Nitrogen metabolism and recycling in Holstein heifers12

Abstract

To study the effect of dietary N level on urea kinetics and recycling, four Holstein heifers (267 +/- 3.6 kg) were used in a Youden square design. Isocaloric diets with a N content of 1.44, 1.89, 2.50, 2.97, and 3.40% were fed at approximately 1.8 times maintenance intake. Increasing the N content of the diet increased urinary N excretion (P < 0.001) and N balance (P < 0.01), but did not affect the fecal N excretion (P = 0.21). Increasing the level of dietary N, increased urea production (P < 0.001) and excretion (P < 0.001), but no effect (P = 0.24) could be detected in the amount of N recycled to the gut. Urea recycled with the saliva, however, increased (P < 0.001) both in absolute and relative terms, with increasing dietary N. No difference could be detected on the amount of recycled N that was used for anabolism or returned to the ornithine cycle, but less (P = 0.001) N originating from urea was excreted in feces as dietary N increased. Ruminal ammonia concentration increased (P < 0.001) with increasing N intake, but total tract neutral detergent fiber digestibility was depressed only on the lowest N intake diet. No difference (P = 0.30) was detected in ruminal microbial yield among diets, but more (P < 0.003) N was derived from blood urea at low N intakes, and the efficiency of use of the recycled N decreased (P < 0.001) with increasing levels of dietary N. Adaptive changes to low-N diets were a decrease (P < 0.003) in the renal clearance of urea and an increase (P < 0.001) in the gastrointestinal clearance of urea. Urea transporters were present in the rumen wall of the heifers and differentially expressed depending on dietary N content, but their role in the transfer of urea into the rumen remains uncertain. Different mechanisms of N salvage and recycling were involved when animals were fed low-N diets that ensured a supply of endogenous N to the gastrointestinal tract and, due to the reduced contribution of dietary N, an increased efficiency of the N recycled was observed.

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

“…In cattle-fed diets >12% of CP, the amount of N recycled (g/d) through the rumen wall may be relatively constant (Marini and Van Amburgh, 2003;Maltby et al, 2005;Reynolds and Kristensen, 2008). Nevertheless, in the current analysis, amount of urea N recycled were linearly associated with increases of N intake, with the increases in recycling accounting for about 35% of increases in N intake.…”

“…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).…”

“…This procedure involves infusion or injection of ( 15 N 15 N)-urea in the jugular vein followed by measurement of the enrichment of ( 15 N 14 N)-and ( 15 N 15 N)-urea excreted in the urine as well as fecal 15 N excretion. However, the original method does not measure the ruminal microbial incorporation of recycled urea N. Supplemental methodologies have been described to measure the ruminal capture of recycled urea N through measurement of 15 N enrichment in bacterial and plasma urea N (PUN; Marini and Van Amburgh, 2003) or in bacterial and duodenal N (Wickersham et al, 2009a).…”

“…However, it seems that expression of UT-B in ruminal epithelium is more affected by dietary carbohydrate (Simmons et al, 2009;Walpole et al, 2015) than dietary protein concentration (Marini and Van Amburgh, 2003;Røjen et al, 2011). Recently, Walpole et al (2015) demonstrated that aquaporins (AQP), which are a family of membrane-spanning proteins (e.g.…”

Aspects of nitrogen metabolism in sheep-fed mixed diets containing tree and shrub foliages

“…It has been suggested that dietary fibre increased excretion of endogenous N resulting from increased mucosal cell turnover when high-fibre diets were fed to rats (42) . Besides, a negative relationship exists between urea recycling to the intestine and NI (43,44) . In the present analysis, intakes of DM and digestible OM were similar among FL-0, FL-L and FL-H diets.…”