his study was conducted to investigate the effect of dairy cow diets containing different protein sources [soybean meal, xylose protected soybean meal (Yasminomax®), and steam-flaked whole soybean] and various partially processed grain on in vitro gas production kinetics and the microbial nitrogen yield. Soybean meal (S) and xylose protected of soybean meal (XS) were provided as 0.56:0.44 (SXS L ) and 0.51:0.49 (SXS H ) ratios. The diets were ground barley (B) + S, B+ SXS L , steam-flaked barley (FB) + S, FB + SXS L , B + steam-flaked whole soybean (WS), B + SXS H , FB + WS and FB + SXS H . An in vitro gas production technique was used to define the differences in the microbial nitrogen production of the diets. Asymptotic gas volume (b) was higher than the others for diets containing steam-flaked whole soybean (P<0.01). The highest constant rate of gas production (c) belongs to diet with ground barley and soybean meal, the highest halftime of gas production (t 1/2 ) was obtained from diet with steam-flaked barley and soybean meal (P<0.01). The microbial nitrogen yield and consequently microbial nitrogen to diet nitrogen ratio were higher in diets containing xylose protected soybean meal than the others, especially, when compared with steam-flaked whole soybean diets (P<0.05). These results showed that WS, when compared with S and XS may not improve rumen fermentation and microbial nitrogen production with both ground and steam-flaked barley grain and the best performance in microbial nitrogen yield belongs to xylose protected soybean meal.

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