By comparison with soybean meal, the energy value and available nutrients contents of rapeseed meals are much more limited. They also contains some anti-nutritionals factors which limit their use in poultry diets. Protein and amino acids contents are by far lower in rapeseed meal than in soybean meal. But rapeseed meal compares favorably with soybean meal for sulfur-amino acids; those two meals tend to complement each other. However, rapeseed meal is known for its lower amino acids digestibility. The reason is frequently related to processing conditions. Overheating the meal during processing may reduce lysine digestibility, values lower than 80% are mentioned (Newkirk et al., 2003; Anderson-Hafermann et al., 1993). Products resulting from Maillard reaction during processing are responsible for those low values. Tannins might also reduce amino acid digestibility (Khajali et al., 2012).
Crude fiber content in rapeseed meal is by far higher than in soybean meal, ADF, NDF, and lignin which is associated with polyphenols (tannins) are also higher. Those values are a consequence of the very small size of the seed and its very high lipid content. During oil extraction, fibrous hulls are concentrated in the meal, thus decreasing the nutrient content. Fiber content is inversely related to metabolizable energy value. As a consequence, metabolizable energy value of rapeseed meal is 10 to 15% lower than that of soybean meal.
In order to improve the nutrient content of rapeseed meal and metabolizable energy value, various approaches have been undertaken to reduce the fiber and polyphenol contents: selection or dehulling. Selection includes the production of yellow-seeds in which lignin and polyphenols contents are reduced (triple-zero varieties). Dehulling is done before oil extraction, but this process is associated to a loss of oil since some kernels particles are removed with hulls. Selection or dehulling process improve equally the nutritive value of the meals. Some other attempts have been tested for improving nutrient availability or reducing encapsulating effect of cell wall in rapeseed meal (Kozlowski et al., 2014), for example the addition of enzymes: proteases, xylanases, phytases.
Main anti-nutritional factors
In poultry, adverse effects of glucosinolates are pungency, bitterness, anti-thyroid activity and as a consequence a reduction of birds’ production: growth and laying performances. Mortality can be increased, especially in laying hens due to hemorrhagic liver syndrome (Fenwick, 1982). In modern rapeseed varieties, glucosinolate content has been much reduced (10-12 fold) and toxic effects are reduced. According to recent publications and glucosinolate levels observed in current rapeseed meals, dietary inclusion of rapeseed meal in poultry diets should not exceed 20% in broilers, and 15% in layers, resulting in a glucosinolate dietary content lower than 1.5 µmole/g.
Rapeseed meal encompass 1% sinapine, a choline ester of sinapic acid. Sinapine is associated with fishy taint of yolk from brown-shelled eggs. The fishy odor comes from trimethylamine which accumulates in the yolk since brown layers are genetically unable (a trimethylamine oxidase deficiency) to convert trimethylamine in odorless components. All sources of choline can be transformed in trimethylamine by the micro-organisms in the gastrointestinal tract of the birds. This genetical deficiency has been recently proved and acceptable level of trimethylamine in the yolk has been quantified (Wang et al., 2013). But tainting effect of rapeseed meal seemed to be more efficient than choline one (Ward et al., 2009). As a consequence, incorporation rates of rapeseed meals should be limited in diets fed to sensitive layers. No off-flavor have been detected in the carcass.
No off-flavors in the meats where observed when canola meal was fed to broilers (Salmon, 1984).
Canola seemed to have no effect on performance in broiler when replacing soybean meal (Salmon, 1981); (Leeson, 1987); (Franzoi, 1998), but other research found that lysine supplementation improved performance (Campbell, 1988). Increasing the dietary level of canola meal was decrease gain, feed intake and increase size of thyroid gland (Baidoo, 1986).
Canola meal was found to be a suitable replacement for soybean meal in diets for layer pullets (Nassar, 1985);(Salmon, 1988). Feed consumption and number of eggs production was reduced when Canola meal replace soybean meal (Summers, 1985), egg shell quality (Summers, 1988) and mortality (Roth-Maier, 1988).
When canola meal was used to replace soybean and fish meals in turkey diets, the canola meal was found to perform similarly to soybean meal (gains and feed conversion), but fish meal showed higher performance (Salmon, 1982);(Borcea, 1996).
No off-flavors in the meats where observed when canola meal was fed to turkeys (Larmond, 1983).