Influence of forage level and corn grain processing on whole-body urea kinetics, and serosal-to-mucosal urea flux and expression of urea transporters and aquaporins in the ovine ruminal, duodenal, and cecal epithelia.

The objectives were to determine the effects of forage level and grain processing on whole-body urea kinetics, N balance, serosal-to-mucosal urea flux (Jsm-urea), and mRNA abundance of urea transporter-B (UT-B; SLC14a1) and aquaporins (AQP) in ovine ruminal, duodenal, and cecal epithelia. Thirty-two wether lambs were blocked by BW into groups of 4 and assigned to 1 of 4 diets (n = 8) in a 2 × 2 factorial design. Dietary factors were forage level (30% [LF] vs. 70% [HF]) and corn grain processing (whole-shelled [WS] vs. steam-flaked [SF]). Four blocks of lambs (n = 4) were used to determine urea kinetics and N balance using 4-d [15N15N]-urea infusions with concurrent fecal and urine collections. Lambs were killed after 23 d of dietary adaptation. Ruminal, duodenal, and cecal epithelia were collected to determine Jsm-urea and mRNA abundance of UT-B and AQP. Lambs fed LF had greater intakes of dry matter (DMI; 1.20 vs. 0.86 kg/d) and N (NI; 20.1 vs 15.0 g/d) than those fed HF (P < 0.01). Lambs fed SF had greater DMI (1.20 vs. 0.86 kg/d) and NI (20.6 vs. 14.5 g/d) than those fed WS (P < 0.01). As a % of NI, total N excretion was greater in lambs fed HF compared to those fed LF (103 vs. 63.0%; P < 0.01), and was also greater in lambs fed WS compared to those fed SF (93.6 vs. 72.1%; P = 0.02). Retained N (% of NI) was greater in lambs fed LF compared to those fed HF (37.0 vs. -2.55%; P < 0.01). Lambs fed SF had a greater (P = 0.02) retained N (% of NI; 28.0 vs. 6.50%) compared to those fed WS. Endogenous urea production (UER) tended (P = 0.09) to be greater in lambs fed HF compared to those fed LF. As a proportion of UER, lambs fed HF had a greater urinary urea-N loss (0.38 vs. 0.22), and lower urea-N transferred to the gastrointestinal tract (GIT; 0.62 vs. 0.78) or urea-N used for anabolism (as a proportion of urea-N transferred to the GIT; 0.12 vs. 0.26) compared to lambs fed LF (P < 0.01). Ruminal Jsm-urea was unaffected by diet. Duodenal Jsm-urea was greater (P < 0.01) in lambs fed HF compared to LF (77.5 vs. 57.2 nmol/[cm2 × h]). Lambs fed LF had greater (P = 0.03) mRNA expression of AQP3 in ruminal epithelia and tended (P = 0.06) to have greater mRNA expression of AQP3 in duodenal epithelia compared to lambs fed HF. Expression of UT-B mRNA was unaffected by diet. Our results showed that feeding more ruminally-available energy improved N utilization, partly through a greater proportion of UER being transferred to the GIT and being used for anabolic purposes.