Improving nitrogen use efficiency of dairy cows in relation to urea in milk - a review

Dietary protein intake is the most important factor determining milk production, milk composition, milk nitrogen efficiency, urinary nitrogen losses, urea content in milk and consequently, ammonia emissions from dairy cow manure. According to the nutrition requirements two main protein sources are available for cows: rumen degradable protein, provided to the animal through ruminally synthesized microbial protein, and rumen undegradable protein that escapes ruminal degradation (but is digested and absorbed in the small intestine). The presence of urea in the milk of cows is caused by metabolic changes in the gastrointestinal tract, resulting in the excess of microbially undigested ammonia in the body. Rumen degradable feed proteins are degraded by rumen microorganisms via amino acids into ammonia and branched chain fatty acids. The bacterial population uses ammonia in order to grow. The extent, to which ammonia is used to synthesize microbial protein, is largely dependent upon the availability of energy generated by the fermentation of carbohydrates. On average, 20 grams of bacterial protein are synthesized per 100 grams of organic matter fermented in the rumen. Bacterial protein synthesis may range from less than 400 g/day to about 1500 g/day, depending primarily on the digestibility of the diet. A highly toxic chemical compound generated during those processes, i.e. ammonia, is detoxified in the liver and converted to urea. The main reason for elevated levels of urea in milk is connected with excess protein contents in feed rations, and energy and protein imbalance. The increase in the percentage of total protein from 13% to 18% DM per ration is accompanied by an increase in the urea level by about 80 mg to over 150 mg in 1 liter of milk. Additional factors affecting the level of urea in milk include the following: frequency of feed administration, number of milkings and length of the interval between milkings, cow's body weight, water intake volume, the level of Na and K ration supplementation, as well as rumen pH. Recent research indicated that an addition of natural plant-origin biologically active compounds, such as tannins, saponins and essential oils, reduce ammonia production and finally urea content in milk. Information on the concentrations of milk urea and nitrogen in dairy cows allows to assess energy balance and protein rations supplied, which in turn can help to reduce both feed costs and nitrogen emission (N) to the environment.