PSX-A-2 - Effects of Dietary Crude Protein Level and Corn Grain Processing on Whole-body Urea Kinetics and in-vitro Urea Secretion into the Rumen of Sheep

Abstract: In ruminants, urea-N salvaging allows secretion of endogenously-produced urea (UER) into the rumen, where it can provide N for microbial growth. Our objective was to determine the effects of dietary crude protein content and corn grain processing on whole-body urea kinetics and in vitro serosal-to-mucosal urea flux (J_sm-urea) across ruminal epithelia. Thirty-two Rideau-Arcott ram lambs were blocked by BW into groups of 4 and then randomly allocated within block to one of 4 dietary treatments (n = 8) in a 2 × 2 factorial design. Dietary factors were crude protein content (10.9% [LP] vs. 15.8% [HP]) and corn grain processing (whole-shelled [WSC] vs. steam-flaked [SFC]). Feeding SFC was expected to provide more ruminally-fermentable energy than WSC. Four blocks of animals (n = 4) were used in 4-d continuous infusions of ¹⁵N-¹⁵N with concurrent collections of urine and feces to determine whole-body urea kinetics. After 23 d of dietary exposure, lambs were killed to collect ruminal epithelia which were mounted in Ussing chambers to determine J_sm-urea. Lambs fed HP had greater N intake (P = 0.01) and total N excretion (P = 0.002) than those fed LP; however, retained N (g/d or % of N intake) was similar between the 2 groups. When expressed as absolute amounts, urea-N secreted into the gut (GER) and urea-N used for anabolic purposes (UUA) were similar across diets; however, lambs fed the LP diet had a greater GER/UER ratio (P = 0.02) and tended to have a greater UUA/GER ratio (P = 0.05) than lambs fed the HP diet. Diet had no effect on ruminal J_sm-urea. Grain processing had no effects on GER, UUA and J_sm-urea. These results suggest ruminants fed protein-restricted diets have a greater dependence on urea secretion into the gut to provide N for anabolic use.