Introduction
The thermal environment can have a negative influence on cattle welfare. Historically, Ames defined the thermoneutral zone as the thermal environment where an animal experiences optimum health and maximum productivity. Whilst cattle comfort and productivity may be compromised during exposure to cold, wet and/or windy conditions, there has been a predominant focus on the influence of hot weather on cattle, and other species. The impact of hot weather on cattle is of increasing importance, particularly in conjunction with the changing global environment.
For livestock production enterprises, climate change has the potential to alter the thermal environment, which may result in the climate having an increasingly negative impact on the welfare and productivity of cattle. Periods of hot weather are already associated with reduced animal health, reduced reproductive efficiency in both males and females, and decreased feed conversion efficiency. Therefore, it is likely that climate change will have a considerable impact on the economic viability of animal agriculture worldwide.
In spite of this, all animals possess the capacity to adapt to their thermal environment. Animals are capable of modifying their behavioral, physiological, and morphological characteristics, or a combination of these, in response to the thermal environment. This review has attempted to provide a rounded overview of the impact that heat stress has on bovines.
Climate Change
The effect of climate change is highly variable globally and is largely influenced by geographical location. Cattle and livestock enterprises have the ability to adapt to an increasing mean global temperature, the primary concern, however, is the ability of livestock to cope with climatic extremes, e.g., heat waves. Climate change has the potential to present as
(i) rapid changes in climate over a couple of years or
(ii) as more subtle changes over decades. However, irrespective of the manifestation of climate change, global warming is likely to have a significant impact on the stability and sustainability of livestock production worldwide. Globally, various climate change models are predicting a 1.1 °C to 6.4 °C increase in temperature by the end of this century.
Numerous species are likely to be negatively impacted by the changing global environment, due to changes in ecosystem microclimates. Many species have adaptations to cope with short-term climate variability, i.e., seasonal changes. However, these adaptations may not be successful for species survival with the predicted climate change.
Predicting the effect of climate change on livestock is somewhat challenging due to the interrelationships that exist between the animal and its surrounding environment, and the impact of human activity on these relationships. It is also important to consider the indirect effects of climate change on soil fertility and degradation, water availability, grain yield, quality and availability, and spread of diseases/pathogens that may potentially impact the cattle producers and their ability to manage periods of hot weather.
Irrespective of livestock productions contribution to climate change, animal production needs to increase to satisfy consumer demand. A challenge regarding the effects of climate change on livestock enterprises is how dependent the enterprise is on the thermal environment and what can be implemented to offset the impacts of increasing temperatures. The current effect of the thermal environment is estimated by the impact of climatic conditions on animal performance, health, and welfare.
Traditionally, the impact of hot weather has been referred to as heat stress. Heat stress is caused by a combination of environmental conditions that result in the effective temperature of the environment to be greater than the temperature range of the thermoneutral zone. However, factors, such as genotype, coat type and coat color, diet type and diet composition, body condition, i.e., fat coverage and deposition, performance, i.e., growth and lactation, health status, and degree of adaptation, are known to influence thermal balance.
Thus, throughout this review, the term heat load will be used rather than heat stress, as the term heat load incorporates the cumulative effects of animal factors and environmental conditions on the thermal comfort of animals and, therefore, becomes a better descriptor of an animal’s thermal balance.
Animals that are adapted to a hot climate generally exhibit reduced growth and reproductive efficiency, which is associated with the adaptive mechanisms that ensure survival. In extensive grazing systems, it has been identified that climatic constraints are not the only factor that negatively influences livestock production. The indirect effects of climate change will also influence pasture resources, potentially depriving grazing animals of nutrient requirements. Similarly, the changing climate may also result in droughts, ultimately resulting in feed and water scarcity for grazing animals. These situations can be associated with a decrease in growth and reproductive efficiency in livestock.
Although the concept of multiple stressors is becoming a focal research topic in small ruminants, the impact of multiple stressors has not been adequately researched, and as such, there is no information on large ruminants. Therefore, it is essential to explore the impact of multiple stressors on both dairy and beef cattle, particularly in conjunction with the changing global environment.
Implication of Hot Environmental Conditions
Animal responses to environmental stressors have been investigated for some time, and although knowledge continues to be developed, managing livestock to reduce the negative impact of hot weather remains challenging. Reductions in dry matter intake (DMI), growth, feed conversion efficiency, reproduction, milk production, and milk quality, are commonly observed when cattle are exposed to thermal stress. Quantifiable measures, such as physiological, behavioral, and biological responses to heat load have been identified as indicators of heat load. Physiological responses to heat load include increased sweating rate, respiration rate, breaths per minute, panting score, and body temperature.
Behavioral responses include alterations to posture, including increasing the proportion of time standing, increased duration in shaded areas or increased shade seeking, including shade provided from other animals, and body splashing at water troughs. Biological markers in the blood are also indicators in determining the level of stress an animal is under. Cattle also use adaptive behaviors to reduce heat load, primarily consisting of shade seeking, under shade structures or other animals, and the alignment of the body in accordance with solar radiation (W/m2) to reduce whole-body exposure to direct sunlight.
Nutrition and Eating Behavior
Heat production has a positive relationship with feed intake in ruminants, and it has been shown that heat production is closely associated with feeding time. Metabolic heat produced during microbial fermentation, accounts for 3 to 8% of the total heat production by cattle. As ambient heat load increases and DMI decreases there is a reduction in metabolic heat production. During hot weather, cattle compensate for the hotter conditions by consuming smaller meals, more frequently, and shifting feed intake to cooler parts of the day. Voluntary feed intake has been reported to commence declining when ambient temperature reaches approximately 25 °C to 27 °C . However, the ambient temperature at which DMI begins to decline is influenced by diet type and composition specifically diets with a greater proportion of roughage exhibit more rapid reductions in DMI. Variations in DMI are also influenced by breed (genotype), production status, health status, body condition, and days on feed.
Water Intake
Water is available to animals in three forms, free drinking water, water in feed, and water produced via oxidation of organic compounds or metabolic water. Water requirements of cattle are influenced by ambient conditions, diet type, breed (genotype), weight, and physiological functions. Daily water intake is also influenced by a number of body functions, including the regulation of core body temperature, growth and development, lactation and reproductive functions, digestion and metabolism, and hydrolysis of proteins, fats, and carbohydrates. Water intake is linked to DMI, with both feed intake and feed type influencing water intake. Furthermore, water intake is influenced by the amount of water gained from drinking, and eating, via metabolic water, and the amount of water lost per unit time through respiration, sweating, faces, urine, and lactation. However, an increase in water intake may also be a reflection of ruminants attempting to compensate for heat loads, particularly in un-shaded grazing systems.
Body Temperature
During periods of hot weather, an increase in core body temperature becomes a function of heat accumulated and dissipated between the animal and the environment. Therefore changes in body temperature can be considered to be a reliable indicator of heat storage and disrupted homeostasis. However, it is important to consider that body temperature is not static and exhibits a circadian rhythm, although is generally regulated within a ± 1 °C gradient.
Under thermoneutral conditions, the core body temperature of cattle is between 38 °C to 38.5 °C and a rectal temperature greater than 42 °C is considered to be lethal.
Conclusion
Climatic conditions are an important regulator in agricultural production systems worldwide. For livestock production, climate change has the potential to alter the thermal environment, which may have a negative impact on welfare and productivity. It is clearly evident that the thermal environment has an influence on the wellbeing and productivity of bovines. Regardless of climate change and the predicted changes to the thermal environment, hot weather will continue to incite heat load responses in cattle worldwide. Therefore, it is imperative that livestock production systems identify and utilize mitigation strategies that are efficient and effective at reducing heat load. In future years, an integrated approach to the adoption and management of mitigation opportunities will become increasingly important to support the sustainability of livestock production systems.
In anticipation of climate change and climate variability, there is a need to develop a greater understanding of the impact global warming is likely to have on biological parameters in cattle. However, this may be somewhat misleading as there is a level of uncertainty in the climate change predictions and what effect the changes will have on livestock in the coming decades. A more achievable objective may be to identify and establish effective management strategies for livestock under suboptimal conditions, rather than selection for maximum productivity and/or adaptability.
Furthermore, there is a need to accurately quantify the indirect effects of climate change on livestock enterprises, such as changing soil quality, water availability, grain, and pasture resources, and the changing distribution of diseases and pathogens. Developing a comprehensive understanding of the factors that influence heat load, including climatic, environmental, and animal, will allow for innovative mitigation strategies to be established. Enhancing mitigation strategies provides an opportunity for the continual improvement of animal welfare and productivity during periods of heat load.
Ravi Ranjan(Ph.D. Scholar)
Department of Animal Husbandry & Dairying,
Field of Specialization Animal Genetics & Breeding, Sam Higginbottom University of Agriculture, Technology and Sciences, Prayagraj, U.P
