[Studies on the effect of phosphoric phenyl ester diamide as inhibitor of rumen urease in dairy cows. 3. Digestibility of the nutrients and bacterial protein synthesis]

Urea Metabolism and Regulation by Rumen Bacterial Urease in Ruminants – A Review

Urea is used as non-protein nitrogen in the rations of ruminants as an economical replacement for feed proteins. Urea transferred from the blood to the rumen is also an important source of nitrogen for rumen microbial growth. It is rapidly hydrolyzed by rumen bacterial urease to ammonia (NH3) and the NH3 is utilized for the synthesis of microbial proteins required to satisfy the protein requirements of ruminants. Urea has commonly become an accepted ingredient in the diets of ruminants. In recent decades, urea utilization in ruminants has been investigated by using traditional research methods. Recently, molecular biotechnologies have also been applied to analyze urea-degrading bacteria or urea nitrogen metabolism in ruminants. Combining traditional and molecular approaches, we can retrieve better information and understanding related to the mechanisms of urea metabolism in ruminants. This review focuses on urea utilization in ruminants and its regulation by rumen bacterial urease in the host. The accumulated research provides foundations for proposing further new strategies to improve the efficiency of urea utilization in ruminants.

Ureases in the gastrointestinal tracts of ruminant and monogastric animals and their implication in urea-N/ammonia metabolism: A review

Urea in diets of ruminants has been investigated to substitute expensive animal and vegetable protein sources for more than a century, and has been widely incorporated in diets of ruminants for many years. Urea is also recycled to the fermentative parts of the gastrointestinal (GI) tracts through saliva or direct secretory flux from blood depending upon the dietary situations. Within the GI tracts, urea is hydrolyzed to ammonia by urease enzymes produced by GI microorganisms and subsequent ammonia utilization serves the synthesis of microbial protein. In ruminants, excessive urease activity in the rumen may lead to urea/ammonia toxicity when high amounts of urea are fed to animals; and in non-ruminants, ammonia concentrations in the GI content and milieu may cause damage to the GI mucosa, resulting in impaired nutrient absorption, futile energy and protein spillage and decreased growth performance. Relatively little attention has been directed to this area by researchers. Therefore, the present review intends to discuss current knowledge in ureolytic bacterial populations, urease activities and factors affecting them, urea metabolism by microorganisms, and the application of inhibitors of urease activity in livestock animals. The information related to the ureolytic bacteria and urease activity could be useful for improving protein utilization efficiency in ruminants and for the reduction of the ammonia concentration in GI tracts of monogastric animals. Application of recent molecular methods can be expected to provide rationales for improved strategies to modulate urease and urea dynamics in the GI tract. This would lead to improved GI health, production performance and environmental compatibility of livestock production.

[Transport of urea nitrogen from the intestines into the stomach in dairy cows]

In two experiments with one dairy cow each the utilisation of urea-N after its ruminal or duodenal infusion was comparatively investigated on two crude protein levels and different urease activities in the rumen. The rations contained 9.6 (I) and 14.3 (II) g crude plant protein/100 g dry matter. After completed adaptation 50 g urea were daily infused in the rumen (R, via cannula) in 3 h resp. the duodemun (D, distal cannula of the reentrance cannula) in 6 h with the morning and evening feeding. In experiment II the urease blocker phosphoric acid phenylester diamide (D/PPD) was applied in an additional experiment synchronously with the duodenal urea application. On the first measuring day in each case the urea in the morning feeding was labelled with 17.4 atom-% 15N-excess (15N'). Measuring results in the sequence I R, I D, II R, II D, II D/PPD: 15N'-passage rate at the duodenum within 72 h in the TCA-soluble N-fraction 29, 18, 24, 13 and 16, in the TCA-precipitable N-fraction 59, 25, 41, 11 and 5% of the application, 15N'-excretion within 96 h in milk protein 6.8, 4.2, 4.6, 3.4 and 1.9, in faeces 20, 12, 19, 8 and 4, in urine 20, 32, 34, 56 and 75% of the application, 15N-balance 59, 56, 47, 36 and 21% of the application, passage rate of non-NH3-N in the duodenum 131, 118, 96, 107 and 99% of the total N-intake. After ruminal infusion there always was a higher NH3-concentration in the rumen and 15N-frequencies in the rumen proteins. One can conclude that urea-N that gets into the intestines is to a low degree used for duodenal protein supply as directly utilisable urea-N from the ration in the rumen. The difference increases with the protein content of the ration and the inhibition of rumen urease. The urea N-balance is to a considerably smaller degree influenced by the place of urea infusion particularly at a low level of N-supply, which is due to a better utilisation of the urea-N transported with intermediary metabolism from the intestines. The role of urease as a regulator of urea transport through the rumen wall cannot be corroborated.