Classification of Potential Sheep Heat-Stress Levels According to the Prevailing Meteorological Conditions

Abstract

Many researchers have found that physiology, welfare, health, and productivity of ruminants are significantly affected when they are exposed to heat-stress conditions. In such cases, heat-stress may also cause a reduction of farmers’ profit in various ways. Heat-stress levels are significantly affected by the prevailing meteorological conditions. This paper aims to study the potential daily peak heat-stress conditions to which sheep are exposed and to classify them according to the prevailing meteorological conditions. For these purposes, temperature, relative humidity, wind speed and incoming solar radiation data were analyzed. The meteorological data were recorded during the summers of the period 2007 – 2012 by an automated meteorological station that operates in a rural area near the east coast of central Greece. Potential sheep’s heat-stress was assessed using the Temperature Humidity Index (THI). The analysis showed that the daily maximum hourly (DMH) THI value exceeded the extreme severe heat-stress threshold (THI ≥ 25.6) in 82% of the days. The average value of the DMH THI values during August was 29.0, being slightly higher than the corresponding value for July (i.e. 28.6) and 2.7 higher than the corresponding value for June. The classification of the DMH THI values according to the prevailing meteorological conditions was achieved by applying cluster analysis (CA). The 4 meteorological parameters mentioned above were used as variables in the CA. CA was performed by using the values of the meteorological parameters recorded when each DMH THI value was observed. CA is considered as an objective technique to group days with similar weather conditions. CA achieved to group extreme hot days (33.8 – 40.8 ^oC) and days characterized by high (28.5 – 32.3 ^oC), moderate (26.1 – 28.9 ^oC), relatively low (24.3 – 26.0 ^oC) and very low (20.1 – 20.6 ^oC) summer temperatures. In brackets, the first number corresponds to the average of DMH THI values calculated for the days included in each cluster. The second number corresponds to the average of temperature values recorded when the DMH THI values were observed, for each cluster of days. The impact of wind speed and incoming solar radiation on peak THI levels was also assessed. It was found that when wind speed and solar radiation increased, peak THI and temperature values also increased. The effect of wind speed to the thermal environment could be mainly attributed to mesoscale circulations that develop in the greater area. This paper showed that CA could be regarded as a useful tool to estimate the range of peak summer THI values, and consequently the magnitude of heat-stress, in relation to the prevailing meteorological conditions.