Nitrogen recycling through the gut and the nitrogen economy of ruminants: An asynchronous symbiosis1

Abstract

The extensive development of the ruminant forestomach sets apart their N economy from that of nonruminants in a number of respects. Extensive pregastric fermentation alters the profile of protein reaching the small intestine, largely through the transformation of nitrogenous compounds into microbial protein. This process is fueled primarily by carbohydrate fermentation and includes extensive recycling of N between the body and gut lumen pools. Nitrogen recycling occurs via blood and gut lumen exchanges of urea and NH(3), as well as endogenous gut and secretory N entry into the gut lumen, and the subsequent digestion and absorption of microbial and endogenous protein. Factors controlling urea transfer to the gut from blood, including the contributions of urea transporters, remain equivocal. Ammonia produced by microbial degradation of urea and dietary and endogenous AA is utilized by microbial fermentation or absorbed and primarily converted to urea. Therefore, microbial growth and carbohydrate fermentation affect the extent of NH(3) absorption and urea N recycling and excretion. The extensive recycling of N to the rumen represents an evolutionary advantage of the ruminant in terms of absorbable protein supply during periods of dietary protein deficiency, or asynchronous carbohydrate and protein supply, but incurs a cost of greater N intakes, especially in terms of excess N excretion. Efforts to improve the efficiency of N utilization in ruminants by synchronizing fermentable energy and N availability have generally met with limited success with regards to production responses. In contrast, imposing asynchrony through oscillating dietary protein concentration, or infrequent supplementation, surprisingly has not negatively affected production responses unless the frequency of supplementation is less than once every 3 d. In some cases, oscillation of dietary protein concentration has improved N retention compared with animals fed an equal amount of dietary protein on a daily basis. This may reflect benefits of Orn cycle adaptations and sustained recycling of urea to the gut. The microbial symbiosis of the ruminant is inherently adaptable to asynchronous N and energy supply. Recycling of urea to the gut buffers the effect of irregular dietary N supply such that intuitive benefits of rumen synchrony in terms of the efficiency of N utilization are typically not observed in practice.

The effect of CP concentration in the diet on urea kinetics and microbial usage of recycled urea in cattle: a meta-analysis

“…The linear regression between N intake and UER showed that net urea N release by the liver accounted for 71.5% of N intake in cattle. The slope estimate was close to those reported by Reynolds and Kristensen (2008), which was 72% for growing cattle and 60.2% for dairy cows. However, the intercept of the linear regression observed by these authors was significant, whereas in this current meta-analytical approach it was not significant.…”

“…This seems to indicate that when dietary CP is >17%, the excess of N intake decreases the efficiency of renal urea reabsorption and therefore decreases the efficiency of urea N recycling, with a resulting increase in the amount of urinary urea N excreted. These results emphasize the inherent ability of ruminants to salvage urea N for recycling and subsequent anabolic purposes when dietary protein is limited (Reynolds and Kristensen, 2008).…”

“…The UUE:TUN may indicate the priority for excretion of excess N in relation to muscle mass, muscle growth or perhaps acid-base regulation (Huntington and Archibeque, 1999). Considering a broad range of dietary CP, UUE:TUN in ruminants increases with dietary CP concentration (Lapierre and Lobley, 2001;Marini and Van Amburgh, 2003;Reynolds and Kristensen, 2008;Wickersham et al, 2008). In accordance with these authors, we observed that UUE:TUN ASD = asymptotic SD of the relationship; UER = urea N entry rate (g/BW 0.75 ); CP = dietary CP concentration (% of dry matter); UER:NI = UER to N intake ratio (g UER/ g N intake); UUE = urinary urea N elimination (g/BW 0.75 ); UUE:UER = urinary urea N elimination relative to entry rate of urea N (g UUE/g UER); UUE:TUN = urinary urea N elimination relative to total urinary N excretion (g UUE/g TUN); GER = gastrointestinal entry rate of urea N (g/BW 0.75 ); ROC:GER = urea N re-entry to ornithine cycle relative to gastrointestinal entry rate of urea N (g ROC/g GER); UFE:GER = urea N to fecal excretion relative to gastrointestinal entry rate of urea N (g UFE/g GER); MNU = microbial incorporation of recycled urea N (g/BW 0.75 ); NI = N intake (g/BW 0.75 ); MNU:GER = microbial incorporation of recycled urea N relative to gastrointestinal entry rate of urea N (g MNU/g GER); CP:DOM = ratio between dietary CP and digestible organic matter concentrations (g CP/kg digestible organic matter).…”

“…Accordingly, relationship between dietary CP and GER:UER was determined as unity minus the model for UUE:UER (Table 3). Thus, the relationships of UUE:UER and GER:UER with dietary CP vary in a reciprocal manner (Figure 2a and b), as highlighted by Reynolds and Kristensen (2008). According to the models, GER:UER was negatively associated with dietary CP concentration.…”

Gene expression in the digestive tissues of ruminants and their relationships with feeding and digestive processes

“…Nutrient transport In addition to SCFA, transport of nutrients such as urea is of particular importance to ruminant nutrition and physiology as the availability of nitrogen and amino acid supply for ruminants is greatly influenced by microbial metabolism of urea in the rumen (Reynolds and Kristensen, 2008). Therefore, knowledge of mechanisms that regulate its supply to the rumen could be used to improve nitrogen balance in production animals.…”