Effects of dietary energy source and level and injection of tilmicosin phosphate on immune function in lipopolysaccharide-challenged beef steers

¹ Texas Tech University, Lubbock 79409
² USDA-ARS Livestock Issues Research Unit, Lubbock, TX 79409
³ The Samuel Roberts Noble Foundation, Agricultural Division, Ardmore, OK 73402

^* To whom correspondence should be addressed. E-mail: rrreuter{at}noble.org.

	Abstract

Twenty-four Angus x Hereford crossbred steers (247 kg BW; SE = 2.4 kg) were used in a completely random design to evaluate the effect of energy source and level with or without antibiotic administration on measures of immune function. Steers were fed 1 of 3 dietary treatments: a 70% concentrate diet ad libitum (70AL); a 30% concentrate diet ad libitum (30AL); and a 70% concentrate diet offered in an amount calculated to provide NEg intake equal to the 30AL treatment (70RES). Half the steers in each dietary treatment received either a s.c. injection of tilmicosin phosphate (ANTI; 1 mL/30 kg of BW) or an equal volume of saline s.c. (SAL). Steers were offered the treatment diets for 28 d before, and administered the ANTI or SAL injections 2 d before indwelling catheters were placed in the jugular vein and 2.0 µg/kg of BW of E. coli lipopolysaccharide (LPS) were administered i.v. Blood serum was collected at 30-min intervals from -2 to 6 h, and at 8, 12, 24, 48, and 72 h relative to the LPS challenge. Increased energy intake (70AL) increased (P 0.04) DMI, ADG, and rectal temperature (RT) following the challenge compared with the 70RES treatment. The 30AL treatment increased the maximum concentrations and area under the response curve of the proinflammatory cytokines (PIC) interferon- (IFN-), tumor necrosis factor- (TNF-), and IL-6 (P 0.05) compared with the average of the 70AL and 70RES treatments. Decreased energy intake (70RES vs. 70AL) increased IL-6 (P 0.003) but did not significantly increase IFN- and TNF- (P 0.14) following LPS administration. Tilmicosin administration decreased the time to attain maximal RT (P = 0.01) by 1 h without altering the peak RT (P = 0.85), and tilmicosin interacted with energy intake to increase pre-challenge PIC in 70RES vs. 70AL (P 0.05). Results indicate that increased PIC response, presumably resulting from a combination of decreased energy intake and from direct effects of roughage, may be a mode of action for the slight decrease in morbidity that often occurs when newly received, stressed calves are fed roughage-based receiving diets. Tilmicosin phosphate might have immunomodulatory capacity beyond its direct effects on pathogenic bacteria, and these effects could interact with dietary energy intake in cattle.

Key Words: beef cattle, energy, immune function, tilmicosin phosphate

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