Reducing Stocking Densities and Using Cooling Systems for More Adapted Pigs to High Temperatures When Reared in Intensive Conditions

Heat stress negatively affects animal welfare, health, and production efficiency, especially in pigs. Pigs have few functional sweat glands and are further hindered by a thick subcutaneous adipose tissue layer, which impedes heat dissipation in warm conditions. Outdoor pigs cope with hot periods by choosing their own environment (e.g., looking for shade and wallowing in mud). However, heat-stressed pigs raised in intensive, confined housing wallow in their manure to cool themselves, despite not being their preferred substrate, while avoiding contact with other pigs. In addition, pigs implement physiological adaptations in an attempt to cope with heat stress, with negative consequences on pigs' performance and carcass quality, causing dramatic losses in the swine industry worldwide. For this reason, to counteract the impact of heat stress in intensive pig farming, the implementation of multiple mitigation strategies is required. Thus far, the effect of the use of cooling systems and/or reducing the stocking density on pig welfare and performance has not been quantified, and the results can be of interest to the swine industry. This study is aimed at evaluating the effect of reducing stocking density and using cooling systems to mitigate the negative effects of high temperatures in growing pigs (females and castrated males) reared in intensive conditions (from 25 to 100 kg) during summer (June to October 2020). The experimental design was a 2 × 2 factorial where pigs were provided with an evaporative cooling system and/or raised at regular or at lower stocking densities (i.e., 0.68 to 0.80 m 2 /animal). Treatments were distributed in four different rooms containing sex-balanced pens with either castrated males or females. Temperature and humidity were recorded throughout the experiment, and the temperature-humidity index was calculated. Heat stress (HS) on pigs was measured through changes in animals' performance, animal-based indicators (dirtiness and activity budget) and physiological indicators (neutrophil/lymphocyte ratio and hair cortisol). The use of cooling, lowering stocking density and the combination of both strategies had positive effects on pigs' final body weight (+5 kg, +3 kg, +9 kg, respectively; p < 0.001). The prevalence of dirtiness was similar at the stocking densities tested, and no clear effect of the cooling system was found. Both mitigation strategies lowered the physiological indicators of stress, although only hair cortisone can be considered an indicator of HS. In conclusion, both mitigation strategies are effective in improving pig welfare and performance, especially when both are combined. The severity of the stocking density effect may depend on the severity of the temperature.