Physiological Responses to Heat Stress and Their Modifications by Acclimatization: A Comparison of Natural And Experimental Conditions

Abstract

Broilers and laying hens of two breeds namely Tetra and Rhode Island Red were used to investigate some physiological responses to heat stress and their modifications by acclimatization. Laying hens exposed to intermittent high ambient temperature of 42oC brought about haemoconcentration during the initial heat intolerance phase. The intravascular water loss was accompanied by an increase of RBCs count, PCV, Hb concentration, plasma protein concentration, colloid osmotic pressure and a decrease in RBCs size. Transfer of laying hens to the residential temperature of 25oC, the pattern was reversed. Acclimatization was evident in all hens after 4 d. Heat tolerance phase of laying hens was characterised by haemodilution. Broilers reared for 8 weeks at high constant temperature (35oC) showed acclamatory haemodilution when compared with broilers reared at ambient temperature of 25oC. Intravascular water gain in both breeds was accompanied by decrease of the above mentioned parameters. Rhode Island Red and Tetra laying hens exposed to one of the following temperature regimes, intermittent (natural or high) and constant, showed no breed difference in tritiated water space and water turnover. Tritiated water space was maximum at the constant temperature when compared with other regimes. The shortest half-life (t½) for tritiated water space (highest water turnover) was observed in laying hens exposed to intermittent high temperature, while the longest t½ (lowest water turnover) was noted in laying hens reared under constant temperature. The initial exposure to intermittent high ambient temperate of 42oC brought about a considerable increase in the level of amylase activity in the duodenum and jejunum of the laying hens and only in the duodenum of broilers. The levels of amylase in the distal parts of the intestine of both breeds sharply decreased. The increase in amylase levels vanished after 3 days and onwards. The pancreatic level of amylase was reversibly related to the levels in the intestine. The response of maltase activity to heat stress and heat acclimation was insignificant.