Immune System Stimulation Increases Nursery Pig Maintenance Energy Requirements

Immune system activation can partition energy and nutrients away from growth, but clear relationships between immune responses and the direction and magnitude of energy partitioning responses have yet to be elucidated. The objective of this study was to determine how lipopolysaccharide (LPS) immune stimulation and b-mannanase supplementation affect nursery pig maintenance energy requirements (ME_m) through changes in immune parameters, digestibility, growth performance, nitrogen and energy balance. In a randomized complete block design, 30 pigs (10.27 ± 0.15 kg) were assigned to either the control treatment (CON; basal corn, soybean meal and soybean hulls diet), the enzyme treatment (ENZ; basal diet + 0.056% b-mannanase), or the immune system stimulation treatment (ISS; basal diet + 0.056% b-mannanase, challenged with repeated increasing doses of Escherichia coli LPS). The experiment consisted of a 10 d adaptation, 5 d digestibility and nitrogen balance measurement, 22 h of heat production (HP) measurements following the final injection, and 12 h of fasting HP measurements in indirect calorimetry chambers. The immune challenge consisted of either LPS (ISS) or sterile saline (CON and ENZ) injections every 48 h, for a total of 4 injections on d 10, 12, 14, and 16. Blood was collected pre- and post-injection for complete blood counts with differential analysis, haptoglobin and mannan-binding lectin, 12 cytokines, and glucose and insulin concentrations. Data were analyzed using the MIXED procedure of SAS (9.4) with blood immune parameters and rectal temperature data analyzed as repeated measures. The ENZ treatment did not differ from CON in growth performance, digestibility, immune parameters, or HP variables (P ³ 0.10). The ISS treatment induced fever, elevated proinflammatory cytokines (TNFa, IL-6, IL-1b), and decreased leukocyte concentrations (P < 0.05). The ISS treatment did not impact nitrogen balance or nutrient digestibility (P > 0.10), but increased total HP (CON = 278.8 vs. ISS = 333.0 kcal∙kg BW^-0.60∙kg DMI^-1∙d^-1; P = 0.04) and estimated ME_m by 23.6% (P = 0.045). This resulted in decreased lipid deposition (CON = 76.2 vs. ISS = 55.5 g/d; P = 0.047) and ADG (CON = 447.1 vs. ISS = 330.7 g/d; P = 0.01). Immune stimulation increased energy partitioning to the immune system by 23% which limited energy available for lipid deposition and weight gain. Understanding energy and nutrient partitioning in immune stressed pigs may facilitate the development of more effective feeding strategies.