Biological and mathematical analysis of desert sheep and goats responses to natural and acute heat stress, in Egypt

الملخص EN

Climate change is likely to be a main challenges livestock production faces in this century.

Climatic projections for arid and semi-arid zones include increasing annual mean and maximum summer temperatures, intensity of solar radiation and frequent of severe climatic events, e.g.

heat waves.

This study aimed to assess variation in physiological responses of desert Barki sheep and goats to both natural and acute heat stresses, under the hot-dry conditions of the Coastal Zone of Western Desert, Egypt.

Natural heat stress represents increasing ambient temperature, while acute one resembles the expected severe weather events.

Fifty nine ewes and twenty-five does were exposed to natural heat stress (NHS) under direct solar radiation in August for three successive days, and biological parameters representing thermal, respiratory and metabolic activities were recorded pre- and post-stress.

NHS significantly affected all parameters studied, in terms of change between post- and pre- exposure values.

Respiration rate (RR) was almost tripled with significant increase in minute ventilation volume (MVV), in response to NHS in sheep reflecting higher gas exchange than in goats.

Increase shallow panting, vasodilatation and decrease heat production (HP) showed to be the primary mechanisms for goats to dissipate heat load.

Animal heat tolerance index (AHTI) was developed to assess individual animal response to NHS based on the changes in rectal temperature (RT), RR, MVV, tidal volume (TV) and HP.

Respiration rate, followed by rectal temperature were the two main physiological parameters contributing to the AHTI, in both species.

To assess variation in body thermal response of the two species to acute heat stress (AHS), 68 desert Barki ewes and 31 Barki does (of which, 40 ewes and 22 does were involved in the NHS trial) were confined in a climatic chamber at>41C for 120 min., in Spring of 2010.

RT was measured at 3-minute intervals and its records were fitted to a linear mathematical model: y=a+b ln(x) + c (ln(x)) 2.

Mathematical fitting of RT, resulted in the classification of the sheep and goats into 4 groups according to the value of two main parameters of curve shape; peak value and post-peak slope.

The “Low peak with post peak Plateau, LPt” sheep and goat animals were proposed to be the most tolerant.

Meanwhile, the “Low peak and Exponential post-peak slope, LEx” goats and the “High peak with post-peak Plateau, HPt” sheep were considered the least tolerant.

Animal groups classified as the most tolerant showed to be more common under both NHS and AHS classes, reflecting possible association between tolerance performance of animals under NHS and AHS in the two species.