The content and standardized ileal digestibility of crude protein and amino acids in rapeseed co-products fed to pigs

An experiment was conducted to determine the effect of rapeseed variety, harvest year, and type of processing on the standardized ileal digestibility (SID) of crude protein (CP), and amino acids (AA) in rapeseed co-products fed to pigs. A number of rapeseed varieties were de-oiled by standard and soft hexane extraction methods, producing rapeseed meal (RSM), and soft rapeseed meal (SRSM), respectively, or by cold pressing, producing rapeseed cake (RSC). Soft hexane extraction was designed to maximize and maintain varietal differences in nutritional composition through seed processing at reduced temperatures. From rapeseed harvested in 2013, SRSM was produced from 3 varieties, i.e. PR46W21 (PR), V2750L (V27) and DK Cabernet (DKC), whilst RSC was produced from DKC only. From rapeseed harvested in 2014, RSM and SRSM were produced from DKC and PR. Thus, 8 rapeseed co-products were evaluated for SID CP and AA through including them at 500 g/kg as the only protein source in semisynthetic diets. Forty-eight growing pigs (initial body weight: 41 ± 2.8 kg) were allotted to these 8 feeding treatments over 6 time periods in a randomized complete block design. Compared to SRSM, the Lys to CP ratio in RSM was reduced from 0.061 to 0.050 for DKC, and from 0.055 to 0.051 for PR (Harvest 2014). The SID of CP and AA in SRSM for both DK and PR was not sensitive to harvest year. Compared to their RSM counterparts, SRSM samples had greater SID of CP, Arg, His, Lys, Met, Thr, and Val for DKC whereas, for PR, this was limited to Lys and Trp (P < 0.05). The SID of CP and AA were negatively correlated with neutral detergent fiber contents, with a coefficient of determination ranging from 0.82 for SID of Lys to 0.95 for SID of Arg. Selection of rapeseed variety and processing method might improve the content of CP and AA in rapeseed co-products. Although the harvest year might also affect the composition of the rapeseed co-products, it had no effect on CP and AA digestibility.