The influence of seed size and hull content on the composition and digestibility of rapeseeds in rats

An investigation was conducted on rapeseed samples consisting of eight double low (low erucic acid and low glucosinolate) and two triple low (+ low crude fibre) cultivars. The cultivar with the smallest seed was further divided by screening into small and large seed, and that with the largest seed was further divided into large, medium and small seed (by weight) respectively. The average seed weight of the resulting eleven samples ranged from 2.9 to 8.8 mg per seed. The flower colours of the double low cultivars varied from yellow through cream to white. The samples were analysed for lipid, protein and amino acids, dietary fibre (DF), and individual glucosinolates. Hull contents were measured by manual physical separation. Nitrogen utilization and energy digestibility were determined in balance trials with rats. Lipid content in whole seeds ranged between 434–504 g kg−1 and protein from 395–549 g kg−1 in oil-free seeds. Glucosinolates were between 4.5–13.8 μmol g−1 seeds, and dominated by progoitrin. The hull proportion ranged from 105–172 g kg−1 in the whole seeds, corresponding to 209–304 g kg−1 in their oil-free dry matter. The larger seeds tended to have lower DF content. Pure hulls contained 150–180 g protein kg−1, in which lysine and threonine contents were considerably higher than in the oil-free meal (6.1 vs. 5.6 for lysine and 5.7 vs. 4.5 for threonine, in g per 16 g N). Thus 7–12% of total lysine and 8–14% of total threonine occurred in hulls. The amino acid-N accounted only for 70% of total N in the hulls. The hull contained 130–200 g lipid kg−1, in which non-fatty acid contents were considerably higher than in seed oil. Digestibilities of the seed meals ranged 67–84% for energy and 82–90% for protein (as true digestibility (TD)); both were negatively correlated to the hull and lignin content (P < 0.05), but TD correlated positively to the protein content (P < 0.01). The results indicate that the hull and protein content in oil-free meal are the main factors influencing digestibility. The observed variations in seed size and hull proportion provide a scope for further improvement of the content of digestible energy and protein.