Abstract #T231

# T231
Increasing the metabolizable protein supply enhanced growth performance and humoral immune response of preconditioning beef steers.
Philipe Moriel1, Luis Felipe Arelaro*1, Matt Poore2, Rodrigo Marques3, Reinaldo Cooke3, 1North Carolina State University, Waynesville, NC, 2North Carolina State University, Raleigh, NC, 3Oregon State University, Burns, OR.

We evaluated the effects of metabolizable protein (MP) supply on growth performance, and post-vaccination measurements of humoral immune response of beef steers. Angus steers (n = 36; 231 ± 21 kg; 184 ± 18 d) were weaned on d 0, stratified by weight and age on d 7, and randomly assigned into 1 of 18 drylot pens (2 steers/pen). Treatments were assigned to pens (6 pens/treatment), and consisted of isocaloric, corn silage-based diets formulated to provide 85, 100 or 115% of the daily MP requirements. Diets were offered at 2.1% of BW (DM basis) from d 7 to 49. Steers were vaccinated on d 21 and 35 with modified-live and killed vaccines (Select-Vac, Zoetis). Blood samples were collected from jugular vein on d 0, 21, 35 and 49 to determine the plasma cortisol concentrations, and serum titers against bovine viral diarrhea virus-1b (BVDV1b), infectious bovine rhinotracheitis (IBR) and Mannheimia haemolytica leukotoxin. Data were analyzed using GLIMMIX procedure of SAS, and pen(treatment) as random effect. Body weight differed only on d 49, and was greatest (P ≤ 0.05) for 115MP, least for 85MP and intermediate (P ≥ 0.21) for 100MP steers (297, 278 and 290 ± 7 kg, respectively). From d 7 to 49, 100MP and 115MP steers had similar ADG (P = 0.13), but both had greater (P < 0.001) ADG than 85MP steers (1.20, 1.35 and 0.83 ± 0.068 kg/d, respectively). Plasma cortisol concentrations were greater (P ≤ 0.03) for 85MP vs. 100MP and 115MP steers on d 21 and 35. Serum BVDV1b titers were greatest (P ≤ 0.02) for 115MP, and similar (P = 0.37) between 85 and 100MP steers (5.8, 3.7 and 3.0 ± 0.60 base 2 log, respectively). Mean serum M. haemolytica leukotoxin titers were greater (P = 0.05) for 85MP vs. 100MP and 115MP steers (3.12, 2.39 and 2.49 ± 0.21 ng/antibody bound, respectively). Hence, a 15% MP deficiency increased plasma cortisol, and both factors may have suppressed innate immune system leading to greater multiplication of endogenous M. haemolytica and a compensatory greater production of serum M. haemolytica leukotoxin titers. In contrast, 15% MP excess enhanced growth performance and increased serum BVDV1b titers of beef steers.

Key Words: metabolizable protein, steer, immune