Effect of solar radiation on thermoregulatory responses of Santa Inês sheep and their crosses with wool and hair Dorper sheep

Abstract

This study aimed to assess the thermoregulatory responses of Santa Inês (SI), Dorper x Santa Inês (CH) and White Dorper x Santa Inês sheep (CW) to direct solar radiation in Southeast Brazil. Thirty adult non-pregnant and non-lactating Santa Inês (SI) hair ewes and their crosses with Dorper (hair ewes) and White Dorper (wool ewes) were allocated into three groups n = 10 and exposed to continuous solar radiation for three consecutive days. Ocular and surface temperatures, measured by infrared thermography, the rectal temperature, respiratory rate and sweating rate were collected at 7:00, 13:00 and 20:00 h. During the experiment, the black globe temperature reached a peak at 13:00 h, reaching mean values of 43.5 °C ± 0.45 °C, representative of severe discomfort for sheep. All genotypes showed an increase in surface temperature, reaching the maximum value at 13:00 h. The wool White Dorper x Santa Inês showed significantly higher surface temperature (dorsal, ventral, and shoulder) than the other genotypes. All the genotypes showed similar rectal temperature increases, peaking at 13:00 h, with values close to 39.4 °C ± 0.12 °C. At 20:00 h, all the genotypes decreased the rectal temperature (RT), albeit not reverting to the 7:00 h values. Changes in ocular temperature values mirrored the RT. All the genotypes presented high levels of evaporative heat loss. Even though all breeds significantly increased the respiratory rate, Santa Inês exhibited significantly higher values (146 bpm) than the others (112 and 117 for CH and CW, respectively). The sweating rate was very high in all genotypes, without differences among them, and exhibiting the same trend with a maximum value at 20:00 h. This behavior reflects the continuous effort to lose heat during the day, despite the decrease in black globe temperature. This study revealed analogous thermoregulatory responses among genotypes groups studied. The three genotypes showed similar heat tolerances, albeit presenting different thermogenesis and thermolysis dynamics, as evidenced by the maintenance of rectal temperatures within physiological limits even when subjected to intense high solar radiation.