Physiological traits as affected by heat stress in sheep—A review

Abstract

The exposure of sheep to elevated ambient temperatures induces an increase in the dissipation of excess body heat, in order to negate the excessive heat load. Dissipation of excess body heat is excluded by evaporation of water from the respiratory tract and skin surface via panting and sweating, respectively. Sweating in woolled sheep is much less effective due to the presence of the wool cover. With the elevation in environmental temperature to 36 °C, a high proportion of heat is dissipated via the ears and legs. When the physiological mechanisms of the animal fail to negate the excessive heat load, the rectal temperature increases. At the same time, such exposure of sheep to heat stress evokes a series of drastic changes in the biological functions, which include a decrease in feed intake efficiency and utilization, disturbances in water, protein, energy and mineral balances, enzymatic reactions, hormonal secretions and blood metabolites. Recent studies indicate that the suprachiasmatic nucleus (SCN) regulates the circadian and seasonal rhythms of most biological functions, particularly the reproductive functions and behaviour in mammals. The regulation includes the phasic and tonic release of hormones, oestrus and in some cases, gonadal size. This is due to the fact that there is evidence suggesting that the SCN is sensitive to changes in ambient temperature—with some cells being more responsive to cold and others more responsive to heat, although photoperiodic changes may also have a strong influence. In conclusion, it can be said that exposure of sheep to elevated ambient temperature negatively affects the biological functions which are reflected in the impairment of their production and reproduction traits.