| In the first part of this study, fourteen commonly used dairy cattle feeds (five forages and nine concentrations) were selected in north part of China. The protein fractions (A,B,C) of these feeds and the in situ degradation characteristics of fraction B were determined with three high producing cows fitted with permanent cannulas in rumen, and the degraded ability of these feeds protein fractions were studied. These data are very important to the breeding of dairy cows of our country.In the second part, an in vitro continous culture system was used to study the effects of four diets with different protein fractions (RDP/RUP: 1.73, 1.58,1.45 and 1.33) on rumen fermentation. The above four diets contain similar same crude protein, energy and forage to concentration ratios. The results showed that there was no significant effects of diets with different protein fractions on the rumen fluid pH, protein yield, neutral protease and endo-glucanase activiy of rumen microbe (p>0.05). However, when RDP/RUP of diet was 1.45, the ammonia nitrogen concentration of rumen fluid was significantly lower than other three diets (P<0.01). When RDP/RUP of diet was 1.73, the concentrations of acetic acid (P<0.05), propionate acid (P<0.05), butyric acid (P<0.01), valeric acid (P<0.05) and total VFA (P<0.01) of rumen fluid were significantly higher than other three diets. When RDP/RUP of diet was 1.33, the a-amylase activiy of rumen microbes was significantly higher man other the release of diets (P <0.01). There was a significant effect of a right proportion of RDP/RUP on rumen fermentation of lactating dairy cows, furthermore, there was a different effect on rumen fermentation with different proportion of RDP/RUP in dietThe third part was mainly conducted to study the effects of synchronizing carbohydrates and proteins (corn + soybean meal, corn + roasted soybean, wheat + soybean meal, wheat + roasted soybean) on rumen fermentation of lactating dairy cows with an in vitro continous culture system. The results showed that there was no significant effects of synchronizing fermentation with carbohydrates and proteins on the concentrations of VFA (except valeric acid), the protein yield and a-amylase activitie of rumen microbes (p>0.05). The pH and ammonia nitrogen concentration of rumen fluid with the diet of corn and soybean meal were significantly lower than other three diets (P<0.01), furthermore, the neutral protease activiy of rumen microbes with this diet tends to be increased (P=0.0783). The endo-glucanase activiy of rumen microbes with the diet of wheat and roasted soybean tends to be increased (P=0.0855). The pH and ammonia nitrogen concentration of rumen fluid with the diet of synchronizing carbohydrates and proteins were significantly reduced, and the the neutral protease activiy tends to be increased, as a result, the speed of rumen fermentation was increased, the efficiency of rumen fermentation was improved, the using efficiency of feed protein was improved, too.The objectives of the forth part were to determine the effects of four diets with different levels of protein fractions (RDP/RUP:1.78, 1.65, 1.54 and 1.44) on milk performance and milk composition of lactating dairy cows. These diets were formulated with similar crude protein, energy and forage to concentration ratios.Thirty-two Holstein cows in middle lactation were selected and randomly allocated four groups according to their parity, milk yield and days in milk. The results indicated that there wasno significant effects of diets with different protein fractions on milk yield, FCM and BUN of lactating dairy cows (P>0.05). However when RDP/RUP of diet was 1.78, there was a numeric increasing trend for milk fat (P=0.071). When RDP/RUP of diet was 1.65, milk protein (P-0.1008) and milk protein yield (P=0.0781) increased numerically. When RDP/RUP of diet was 1.54, there was a decreasing trend for MUN (P=0.1433). Milk fat, milk protein and milk protein yield were improved at a certain extent by a right proportion of RDP/RUP, but the MUN tends to be decreased.
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