Nitrogen Utilization in Cattle Using a Nitrogen Depletion-Repletion Technique3

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Abstract
A NITROGEN (N) depletion-repletion feeding technique was employed in a metabolism trial designed to evaluate N utilization in ruminants. Three consecutive 28-day periods were used to standardize, to deplete and to replete body N in four Jersey steers with an average weight of 354 kilograms. Dietary protein equivalent averaged 15, 5 and 14% per period, respectively, and was supplied by either corn gluten or urea. Metabolism collections were made during week 4 of standardization (S), weeks 1, 3 and 4 of depletion (D) and weeks 1, 2 and 4 of repletion (R). Blood samples were obtained 1 hr. post-prandial from each animal on the first day of every metabolism collection. Although changes in body weights and ingested dry matter (DM) were not affected by altering the level of dietary N offered, digestibility of DM was reduced (P<.05) during the D period. Apparently digested N was defined by the equation: Y=0.94x−0.62, r=0.99, CV=8%; in which truly digested N was 94% and metabolic fecal N totalled 0.62 g N/100 g DM consumed, where X equalled ingested N. Total fecal and urinary N losses were reduced (P<.05) during the D period, and retained N (percent of intake) reached a low of −16% after 1 week of restricted N intake. However, by week 3 of D retention increased to 9%. Realimentation with higher N diets produced an immediate improvement in retained N. reaching a maximal value of 50% after 2 weeks of R. Fluctuations in retained N were mediated through changes in the excretion of total urinary N, but even more specifically through alterations in urinary urea N. Urinary ammonia N elimination was relatively constant (P<.05) at 21% of the total urinary N voided for all levels of ingested N. Plasma albumin concentrations were reduced (P<.05) during D, but plasma total N levels remained unchanged throughout the trial. Following D, approximately 2 weeks of R feeding were required before plasma albumin levels were increased. Changes in plasma urea N were directly proportional to the level of ingested N. The N depletion-repletion technique, which has been used in studies in N metabolism with non ruminants, served as a satisfactory method for assessing the effects of variations in ingested N in body N metabolism in growing cattle. Copyright © 1973. American Society of Animal Science . Copyright 1973 by American Society of Animal Science.

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