Impact of heat stress on milk and meat production

Global warming is a major concern during recent years, and the livestock sector will be one of the most affected segments of the agricultural industry. However, the effects of increasing temperatures on livestock will be different worldwide, based on latitude and farming systems. In addition to the direct effects of heat stress on animal productivity, global warming will also affect soil fertility, water availability, crop yield, and pathogen circulation (Thornton et al., 2009; Nardone et al., 2010). Therefore, in addition to the arid and tropical areas where heat already represents a major constraint, the most affected areas will be those of the subtropical–Mediterranean zones, which are exposed to considerable heat stress for 3 to 5 mo per year (Silanikove, 2000). Other critical factors in these areas are linked to intensive production systems, which are characterized by high farm animal density of high-producing selected breeds and managed in specialized farms. However, the pasture-based system is equally at risk mainly due to the indirect effects of climate change on pasture growth and water availability (Nardone et al., 2010). Exposure to uncomfortable thermal conditions (due to the combination of high temperature and humidity) overcomes the capacity of cattle to dissipate heat and leads to an increase in body temperature that exceeds the physiological limits (Ronchi et al., 1997). Such condition is called heat stress and impairs the welfare and productive performance of dairy and beef cattle. In this condition, efficiency of nutrient conversion to energy reduces dry matter intake and increases water consumption, and there is a reduction of efficiency in nutrient absorption. In this scenario, cattle performance worsens rapidly (Collier et al., 1982). To evaluate the simultaneous effect of temperature and humidity factors and to assess the risk of heat stress in cattle, the temperature–humidity index is used. In the case of dairy cows, climate change has an important effect on milk organic and inorganic composition (Mariani et al., 1993, 1998). Climate change also influences the efficiency of cheese manufacturing processes (Sommerfeldt and Baer, 1986), both on cheese yield and on quality and especially for those cheeses produced using raw and not standardized milk. Heat stress in beef cattle is usually considered less severe than in dairy cattle because beef cattle have a higher average temperature–humidity index threshold due to their lower metabolic rate and lower body heat production (St-Pierre et al., 2003; Nardone et al., 2010). However, beef cattle also will compensate for increased body temperature by homeostatic mechanisms (panting, sweating, and urination) and behavioral alterations such as reduced activity, increased water intake, and reduced feed intake, which will take place preferentially during the coolest hours of the day (Magrin et al., 2017; Marchesini et al., 2018). The changes in feeding behavior will reflect on the efficiency of rumen function up to the onset of metabolic disorders such as ruminal acidosis (Marchesini et al., 2018). The main consequences are generally lower growth rate and reduced fertility of both males and females (St-Pierre et al., 2003). The economic losses due to heat stress were estimated by St-Pierre et al. (2003) for the major livestock industries in the United States. In the dairy and beef industries, heat stress had a negative economic impact of $897 million and $369 million per year, respectively. Therefore, this article focuses on the main aspects linked to the effects of heat stress on dairy and beef production. Implications