Role of Spirulina platensis and Humic Acid in Mitigating Acute Cyclic Heat Stress: Effects on the Growth Performance, Meat Quality, Immunological Responses, and Tissue Histomorphology in Broiler Chickens

Elevated ambient temperature is the primary abiotic element that possibly diminishes production and economic profitability in the chicken industry. The current study evaluated the role of dietary addition of <i>Spirulina platensis</i> (SP) and humic acid (HA) in mitigating the effects of acute cyclic heat stress on growth performance, meat quality, immune status, and intestinal morphology in broiler chickens. Five hundred three-day-old male Ross 308 broiler chicks (average body weight 101.42 ± 3.22 g) were assigned to five experimental groups, each with 10 replicates (10 birds per replicate). The chicks in the first group received a basal diet and were maintained in thermoneutral conditions (NEG CON). The remaining four groups received either a basal diet only (POS CON) or a basal diet added with SP (2 g/kg of feed; SP group), HA (5 g/kg of feed; HA group), or a mix of SP and HA by the same doses (SP+HA group). The four groups were exposed to acute cyclic heat stress (36 °C ± 2 °C) from the 22nd to the 25th day of age for 6 h/day. The HA group showed increased body weight and body weight gain, and improved feed conversion ratio compared with other groups (<i>p</i> < 0.001). The addition of SP and HA improved sensory characteristics and reduced the dripping losses of the breast muscles. The phagocytic % and phagocytic index were higher in the SP group compared with the NEG and POS CON. The serum levels of triiodothyronine and thyroxine were higher in the HA and SP+HA groups compared with the control groups. The serum concentrations of interleukin-10, complement 3, and lysozymes, as well as the liver concentrations of HSP90A and HSP90B, were higher in the SP, HA, and SP+HA groups compared with the NEG and POS CON. The duodenal villous height and width were significantly greater in the HA group compared to the other groups. Spleen histomorphology in the SP and HA groups was better than that of the POS CON. The HA group showed up-regulation in the immune expression of clusters of differentiation 3 (CD3) and 20 (CD20) proteins in the spleen tissues. In conclusion, both HA and SP, individually but not in combination, mitigated the detrimental effects of acute cyclic heat stress on growth and immunity. Humic acid addition provided the most pronounced improvements in performance and intestinal morphology. Further studies are warranted to clarify the biochemical interactions between SP and HA under different stress intensities.