Influence of the grinding level and extrusion on the nutritional value of lupin seed (Lupinus albus) for cattle in the context of the Dutch protein evaluation system

The experiment was conducted on double-muscled Belgian Blue bulls, fitted with rumen, duodenal and ileal cannulas, according to a 5 × 5 Latin square design with one missing animal. All diets contained 320 g/kg of lupin seed (Lupinus albus, var. Lublanc) on a dry matter (DM) basis. Four grinding treatments of raw lupins (RAW) were used to obtain median particle sizes of approximately 0.5, 2.0, 4.2 or 6.0 mm. One extrusion treatment (EXTR) was also used and consisted of heating to 180 °C for 30 s. The pH and ammonia nitrogen (NH₃–N) concentration kinetics in the rumen liquid differed between RAW and EXTR. The latter induced larger variation in these parameters after the meal. Quadratic effects of the grinding level on rumen fermentation parameters were observed at several sampling times for RAW. The grinding level had also quadratic effects on the intestinal digestibility of DM, organic matter (OM) and N, reaching a maximum with treatment 4.2 mm. The treatment 0.5 mm tended to reduce the microbial N flow at the duodenum whereas EXTR tended to increase the non NH₃–N flow. The total digestible flow of N tended to increase with the intermediate grinding levels and EXTR. Lupin protein degradability was calculated at 0.78, 0.74, 0.65, 0.82 and 0.70 for the 0.5, 2.0, 4.2, 6.0 mm treatments and EXTR, respectively. Treatments 2.0 and 4.2 mm increased the digestible protein in the small intestine (DVE) content by more than 40% compared with treatment 0.5 mm. The balance between rumen fermentable N and energy (OEB) was reduced by 15% with the 4.2 mm and EXTR. Compared with treatment 4.2 mm, EXTR did not greatly improve the nutritional value of lupin seed. The results suggested that lupin seed should be coarsely ground or flattened to obtain a mean particle size between 2.0 and 4.2 mm for cattle feed, but that an insufficient grinding level (treatment 6.0 mm) induced a higher degradability of lupin protein, probably due to more intense rumination. Under our experimental conditions, the extrusion did not sufficiently improve the nutritional value of the seed to be economically viable. These results show that feeding standards should consider the influence of the grinding level of legume seeds in order to assess their nutritional value accurately.