ARTICLE 68 FEED & ADDITIVE MAGAZINE August 2022 testine of oxygen, leading to ischemia and consequently oxidative stress. This further may lead to a reduction in small intestinal weight, the number of villi, and enterocyte proliferation, which considerably reduces performance due to inadequate intestinal development and nutrient absorption (Ayo and Ogbuagu, 2021). Heat stress also changes ion pump activity and increases the permeability of tight junctions of the small intestinal epithelium (Goel, 2020). This condition tends to compromise the tight junction barrier, which leads to leakage of luminal substances onto the bloodstream, the so-called "leaky gut." During stressful situations, microbial communities in the intestine can become dysfunctional, increasing the risk of colonization of harmful pathobionts. Increased production of ROS during heat stress can raise intestinal permeability leading to the translocation of bacteria and their endotoxins from the intestinal tract into the circulation. This will likely enable the penetration of potentially harmful bacterias such as Salmonella sp., Clostridium sp., and Escherichia coli through the gastrointestinal tract. PHYTOGENIC FEED ADDITIVES (PFA): A PROVEN TOOL TO OVERCOME HEAT STRESS Several nutritional strategies effectively reduce the negative effect of heat stress by maintaining energy intake and electrolytic/water balance. Vitamin E and C were found to be the foremost compelling, enhancing feed intake and body weight of broilers reared under heat stress conditions (Nawaz et al., 2021). Apart from these common ingredients, other additives such as probiotics, prebiotics, botanical substances, betaine, etc., have also been presented to alleviate some of the adverse effects of heat stress (Goel, 2020) Bioactives in PFA with antioxidant activity, such as polyphenols, could serve as a means to alleviate heat stress. By elevating antioxidant enzyme activities and inhibiting lipid peroxidation, these substances can reduce intestinal mucosal oxidative injury and ROS‐associated gene expression (Vandana et al., 2020). Specific PFA bioactives can also module the heat shock proteins to protect the integrity of the intestinal mucosa of heat‐stressed broilers. They can boost the expression of stress response proteins such as HSP via HSP70-leptin signaling pathways and antioxidant enzymes to improve feed intake in heat stress in broilers (Surai, 2015). By Tang et al. (2018), purified rosemary extract could induce high levels of HSP70 and CRYAB in chicken in vivo myocardial cell model and, therefore, could be used to alleviate heat stress in broiler chickens. Resveratrol could inhibit the higher HSP expression, antagonise the NF-kB and initiate the expression of tight junction mRNA, resulting in alleviation of heat stress-induced impairment of the intestine and better performance of broilers (Zhang et al., 2017). The incorporation of cinnamon powder improves oxidative stability by increasing superoxide dismutase (SOD), catalase, and total antioxidant capacity and lowering corticosteroid concentrations (Vandana et al., 2020). Phytogenic feed additives such as essential oils or spices can improve nutrient digestion and utilization by stimulating digestive enzyme production and activity, higher secretion of bile acids, and enhanced activities of trypsin and amylase. PFA can thus effectively counteract heat stress-related reduction in feed intake and enhance nutrient digestibility (Windisch et al., 2008). PROTECTIVE EFFECTS OF PFA ON GROWTH PERFORMANCE IN CYCLIC HEAT-STRESSED BROILERS A commercial PFA ("Comfort", Delacon Biotechnik GmbH, Austria) has been evaluated for growth parameters and carcass quality traits, as well as its potential underlying molecular mechanisms in broilers exposed to chronic cyclic heat stress in a trial at the University of Arkansas, USA. Greene et al. (2020) study, is published and available in the
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