253. Inflammation Induced by 4 Days of Moderate-intensity Heat Stress Was Not Improved by ω-3 Polyunsaturated Fatty Acid Supplementation in Growing Steers.

Abstract Heat stress causes deficits in growth and metabolism that compromise the efficiency of meat animal production. In addition to elevated adrenergic tone, many of the deficits during chronic heat stress are mediated by heightened inflammation, which can be ameliorated by supplementation of anti-inflammatory nutraceuticals. Thus, our objective was to evaluate whether an anti-inflammatory ω-3 polyunsaturated fatty acid Ca2+ salt (ω-3 PUFA) supplement would improve markers of inflammatory and adrenergic tone in acutely heat-stressed beef steers. Growing steers were housed under thermoneutral (19 ± 1 °C) or hyperthermic (31 ± 1 ºC; THI 77) conditions for 4 days to produce control (n = 11) and heat-stressed steers. Heat-stressed steers were supplemented daily with 0.42 g/kg ω-3 PUFA Ca2+ salts (Heat Stress+ω-3; n = 12) or placebo (Heat Stress; n = 12) beginning 2 days prior to initiating the heat-stress period. Rectal temperatures were recorded daily at 1500, coinciding with peak heat. Venous blood was drawn on days -2, 0, 2, and 4. Muscle biopsies were performed on day 4, immediately after terminating the heat stress period. Feed intake (5.9 ± 0.1 kg/day) did not differ among groups, per study design. Rectal temperatures did not differ among groups on day 0 (39.2 ± 0.1 °C) and were on average greater (p ≤ 0.05) for Heat Stress (39.8 ± 0.2 °C) and Heat Stress+ω-3 (39.9 ± 0.1 °C) steers than for controls (39.0 ± 0.1 °C) on days 1, 2, 3, and 4. Blood lymphocytes were 13% less (p ≤ 0.05) for Heat Stress steers than for control or Heat Stress+ω-3 steers. Total white blood cells (8.00 ± 0.43 cells/μL), granulocytes (1.58 ± 0.15 cells/μL), and monocytes (0.69 ± 0.03 cells/μL) did not differ among groups. Plasma interleukin-6 (IL-6) was 11–17% greater (p ≤ 0.05) and tumor necrosis factor (TNF)-α was 6–13% greater (p ≤ 0.05) for Heat Stress and Heat Stress+ω3 steers than for controls. Plasma epinephrine (0.19 ± 0.02 ng/mL) did not differ among groups. Muscle IL-6 receptor protein content tended to be 13% greater (p ≤ 0.10) for Heat Stress but 21% less (p ≤ 0.10) for Heat Stress+ω3 steers than for controls. Muscle TNF receptor 1 (1.08 ± 0.08 AU) and β2-adrenergic receptor (11.29 ± 1.04 AU) protein content did not differ among groups. These findings demonstrate that heat stress of moderate intensity and duration was sufficient to raise core body temperature and induce inflammation but did not elevate adrenergic tone. Further, beginning supplemental ω-3 PUFA just prior to a heat event only modestly improved inflammatory tone in growing steers.