Rumen degradable protein can be decomposed into peptides, ammonia and amino acids by microorganisms for growth and reproduction. The protein content of microorganism itself is about 60%. The amount of protein available to dairy cows depends on the intake from the diet, the amount of rumen passing microorganisms and the digestion in the intestine. When energy (carbohydrate) is deficient or excessive, the efficiency of ammonia utilization by microorganisms will be reduced.
Milk urea nitrogen (MUN) can be used as a tool to evaluate protein utilization efficiency. When the rumen microbes can not make full use of ammonia to synthesize microbial protein, ammonia can be absorbed through the rumen wall, and enter the liver with blood circulation, which is converted into urea nitrogen and then into circulation. Urea nitrogen in the blood can enter the urine and milk, or return to the rumen for microbial reuse. Blood and milk urea nitrogen content is highly correlated, so Mun concentration can reflect the urea concentration in blood and other liquids.
The ideal Mun may vary between different pastures, depending on the nutritional management of dairy cows. According to field observation, it is suggested that Mun lower than 6-8 mg / dl may mean the lack of degradable protein in rumen, which limits milk production and milk protein synthesis. Mun higher than 12-14 mg / dl may indicate that the rumen available protein is too much, the protein composition is not balanced or the energy and nitrogen do not match.
In the case of poor quality corn silage, it is important to monitor Mun to assess the balance between protein and carbohydrate in the diet. The content of fermentable starch in rumen was decreased and the concentration of Mun was increased. In assessing Mun concentrations, it is important to remember that it is affected by milking time (relative to feeding), the number of milking per day, and the sampling time (morning or afternoon).