Review
Impact of heat stress on milk production, immunity and udder health in sheep: A critical review

https://doi.org/10.1016/j.smallrumres.2012.07.012Get rights and content

Abstract

The effect of heat stress on sheep milk production and on milk quality, in terms of fatty acid composition and coagulation properties, were discussed. The changes in sheep milk production in relation to some nutritional and management strategies, as feed supplementation, time of feeding and ventilation regimen, provision of shade and housing system were analyzed. The role of heat stress and nutritional and management strategies in immune response and mammary gland health were also discussed.

Introduction

Mediterranean countries including France, Greece, Italy, Spain and Turkey account for sheep milk production of about 2,900,000 MT (million tonnes), and were among the first twelve countries in the world for sheep milk production in 2009 (Faostat, 2012). The Mediterranean basin is characterized by hot, dry summers and cool, wet winters. Sheep are believed to be one of the most resistant species to climatic extremes, especially to high ambient temperatures. Temperatures in the Mediterranean basin, however, can often exceed sheep thermoneutral zone, which is comprised between 5 °C and 25 °C (Curtis, 1983). Temperatures higher than the upper critical point can affect both physiological and production performance of lactating sheep. Hot climates may increase energy requirements for maintenance by 7–25%, by inducing a rise in body temperature and breath rate. Feed intake decreases in heat stressed sheep (Marai et al., 2007), especially when they are offered low quality feed (Costa et al., 1992), due to both the attempt of reducing heat production and the reduction of feed transit through the digestive tract. Under these conditions, body reserves of fat and nitrogen are used to supply energy through gluconeogenesis at the expense of the mammary gland, especially in early lactating animals (Amaral-Phillips et al., 1993). In the Mediterranean basin, the rise in ambient temperatures occurs together with the late stage of lactation of dairy ewes. Both heat stress and the advancement of lactation concur to reduce mobilization of body reserve for milk synthesis, thus inducing a worsening of milk yield and quality. Milk from sheep is entirely destined for cheese production; as a consequence, sheep milk is requested to contain high contents of fat and protein to produce high-quality cheese. Heat stress can affect milk composition through a reduction of fat and protein content. High ambient temperatures can also result in plasma mineral imbalance, especially due to reduction in sodium, potassium, calcium and phosphorus and increase in chloride concentrations (Kume et al., 1987, Schneider et al., 1988, Caroprese et al., 2012). Published evidence does not prove that ambient temperature is directly related to udder health, but indicates that it may interact with other predisposing conditions to exert an influence (Klastrup et al., 1987). In the present review the effects of heat stress on the sheep production performance and on milk quality are discussed together with the effects on immunological state and udder health. Strategies to reduce the negative effects of heat stress in sheep are proposed.

Section snippets

Heat stress and sheep production responses

In sheep the efficacy of thermoregulatory mechanisms largely depends on sheep breeds and individual animal genetics. Thermoregulation in newborn lambs can be influenced by the environmental temperature the mothers were exposed to during pregnancy (Stott and Slee, 1985). Furthermore, lambs born from ewes of the same flock and breed, which were able to maintain low rectal temperature have higher birth weight than lambs born from ewes with higher rectal temperature, suggesting that selection of

Heat stress and sheep immune responses

Environmental factors such as ambient temperature have been implicated in alterations of livestock immune function. Heat response at cellular levels includes the synthesis of heat shock proteins, which are divided into a set of different families according to their molecular weights. The most studied of these proteins have molecular weights of approximately 90, 70 and 27 kDa and are referred to as HSP90, HSP70 and HSP27 (Guerriero and Raynes, 1990). HSP, whose expression and activity is strictly

Heat stress and sheep udder health

The incidence of udder health problems in sheep increases during summer because heat stress itself can negatively influenced sheep health by altering sheep normal physiological functions. It is apparent that microorganisms responsible for clinical or sub-clinical infections of ewe udder can increase in summer because of the warm environmental conditions and high relative humidity which occur in the Mediterranean basin.

Exposure to solar radiation has a detrimental effect on the hygienic quality

Conclusions

Field trials have clearly demonstrated that, even if sheep are considered to be among the most heat tolerant species, exposure to high ambient temperatures has a detrimental impact on their production performance, including milk nutritional and technological properties. Evidence exists that management and nutritional strategies able to reduce the negative effects of thermal stress on sheep milk production can also improve sheep immunological functions and udder health. So the reduction of

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