Pigs' Physiological Responses at Different Relative Humidities and Increasing Temperatures

Abstract

Effects of relative humidity and high temperature on physiological responses and performance was assessed using twelve groups of ten young female pigs each were housed in a pen inside respiration rooms. Daily temperature increased with 20 C from low (160 C) to high (320 C), but remained constant within a day. Relative humidity (RH) was kept constant at one of three levels of 50, 65 or 80 %. Average initial body weight (BW) was 61.7 kg ( 4.4 kg) and the average end weight of the pigs was 70.2 kg ( 4.9 kg). Respiration rate (RR), rectal temperature (RT), skin temperature (ST), voluntary feed intake (VFI), ratio water to feed (rW:F), heat production (HP) and animal gain were analysed. Animals were fed ad libitum and had free access to water. We determined the so-called inflection point temperatures (IPt). These are the room temperatures above which animal physiological changes occured. Results showed clear IPt for RR, RT, rW:F, VFI, HP. Respiration rate was the first physiological parameter that started to change. Its IPt ranged from 21.3 to 23.40 C. Heat production and feed intake reduced sharply above Ipts of 22.9 and 25.50 C, respectively (p < 0.001). Eventually, IPt for rectal temperature ranged from 24.6 to 27.10 C. Average daily gain was highest in treatment 50 %RH (p < 0.05). It is concluded that relative humidity has a, relatively, minor effect on heat stress in growing pigs. However, room temperature strongly affects pigs physiological changes and their performance. We conclude that 60 kg pigs housed in groups and fed ad libitum experience upper critical temperature between 21.3 and 22.40 C for respiration rate, between 22.9 and 25.50 C for heat production and feed intake, and 24.6 and 27.10 C for rectal temperature.