Aim : There is little regarding the effect of dietary protein degradability and energy sources on ruminal microbial protein production and utilization in the duodenum of dairy cows. Materials and methods : Two in vitro experiments were conducted to verify the e ffect of glucogenic or lipogenic diet provided with soybean meal (SB) versus a common xylose - protected SB (Yasmino max soy ® , XPS) on the ruminal fermentation potential, the ruminal - N yield, and the utilizable CP in the duodenum of dairy cows. Barley and corn were used as ground and steam - flaked as the main source of energy in glucogenic diets while in lipogenic diets, corn sil age, sugar beet pulp, linseed, and fat prill were included. In order to estimate the microbial - N yield, after a test to evaluate the ruminal fermentation responses for each diet, a second culture was conducted and stopped at the of half time gas production . Some selected diets were incubated to estimate the amount of utilizable CP in the duodenum. Results : Gas produced from the fermentable part (b) was higest in the corn grain and XPS diet. Corn silage+ soybean meal (CSSB) had the lowest amount of microbia l - N to dietary - N. Steam flaked barely+ SB (SFBSB) diet had the highest amount of microbial - N to dietary - N, Utilizable crude protein (uCP) and relative uCP in 8 and 24 hours after incubation. Glucogenic diets had higher EuCP than L diets. Conclusion : Overal l, the results demonstrate that the glucogenic di et al ong with the xylose - protected soybean meal improve the microbial nitrogen production and the cp utilization after 8 and 24 hours of incubation.

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