Oil rapeseeds used to have a high content of erucic acid and glucosinolates that are of concern for animal and human health, but those problems have been eliminated or largely reduced through traditional genetic selection since the 1970s (Przybylski et al., 2005; Pinochet et al., 2012; Snowdon, 2006).
Erucic acid
Erucic acid (cis 13-docosenoic acid, 22:1n-9) used to be a major component of rapeseed oil (up to 50% in earlier cultivars). Erucic acid causes a bitter taste and has adverse effects on heart health and animal performance that were demonstrated in early studies with rats, ducklings, poultry, and pigs. This concern led to the development of low-erucic cultivars ("0") in the 1970s, and by 2005 most rapeseed oil produced worldwide contained less than 2% of erucic acid (Snowdon, 2006; Przybylski et al., 2005). The content in erucic acid of foods is now regulated: in Canada, canola oil must contain less than 2% erucic acid (CCC, 2014); in the EU the fat content of foodstuffs must contain less than 5% erucic acid (less than 1% for infant formulae) (EFSA, 2016).
However, the scientific consensus on erucic acid has softened since the 1970s. An extensive literature review (EFSA, 2016) found that while a high intake of erucic acid does result in myocardial lipidosis in pigs and rats, this lipidosis regresses after animals return to a low-fat diet without erucic acid. For pigs, a NOAEL (no observed adverse effect level) of 700 mg/kg BW per day was identified. For dairy cows, a reduction in feed intake and milk yield by dairy cows was reported at an intake of 0.4 g erucic acid/kg BW per day from rapeseed meal. In fish, rabbits and horses, no conclusion could be drawn due to the limited studies available. The most severe effects were observed in poultry: high-erucic diets resulted in growth retardation, cardiac lipidosis, and other adverse effects on health and production. A LOAEL (lowest observed adverse effect level) of 0.02 g/kg BW per day was defined, and a health risk exists in poultry when maximum inclusion rates are applied. It is important to note that, in all livestock species, it is difficult to distinguish the adverse effects of erucic acid from those of other dietary factors such as glucosinolates. Still, erucic acid remains a concern, as dietary erucic acid is transferred to products of animal origin and a dose-related increase in erucic acid in food of animal origin has been shown. As of 2019, the question of the toxicity of erucic acid for livestock remains elusive.
Since the late 1990s, high-erucic acid rapeseed (HEAR) cultivars are grown for niche industrial purposes to produce erucamide (an additive in polyethylene and polypropylene manufacture and a surfactant) and behenic acid (Snowdon, 2006; Tonin, 2018).
Glucosinolates
Glucosinolates are a family of sulphur-rich glucosides that are characteristic of Brassicaceae plants. The hydrolyzed products of glucosinolates (isothiocyanates and other sulphur-containing compounds) give a pungent and bitter taste to the oil that is often enjoyed by humans (mustard) but tends to reduce the palatability of brassica products for livestock, affecting feed intake (Przybylski et al., 2005). Glucosinolates also interfere with iodine metabolism. In monogastrics, they cause physiological disorders in the liver, kidneys, and thyroid glands, and, as a consequence, reduce growth and performance. Mortality can increase, especially in laying hens, due to hemorrhagic liver syndrome (Fenwick, 1982).
Since the 1970s, glucosinolates have been mostly bred out of rapeseed cultivars, though not eliminated. Modern 00 rapeseed/canola cultivars have very low levels of glucosinolates. In Canada, canola oil must contain less than 30 µmoles of glucosinolates per gram of air-dried oil-free meal (CCC, 2014). The glucosinolate content of rapeseeds has been declining steadily, and is now often below 10 µmol/g vs. 120 µmol/g for former non-00 cultivars (Peyronnet et al., 2014; Khajali et al., 2012). Surveys conducted since 2010 reported averages of 3.9 µmol/g (Canadian canola meal), and 10 µmol/g (French rapeseed meal) (Mejicanos et al., 2016). The use of rapeseed meal in pigs and poultry diets can now be increased without affecting feed intake or the physiological functions of livestock (Cetiom, 2001). In poultry the limitation is not due to glucosinolates but to the high fibre content (Cetiom, 2001).
Tannins
Tannins are phenolic compounds that bind with various compounds, including proteins, making them less available to the animal (Bell, 1993). In rapeseeds, most tannins are contained in the seed coat (Lipsa et al., 2012). With pigs it was found that dark hulled seeds were nearly indigestible whereas yellow hulled seeds were reasonably well digested. This was attributed to a lower tannin and lignin content in the brighter seeds (Bell et al., 1982). Lighter varieties of rapeseeds were reported to contain less tannins ("000" varieties) (Auger et al., 2010). Some breeding programmes aim at reducing the thickness of the seed coat, and thus the level of tannins (Lipsa et al., 2012). Dehulled rapeseed meal and rapeseed meal from light-coloured varieties may thus have a lower tannin content.
Phytic acid
The phosphorus of rapeseed meal is mostly in the form of phytic acid, with a phytic P:total P ratio comprised in the range of 67-95% (Selle et al., 2003; Spragg et al., 2007). Phytic acid binds to cations such as Zn, Ca, and Fe, thus reducing their bioavailability (Mejicanos et al., 2016).
Sinapine and choline
Rapeseed meal contains about 1% sinapine, an alkaloidal amine found in the seeds of Brassica species including rape. Sinapine is a choline ester converted into trimethylamine by the micro-organisms in the gastrointestinal tract of birds. Trimethylamine is then converted by an enzyme into an odourless form later excreted through urine. Hens with brown-shelled eggs lack that enzyme and, in these breeds, trimethylamine accumulates in the egg, causing them to have a fishy taste (Newkirk, 2009; Bell, 1993). The amount of rapeseeds should thus be limited for this category of layers (Pickard, 2005). All sources of choline (choline chloride and sinapine from canola meal) can be transformed in trimethylamine, but the tainting effect of rapeseed meal seemed to override the one with choline chloride (Wang et al., 2013; Ward et al., 2009). Sinapine also reduces palatability and has a depressing effect on feed consumption (Mejicanos et al., 2016).