The Influence of Previous Nitrogen Levels on Nitrogen Metabolism and Rumen Gene Expression of Grain Or Forage-Fed Finished Steers.

The objective of this study was to evaluate how the previous nitrogen levels (PNL) in the diet might affect nitrogen balance and nitrogen-related genes in the rumen of either forage-fed or grain-fed steers during finishing. Twenty-four crossbred Angus steers were randomly distributed into one of two backgrounding nutritional plans: moderate (MP; crude protein (CP): 9.10%) or high (HP; CP:12.6%). After 85 d, animals were blocked by PNL, transitioned, and fed a finishing diet for 105 d, which consisted of either alfalfa only (forage-fed; CP: 21.3%) or predominantly whole grain (grain-fed; CP: 10.8%). Feed, orts, feces, and urine samples were collected in both phases. By the end of the finishing phase, animals were slaughtered, and ventral sac rumen tissue was collected. Gene expression of Aquaporins (AQP) -3 and -7, and UT-B from rumen tissue were assayed via real-time qPCR; 18S rRNA was used as an endogenous control. Animals on MP diets excreted less N through urine (P < 0.01) and feces (P < 0.01), while retaining more N (P < 0.01), which resulted in an increased N efficiency (P < 0.01) compared with HP animals. No differences were observed for microbial N production (P > 0.05). Grain-fed animals consumed less N (P < 0.01), but still had a greater amount of N being digested compared with forage-fed animals (P < 0.01), as well as the least excretion of N through feces (P < 0.01) and urine (P < 0.01), and consequently a greater rate of N being retained (P < 0.01) and a greater efficiency on utilizing the ingested N (P < 0.05). Further, an interaction was also observed between treatment and PNL, where animals coming from a MP had the least dry matter intake, greatest amount of N being digested and least N excretion, fecal output, and excretion of N through the feces (P < 0.05) when finished on a grain-based diet. In the rumen, steers coming from a low PNL that were finished as forage-fed, had the greatest expression of AQP7, with no statistically significant differences observed for the other groups. When comparing only animals backgrounded on MP, AQP7 (P < 0.01) expression was greater in forage-fed animals, with a trend for greater expression of UT-B in grain-fed animals (P < 0.1). Overall, these results indicate that animals receiving MP will reduce their excretion of N while producing the same amount of microbial N due to more efficient N recycling. Further, carried-over effects were also observed after these animals were transitioned to grain-fed finishing systems. In the rumen, PNL affects expression of AQP7 and UT-B, which may ultimately impact nitrogen metabolism, since both genes are also related with urea transport. [ABSTRACT FROM AUTHOR]