Effects Of Dietary Protein And Ruminal Undegradable Protein On Milk Performance And Nitrogen Excretion Of Dairy Cows

This experiment was conducted to study the effects of different levels of Crude Protein(CP) and Ruminal Undegradable Protein(RUP) in ration on the rumen fermentation and milk performance and nitrogen excretion of dairy cows, and furthermore, to study the feasibility of reducing nitrogen release of dairy cows by nutritional control.The experiment was divided into 4 parts. Trial 1 choosed three Holstein dairy cows fitted with ruminal cannulas to study the effects of different levels of CP and RUP on the rumen fermentation. This experiment was a 2×3 factorial arrangement (two RUP levels and three CP levels) with total 6 diet combinations, and the ratio of concentration to forage was 30:70. Pre-trial period was 10 d for each diet. The ruminal fluid samples were collected at the time points of 2, 4, 8, 12, and 24h.The trial 2 to 4 were conducted to study the effects of different levels of CP and RUP on production of milk, content of urea nitrogen and output of nitrogen of dairy cows. The experiment was a was a 2×3 factorial arrangement (two RUP levels and two CP levels) with total 4 diet combinations, and the ratio of concentration to forage was 48:52.Thirty two dairy cows which were similar in weight, milk yield and lactation stage were randomly divided into 4 groups , with 8 each group. Pre-trial period was 15 d and the experimental period was 60 d.Under the present experimental conditions, the results showed as follows:1. The rumen NH3-N contents were significantly affected by CP levels in diets, and 10.5% CP levels of diet would result in the highest NH3-N level in rumen.The rumen fermentation for 9% CP level was slightly better than other levels;2. The high levels ofRUP(55%cp)can reduce the pH value and NH3-N content in rumen, and increase the concentrations of BCP and VFA in rumen ;3. There was no significant effects on milk production ,when the dietary CP level was reduced from 14.5% to 12.5%;4. Increasing RUP levels in diet would promote milk yield of dairy cows;5. The contents of MUN and BUN were significantly affected by CP levels in diet. Lower CP level can reduce the contents of MUN and BUN;6. Improving the proportion of RUP in diets could obviously decrease the contents of MUN and BUN; 7. When the CP level aws reduced from 14.5% to 12.5%, the outputs of FN and UN were significantly decreased , but there was no obviours change in apparent nitrogen degestability and apparent nitrogen metabolic rate in diets;8. Improving the proportion of RUP in diets could increase the nitrogen utilization rate and nitrogen metabolic rate ,and reduce the nitrogen output.