Canola is an offspring of rapeseed (Brassica napus and Brassica campestris/rapa), canola seed is rich in oil, and after oil extraction, the remaining “canola meal” (CM), is a rich protein source used as feedstock to different animal species, mainly dairy cows in North America and in Europe. Despite the positive responses in milk production and nitrogen (N) utilization efficiency observed when soybean meal (SBM) is replaced with CM as the main protein supplement in dairy cow diets, it is unclear if the responses are due to a ruminal effect, a post ruminal effect, or a combination of both. The objectives of the research presented here were: 1) to evaluate whether the positive responses in milk production and N utilization efficiency are due to a better ruminal digestibility and N metabolism when CM is compared to SBM in dairy cow diets; 2) to evaluate whether these positive responses may be due to a greater contribution of the rumen undegraded protein (RUP) fraction and/or an increase of microbial protein synthesis to the metabolizable protein supply; and 3) to assess the potentially digestible neutral detergent-fiber (pdNDF) and the energy content of CM in a large sample set. To assess these objectives, first an in vitro study was performed (Chapter 2) evaluating the effects of feeding CM with different RUP content on ruminal fermentation, nutrient digestion, and microbial growth using a dual-flow continuous culture system. For this study, it was observed that CM with RUP varying from 38 up to 50% of CP did not affect ruminal N metabolism. Furthermore, no major differences in ruminal N metabolism and digestibility between SBM and CM diets were observed, which indicate that there are no major ruminal effects of replacing SBM with CM. Then a follow up in vivo study was performed (Chapter 3) to evaluate whether treating CM by extrusion to increase its RUP content would improve RUP flow to the small intestine, N utilization and performance of dairy cows compared to regular CM and SBM. For this study, our results indicate that treating CM by extrusion was not effective in improving CM utilization by lactating in dairy cows. Nonetheless, when compared to the SBM diet, both CM diets decreased milk urea nitrogen (MUN) and N excretion in feces and urine. A third study was performed (Chapter 4) to assess whether the pdNDF of CM is underestimated based on current prediction models, and consequently its energy content is also underestimated. For this study, our results indicate that the pdNDF and the energy content are underestimated in current nutritional models. As an overall conclusion, our results indicate that the positive production responses previously observed when CM replaced SBM may have been due to post-ruminal effects and/or dry matter intake. Furthermore, treating CM by extrusion was not effective in improving CM utilization by lactating in dairy cows. However, CM may reduce the environmental impact compared to SBM, due to a lower urea N excretion as a proportion of total urinary N. More accurate information on CM NDF digestibility may improve energy content estimation, thus improving diet formulation accuracy
