Effects of dietary protein and bacterial lipopolysaccharide infusion on nitrogen metabolism and hormonal responses of growing beef steers1

Purified lipopolysaccharide (LPS) infusion in cattle induces clinical and metabolic responses similar to gram-negative bacterial infection. Effects of LPS and dietary protein on rectal temperature, serum hormones, haptoglobin, plasma urea N and AA, and N balance were evaluated in 24 steers (250 ± 2.8 kg of BW). Treatments were a 2 × 3 factorial of LPS (0 vs. 1.5 μg/kg of BW; –LPS vs. +LPS) and diets containing (DM basis) 1) 14.5% CP, 11.6% ruminally degradable protein (RDP), and 2.9% ruminally undegradable protein (RUP; CP14.5CON); 2) 16.3% CP, 13.4% RDP, and 2.9% RUP (CP16RDP); and 3) 16.1% CP, 11.2% RDP, and 4.9% RUP (CP16RUP). Diet RDP and RUP were altered using casein, fish meal, and corn gluten meal. Steers were adapted to diets (1.1 Mcal/kg of NEg; DM fed at 1.8% BW) for 14 d and were infused (intravenously 1 mL/min) with LPS (in 100 mL of saline) on d 15. Rectal temperature and serum cortisol, prolactin, haptoglobin, and insulin increased, glucose initially increased and then declined, and serum thyroxine and triiodothyronine decreased for +LPS vs. –LPS steers (LPS × hour; P < 0.01). Serum IGF-I was less (P < 0.01) for +LPS vs. –LPS steers. Plasma urea N increased in response to LPS (LPS × hour; P = 0.02) and was greater for +LPS steers fed CP16RDP and CP16RUP vs. CP14.5CON, but greater in –LPS steers fed CP16RUP vs. CP16RDP and CP14.5CON (LPS × diet; P = 0.04). Plasma Met, Thr, Leu, Ile, Phe, Trp, Gly, Ser, Asn, and Tyr decreased, and plasma Ala increased in response to LPS (LPS × hour; P < 0.01). Plasma Orn initially increased and then decreased in +LPS vs. –LPS steers (LPS × hour; P < 0.01). No LPS × diet interactions (P ≥ 0.15) occurred for DM, OM, NDF and N intake, fecal excretion, or apparent digestibility. Dietary DM, OM, NDF, and N intake, and retained N were less (P < 0.01) for +LPS than –LPS steers. Total N intake, apparent N digestibility, and retained N were greater (P ≤ 0.05) for steers fed CP16RDP and CP16RUP vs. CP14.5CON. An LPS × diet interaction (P = 0.05) occurred for N retention (% N intake) because N retention was less for +LPS than –LPS steers when fed CP14.5CON, but not different between +LPS and –LPS steers when fed CP16RDP and CP16RUP. These results demonstrate that LPS infusion alters serum hormones, plasma AA, and N balance in cattle and imply that growing steers exposed to LPS may require greater dietary protein concentrations to account for altered intake and metabolic AA demand.