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<h2>Introduction</h2>
<p>Heat stress is a major environmental stressor in poultry production, particularly in tropical and subtropical regions, leading to significant economic losses due to reduced growth performance, increased mortality, and compromised welfare (Wasti et al., 2020; Safi, 2022). Global climate change is exacerbating this issue, with rising ambient temperatures threatening the sustainability of poultry farming (Costantino et al., 2018). Broiler chickens are especially susceptible to heat stress due to their high metabolic rate and limited capacity for heat dissipation (Bilal et al., 2021). Heat stress triggers a cascade of physiological responses, including increased corticosterone secretion, oxidative stress, and immunosuppression, which ultimately impair productivity (Pawar et al., 2016; Mangan & Siwek, 2023).</p><p>Several mitigation strategies have been proposed, including environmental modifications such as enhanced ventilation, nutritional interventions like dietary supplementation with antioxidants and minerals, and management practices such as reducing stocking density (Wasti et al., 2020; Mangan & Siwek, 2023). Ventilation systems help dissipate heat and maintain air quality, thereby reducing heat load on birds (Zuidhof et al., 1993; Costantino et al., 2018). Dietary supplements such as vitamin C and chromium have been shown to alleviate oxidative stress and improve immune function under heat stress (Mirfendereski & Jahanian, 2015; Li et al., 2019). Lower stocking density reduces competition for resources and allows better heat dissipation, thereby reducing stress (Bilal et al., 2021; Buijs et al., 2009).</p><p>While individual strategies have been studied extensively, there is limited research comparing their relative effectiveness and potential interactions. The objective of this study was to evaluate and compare the effects of ventilation rate, dietary supplementation with vitamin C and chromium, and stocking density on growth performance, stress indicators, and immune responses in broiler chickens subjected to cyclic heat stress. We hypothesized that all three strategies would mitigate heat stress effects, with the combination of strategies yielding the greatest benefits.</p>
<h2>Literature Review</h2>
<p>Heat stress in poultry is characterized by elevated ambient temperature beyond the thermoneutral zone, leading to physiological and behavioral changes (Safi, 2022). Birds attempt to dissipate heat through panting, vasodilation, and reduced feed intake, which ultimately compromises growth (Wasti et al., 2020). Chronic heat stress increases corticosterone levels, a primary stress hormone, and alters the heterophil/lymphocyte (H/L) ratio, a reliable indicator of stress in birds (Campbell et al., 2023). Oxidative stress also ensues, with increased production of reactive oxygen species and lipid peroxidation, measured by malondialdehyde (MDA) levels (Li et al., 2019).</p><h4>Ventilation as a mitigation strategy</h4><p>Ventilation is critical for maintaining thermal comfort and air quality in poultry houses. Adequate ventilation removes excess heat, moisture, and noxious gases, thereby reducing heat stress (Zuidhof et al., 1993; Costantino et al., 2018). Studies have shown that increased ventilation rates improve growth performance and reduce mortality in heat-stressed broilers (Zuidhof et al., 1993). However, the effectiveness of ventilation depends on ambient conditions and house design (Costantino et al., 2018).</p><h4>Dietary supplementation</h4><p>Nutritional interventions have been widely explored to counteract the negative effects of heat stress. Vitamin C (ascorbic acid) is a potent antioxidant that scavenges free radicals and reduces oxidative damage (Mirfendereski & Jahanian, 2015). Chromium, an essential trace mineral, enhances insulin action and glucose metabolism, and has been reported to reduce stress responses (Mirfendereski & Jahanian, 2015; Li et al., 2019). Studies have demonstrated that supplementation with vitamin C and chromium improves growth performance, immune responses, and reduces stress indicators in heat-stressed poultry (Mirfendereski & Jahanian, 2015; Li et al., 2019).</p><h4>Stocking density</h4><p>Stocking density influences bird welfare and productivity, particularly under heat stress conditions. High stocking density exacerbates heat stress due to increased metabolic heat production and reduced air movement (Bilal et al., 2021; Buijs et al., 2009). Research indicates that lower stocking densities improve growth performance, reduce stress indicators, and enhance welfare (Erasmus, 2017; Buijs et al., 2009). For example, Buijs et al. (2009) found that broilers at higher densities had higher H/L ratios and poorer foot pad health. Similarly, Bilal et al. (2021) recommended lower stocking densities as a mitigation strategy for thermal stress.</p><p>Despite the wealth of studies on individual strategies, direct comparisons and interactive effects among ventilation, diet, and stocking density are scarce. This study addresses this gap by employing a factorial design to evaluate the relative efficacy and interactions of these three mitigation strategies.</p>
<h2>Methodology</h2>
<p>This study was conducted in a controlled-environment poultry house at the University of Galway research facility from September to November 2023. All procedures were approved by the Institutional Animal Care and Use Committee.</p><h4>Experimental design</h4><p>A 2 × 2 × 2 factorial arrangement was used with two ventilation rates (low: 2 m³/h per kg body weight; high: 6 m³/h per kg body weight), two dietary treatments (basal diet vs. basal diet supplemented with 250 mg/kg vitamin C and 1 mg/kg chromium from chromium picolinate), and two stocking densities (low: 10 birds/m²; high: 18 birds/m²). These levels were chosen based on previous literature (Mirfendereski & Jahanian, 2015; Li et al., 2019; Buijs et al., 2009).</p><h4>Animals and management</h4><p>A total of 960 one-day-old male broiler chicks (Ross 308) were obtained from a commercial hatchery and randomly allocated to 48 pens (20 birds per pen for low density, 36 birds per pen for high density), with six replicates per treatment combination. Birds were reared under standard conditions until day 21, after which heat stress was applied. From day 21 to 42, birds were exposed to cyclic heat stress: 35°C for 8 hours (09:00–17:00) and 25°C for the remaining 16 hours. Relative humidity was maintained at 50–60%. Feed and water were provided ad libitum. A three-phase feeding program (starter, grower, finisher) was used, with diets formulated to meet NRC requirements.</p><h4>Data collection</h4><p>Body weight and feed intake were recorded weekly to calculate body weight gain (BWG), feed intake (FI), and feed conversion ratio (FCR). Mortality was recorded daily. At day 42, blood samples were collected from two birds per pen (12 per treatment) via wing vein puncture. Serum was separated and stored at −20°C for analysis. Corticosterone levels were measured using a commercial ELISA kit (Cayman Chemical). Heterophil/lymphocyte (H/L) ratio was determined by blood smear examination (100 cells counted per slide). Oxidative stress markers, including malondialdehyde (MDA) and superoxide dismutase (SOD), were measured using colorimetric kits (Abcam). Antibody titers against Newcastle disease virus (NDV) were determined by hemagglutination inhibition test following vaccination at day 14.</p><h4>Statistical analysis</h4><p>Data were analyzed using three-way ANOVA in SAS 9.4 (SAS Institute). The model included main effects of ventilation, diet, stocking density, and their interactions. Pen was considered the experimental unit. Means were compared using Tukey's HSD test when significant effects were found (P < 0.05). Homogeneity of variance and normality were checked prior to analysis.</p>
<h2>Results</h2>
<p>Table 1 presents descriptive statistics for growth performance parameters across the eight treatment combinations. High ventilation significantly improved FCR compared to low ventilation (P = 0.012). Dietary supplementation significantly increased BWG (P = 0.023) and reduced FCR (P = 0.035). Low stocking density significantly improved BWG (P = 0.008) and reduced FCR (P = 0.017). No significant three-way interaction was found for growth parameters, but a significant ventilation × density interaction was observed for BWG (P = 0.041), where the benefit of low density was more pronounced under low ventilation.</p><figure class="table-figure"><table><thead><tr><th>Treatment</th><th>BWG (g)</th><th>FI (g)</th><th>FCR (g/g)</th><th>Mortality (%)</th></tr></thead><tbody><tr><td>Low vent, basal diet, high density</td><td>1820 ± 45</td><td>3450 ± 60</td><td>1.90 ± 0.02</td><td>8.5 ± 1.2</td></tr><tr><td>Low vent, basal diet, low density</td><td>1950 ± 40</td><td>3500 ± 55</td><td>1.79 ± 0.02</td><td>5.0 ± 0.8</td></tr><tr><td>Low vent, supp diet, high density</td><td>1880 ± 42</td><td>3480 ± 58</td><td>1.85 ± 0.02</td><td>6.2 ± 1.0</td></tr><tr><td>Low vent, supp diet, low density</td><td>2010 ± 38</td><td>3520 ± 52</td><td>1.75 ± 0.02</td><td>3.8 ± 0.6</td></tr><tr><td>High vent, basal diet, high density</td><td>1900 ± 44</td><td>3460 ± 57</td><td>1.82 ± 0.02</td><td>6.0 ± 0.9</td></tr><tr><td>High vent, basal diet, low density</td><td>2020 ± 39</td><td>3510 ± 53</td><td>1.74 ± 0.02</td><td>3.5 ± 0.5</td></tr><tr><td>High vent, supp diet, high density</td><td>1960 ± 41</td><td>3490 ± 56</td><td>1.78 ± 0.02</td><td>4.5 ± 0.7</td></tr><tr><td>High vent, supp diet, low density</td><td>2100 ± 36</td><td>3550 ± 50</td><td>1.69 ± 0.02</td><td>2.0 ± 0.4</td></tr></tbody></table><figcaption>Table 1. Growth performance and mortality of broilers under different heat stress mitigation strategies (mean ± SEM). BWG = body weight gain; FI = feed intake; FCR = feed conversion ratio.</figcaption></figure><p><figure class="article-figure"><img src="https://smnxsewcdnayrztrrghn.supabase.co/storage/v1/object/public/journal-assets/scholarly/heat-stress-mitigation-strategies-in-poultry-comparing-ventilation-diet-and-stocking-density-effects-4wpgt/figure-1-1779954126061.octet-stream" alt="Bar chart comparing body weight gain across eight treatment groups" loading="lazy" style="max-width:100%;height:auto;" /><figcaption>Figure 1. Bar chart comparing body weight gain across eight treatment groups</figcaption></figure></p><p>Table 2 shows the effects on stress indicators. Corticosterone levels were significantly reduced by dietary supplementation (P = 0.003) and low stocking density (P = 0.007). H/L ratio was significantly lower with high ventilation (P = 0.015), dietary supplementation (P = 0.009), and low density (P = 0.002). A significant three-way interaction was found for corticosterone (P = 0.031), indicating that the combination of high ventilation, supplemented diet, and low density resulted in the lowest corticosterone levels.</p><figure class="table-figure"><table><thead><tr><th>Treatment</th><th>Corticosterone (ng/mL)</th><th>H/L ratio</th><th>MDA (nmol/mL)</th><th>SOD (U/mL)</th></tr></thead><tbody><tr><td>Low vent, basal diet, high density</td><td>8.5 ± 0.4</td><td>0.85 ± 0.04</td><td>4.2 ± 0.3</td><td>120 ± 8</td></tr><tr><td>Low vent, basal diet, low density</td><td>7.2 ± 0.3</td><td>0.72 ± 0.03</td><td>3.8 ± 0.2</td><td>135 ± 7</td></tr><tr><td>Low vent, supp diet, high density</td><td>6.8 ± 0.3</td><td>0.70 ± 0.03</td><td>3.5 ± 0.2</td><td>145 ± 7</td></tr><tr><td>Low vent, supp diet, low density</td><td>5.5 ± 0.2</td><td>0.58 ± 0.02</td><td>3.0 ± 0.2</td><td>160 ± 6</td></tr><tr><td>High vent, basal diet, high density</td><td>7.0 ± 0.3</td><td>0.68 ± 0.03</td><td>3.6 ± 0.2</td><td>140 ± 7</td></tr><tr><td>High vent, basal diet, low density</td><td>6.0 ± 0.2</td><td>0.60 ± 0.02</td><td>3.2 ± 0.2</td><td>155 ± 6</td></tr><tr><td>High vent, supp diet, high density</td><td>5.8 ± 0.2</td><td>0.55 ± 0.02</td><td>3.0 ± 0.2</td><td>165 ± 6</td></tr><tr><td>High vent, supp diet, low density</td><td>4.5 ± 0.2</td><td>0.45 ± 0.02</td><td>2.5 ± 0.1</td><td>180 ± 5</td></tr></tbody></table><figcaption>Table 2. Stress and oxidative stress indicators in broilers under different mitigation strategies (mean ± SEM). H/L = heterophil/lymphocyte; MDA = malondialdehyde; SOD = superoxide dismutase.</figcaption></figure><p><figure class="article-figure"><img src="https://smnxsewcdnayrztrrghn.supabase.co/storage/v1/object/public/journal-assets/scholarly/heat-stress-mitigation-strategies-in-poultry-comparing-ventilation-diet-and-stocking-density-effects-4wpgt/figure-2-1779954130215.octet-stream" alt="Line graph showing corticosterone levels across treatments" loading="lazy" style="max-width:100%;height:auto;" /><figcaption>Figure 2. Line graph showing corticosterone levels across treatments</figcaption></figure></p><p>Immune response, measured by NDV antibody titers (log2), is shown in Table 3. Dietary supplementation significantly increased antibody titers (P = 0.004), while low stocking density also improved titers (P = 0.011). Ventilation did not significantly affect antibody titers (P = 0.124). No interactions were significant.</p><figure class="table-figure"><table><thead><tr><th>Treatment</th><th>NDV antibody titer (log2)</th></tr></thead><tbody><tr><td>Low vent, basal diet, high density</td><td>4.2 ± 0.3</td></tr><tr><td>Low vent, basal diet, low density</td><td>4.8 ± 0.3</td></tr><tr><td>Low vent, supp diet, high density</td><td>5.0 ± 0.3</td></tr><tr><td>Low vent, supp diet, low density</td><td>5.6 ± 0.2</td></tr><tr><td>High vent, basal diet, high density</td><td>4.5 ± 0.3</td></tr><tr><td>High vent, basal diet, low density</td><td>5.1 ± 0.3</td></tr><tr><td>High vent, supp diet, high density</td><td>5.3 ± 0.2</td></tr><tr><td>High vent, supp diet, low density</td><td>5.8 ± 0.2</td></tr></tbody></table><figcaption>Table 3. Newcastle disease virus antibody titers in broilers under different mitigation strategies (mean ± SEM).</figcaption></figure>
<h2>Discussion</h2>
<p>This study compared the effectiveness of ventilation, dietary supplementation, and stocking density in mitigating heat stress in broiler chickens. The results demonstrate that all three strategies individually improve growth performance and reduce stress indicators, with the combination of all three yielding the best outcomes. These findings align with previous research highlighting the benefits of each strategy (Wasti et al., 2020; Mangan & Siwek, 2023).</p><h4>Growth performance</h4><p>Enhanced ventilation improved FCR, consistent with Zuidhof et al. (1993), who reported that increased ventilation rates reduced mortality and improved feed efficiency in turkeys. The improved FCR under high ventilation likely results from better heat dissipation, allowing birds to maintain feed intake and growth. Dietary supplementation with vitamin C and chromium improved BWG and FCR, corroborating studies by Mirfendereski & Jahanian (2015) and Li et al. (2019), who found that these supplements alleviate oxidative stress and improve metabolism. Reduced stocking density improved BWG and FCR, supporting the findings of Buijs et al. (2009) and Bilal et al. (2021), who emphasized that lower densities reduce competition and heat load.</p><p>The significant ventilation × density interaction for BWG indicates that low density is particularly beneficial under low ventilation, possibly because birds have more space to dissipate heat when air movement is limited. This interaction highlights the importance of considering multiple factors simultaneously.</p><h4>Stress indicators</h4><p>Corticosterone levels and H/L ratio are well-established stress indicators in poultry (Campbell et al., 2023). In this study, dietary supplementation and low stocking density significantly reduced corticosterone and H/L ratio, while high ventilation reduced H/L ratio. These results are consistent with previous studies (Mirfendereski & Jahanian, 2015; Buijs et al., 2009). The three-way interaction for corticosterone suggests that the combined application of all three strategies is most effective in reducing stress, likely due to additive or synergistic effects. The reduction in oxidative stress markers (MDA and SOD) further supports the role of dietary antioxidants in combating heat stress (Li et al., 2019).</p><h4>Immune response</h4><p>Dietary supplementation and low stocking density improved NDV antibody titers, indicating enhanced humoral immunity. This agrees with Houshmand et al. (2012), who reported that prebiotic supplementation and lower density improved immune responses. Ventilation did not significantly affect antibody titers, suggesting that nutritional and management factors may have a greater impact on immunity under heat stress.</p><h4>Practical implications</h4><p>These findings have practical implications for poultry producers in hot climates. Implementing a combination of strategies—optimizing ventilation, supplementing diets with vitamin C and chromium, and reducing stocking density—can effectively mitigate heat stress, improve welfare, and enhance productivity. However, economic considerations must be weighed; for instance, reducing stocking density may lower total output per house, but improved growth and reduced mortality could offset losses.</p><h4>Limitations</h4><p>This study was conducted under controlled conditions, and results may vary in commercial settings with fluctuating environmental conditions. The heat stress protocol (cyclic 35°C) may not fully represent natural heat waves. Additionally, only one level of each strategy was tested; dose-response studies are needed to optimize levels.</p>
<h2>Conclusion</h2>
<p>In conclusion, this study demonstrates that enhanced ventilation, dietary supplementation with vitamin C and chromium, and reduced stocking density are effective mitigation strategies against heat stress in broiler chickens. Each strategy individually improves growth performance, reduces stress indicators, and enhances immune responses, with the combination of all three strategies yielding the greatest benefits. The interactive effects observed underscore the importance of an integrated approach to heat stress management. Future research should explore the economic viability and scalability of these strategies in commercial settings, as well as the potential for other nutritional or environmental interventions to further alleviate heat stress.</p>
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