Evaluation of a heating protocol and stocking density impact on heatstressed fattening pigs

As climate change intensifies, heat stress mitigation for pigs becomes more important. Trials involving induced heat waves are useful to test several measures (e.g. reduced stocking density) at a faster rate, but only when accurately evaluated and validated. In the present study, we investigated the suitability of an artificial heating protocol at different pig weights (experiment 1). The impact of different stocking densities on fattening pigs during an artificial heat wave (experiment 2) was also investigated. Experiment 1: Forty 20-week-old pigs weighing 96.5 ± 7.3 kg (W₁₀₀) and forty 17-week-old pigs weighing 72.7 ± 9.9 kg (W₇₀) were housed in two compartments. An artificial heat wave (heat load) was induced for 3 days. During 3-day periods before, during and after the heat load, physiological parameters (respiration rate (RR), rectal temperature (T_rectal), skin temperature (T_skin) and behavior) were measured and average daily feed intake was observed. Ambient temperature, relative humidity and temperature-humidity index (THI) were monitored. Experiment 2: A total of 150 fattening pigs were randomly divided into three treatment groups: SD_1.3 (1.3 m²/pig), SD_1.0 (1.0 m²/pig) and SD_0.8 (0.8 m²/pig). All pens had a total pen surface of 4.88 m², corresponding with 4, 5 and 6 fattening pigs in the SD_1.3, SD_1.0 and SD_0.8 groups, respectively. The heat load was induced for 7 days on week 21. Respiration rate and T_rectal were observed as in experiment 1. Average daily gain and average daily feed intake were also noted. During the heat load, THI reached ≥ 75 (78.4 (experiment 1) and 78.6 (experiment 2)), even when relative humidity decreased to ± 45%. Every physiological parameter showed significant increases during the heat load. The prolonged heating protocol in experiment 2 also provoked significant decreases in average daily feed intake (15%) and average daily gain (19%) for all groups. Weight within the studied range of 70-100 kg did not have a significant impact on any of the parameters. However, T_skin was affected by both weight and heat load (P < 0.05), where T_skin from W₁₀₀ was always lower in comparison to W₇₀. In addition, we found that 0.8 m²/pig doubled the increase of T_rectal during the heat load, namely SD_0.8 (0.22 °C) compared to SD_1.0 (0.12 °C) (P = 0.033) and SD_1.3 (0.13 °C) (P = 0.053). This suggests that pigs housed at higher densities are less able to regulate their internal heat production. However, RR and performances were not significantly affected by heat load in this experimental set-up. A stocking density of 1.0 m²/animal may be sufficient to mitigate some negative effects of heat stress.