Nitrogen metabolism and recycling in Holstein heifers12

TLDR

Let's gather content: Purpose, Mechanism: "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." Mechanism: "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." So Purpose+Mechanism combined: The purpose is to study effect of dietary N level on urea kinetics and recycling in Holstein heifers; mechanism: using a Youden square design with isocaloric diets at 1.8 times maintenance. Background: "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." Findings: multiple sentences. Summarize: Increasing dietary N increased urinary N excretion, N balance, urea production and excretion, urea recycled with saliva increased, ruminal ammonia increased, but fecal N excretion unchanged, no effect on N recycled to gut, no difference in recycled N used for anabolism or returned to ornithine cycle, less N from urea excreted in feces as dietary N increased, total tract NDF digestibility depressed only on lowest N diet, no difference in ruminal microbial yield, more N derived from blood urea at low N intakes, efficiency of use of recycled N decreased with increasing dietary N, adaptive changes: decreased renal clearance of urea, increased gastrointestinal clearance. Also: "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." So findings: higher dietary N increases urinary excretion, urea production, saliva recycling, ruminal ammonia, but reduces fecal N from urea, decreases efficiency of recycled N, etc.

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.