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Nitrogen Metabolism in Crossbred Steers with Varying Levels of Brahman using a Nitrogen Depletion-Repletion Regimen¹

U.S. Department of Agriculture, El Reno, OK 73036 and Oklahoma State University, Stillwater 74074

Abstract

Nine N metabolism trials were conducted in a N-depletion-repletion regimen using five breed crosses of steers with 0, 25 or 50% Brahman. Three steers from each breed cross (avg initial wt 239 kg) were standardized for 3 wk on an 18% protein diet and depleted for 5 wk on a 7.5% protein diet and then repleted for 4 wk on the 18% protein diet. Nitrogen balance and blood and urinary N metabolites were monitored to determine changes in metabolic status due to changes in dietary N intake. Reduced N intake during the depletion period almost completely inhibited fiber digestion (P<.01), while ruminal ammonia concentration fell from an average concentration of 156 during standardization to 22 mg/liter (P<.01) during depletion. Organic matter digestion was depressed (P<.05) due to the reduced fiber digestion. Absolute amounts of intake, fecal, urinary, absorbed and retained N were severely depressed during depletion. Apparent digestibility of N was reduced from an average of 68% during standardization to 42% (P<.05) during depletion, but truly digested N was not affected. Nitrogen retained, as a percentage of intake, or absorbed was reduced only during the first week of depletion. Urea N comprised the major identified urinary fraction during standardization and repletion, whereas creatinine was the major fraction during depletion. Brahman-cross steers excreted more urinary N (P<.1) and retained less N (P<.13) during repletion than Angus x Hereford steers. Creatinine excretion was also higher (P<.01) during depletion for Brahman-cross steers. Blood urea and albumin N concentrations were higher (P<.05) during both depletion and repletion among Brahman crosses. These experiments suggest that blood measurements were more sensitive than N balance to changes in dietary N state and that faster recharge of blood N measurements in Brahman cattle may help mitigate adverse effects of N depletion stress.

¹ This research was conducted by USDA, Agric. Res. Serv. in cooperation with Oklahoma State Univ., Stillwater 74074.

² USDA, Agric. Res. Serv., Forage and Livestock Res. Lab., P. O. Box 1199, El Reno, OK 73036.

³ Dept. of Anim. Sci., Oklahoma State Univ.

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