Nutritional and physiological responses of growing pigs exposed to a diurnal pattern of heat stress

Abstract

The effects of continuous heat stress (HS) on pig performance have been reported but how a diurnal pattern of HS affects the nutritional and physiological status of the pig is unclear. Two replicates of 6 pigs each (3 pairs of littermates; ∼25 kg BW) were assigned to one of two temperature treatments: control (20 °C; C) or diurnal HS (range 20 to 38 °C within 24 h; DHS) to provide six observations per temperature treatment. Pigs were housed in individual metabolism crates to facilitate separate collection of urine and faeces. After a 10-day acclimation period, pigs were fitted with indwelling arterial cannulae and a Foley bladder catheter for blood and urine collection, respectively. A wheat, barley and soybean meal-based diet was used and feed intake was paired between littermates in different treatment groups. After a 5-day recovery period, observations and sample collection were made during an 11-day experimental period. Plasma glucose and urea levels and the amount of minerals retained as a percentage of intake were similar between the treatments (P>0.05). Likewise, faeces DM and mineral digestibilities did not differ between the two temperature treatments (P>0.05). Diurnal heat stressed pigs had higher (P<0.05) respiratory rate (105.2 vs. 15.0 breaths/min), rectal temperature (39.7 vs. 38.4 °C), heart rate (104.0 vs. 90.1 beats/min) and mean blood pressure (97.4 vs. 81.7 mm Hg) compared to C pigs. Arterial pO₂ rose (89.73 vs. 77.91 mm Hg; P<0.05) and bicarbonate levels declined (25.82 vs. 27.24 mmol/L; P<0.05) in the DHS compared to C pigs, but no differences in pH, pCO₂ and base excess levels were observed between the two groups. Blood osmolality was lower (P<0.05) in DHS than in C pigs (287.1 vs. 291.3 mOsmol/kg) over the 11-day study period. Diurnal heat-stressed pigs had lower urine pH at 0700 (6.065 vs. 6.599; P<0.05) compared to C pigs. Diurnal heat-stressed pigs had higher (P<0.05) urinary levels of titratable acid (22.84 vs. 14.95 mmol/L), net acid concentration (33.36 vs. 23.49 mmol/L), and ammonium (11.05 vs. 9.88 mmol/L) compared with C pigs at 0700 but not at 1430 and 1900. Plasma T4 and aldosterone levels were not affected by temperature (P>0.05) but T3 levels were lower (16.19 vs. 23.46 ng/dL; P<0.05) in DHS compared to C pigs. It was concluded that although the diurnal pattern of heat stress used in the current study had no effect on mineral metabolism in growing pigs, it altered the physiological status, including acid–base homeostasis, as well as the more obvious thermal homeostasis.