid and poorer thermoregulation
at birth However, all of these consequences are negated if GL
intake was below 2 mol/g of diet or 5mM Gl/day (Etienne and Dourmad
1994 {1666}).
c. Elimination of Glucosinolate Effects
Various approaches have been undertaken to lessen or eliminate the effects of GL, the most obvious being iodine supplementation with either free iron or iodinated casein. Despite improved iodine status, antithyroid effects were only reduced but not diminished. Serum T4 levels did not respond consistantly to increased dietary iodine in agreement with only marginal improvement in growth performance with VLGL rations (Bell et al. 1988 {1850}; Schone et al. 1993 {1713}; Schone et al. 1994 {1674}).
Copper sulphate solution inactivates isothiocyanates
and oxazolidinethiones in RSM. Although animal performance with
copper supplementation matched that of the control, goitrogenic
responses were still observed when copper-treated RSM was fed
without iodine supplementation (Schone et al. 1988 {1859};
Ludke and Schone 1988 {1858}). Endogenous thiosulfate is necessary
for the transformation of toxic nitriles to less toxic thiocyanate.
However, sodium thiosulphate addition to 00 RSM was not successful
in synthesis of the less toxic thiocyanates from nitriles derived
from GL, possibly nitrile formation was reduced during RSM heating
or maybe a better sulfur donor (eg. cysteine) is necessary (Boltshauser
et al. 1993 {1701}).