Climate change and impact on livestock: Part Two …effect of heat stress on cattle productivity

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EVERY year, throughout Zimbabwe, several reports of the alarming incidence of infectious, non-infectious and reproductive health issues of livestock come to my attention.

Factors that influence livestock health are extremely complex, involving environmental forces, including ecological, social, economic interest, individual or community behaviour.

Climate change will also have many knock-on effects on livestock health, both direct and indirect. 

The direct effect of climate change on livestock health includes temperature related issues to frequent disease incidence and death. 

Climate change is defined as an average weather condition of an area that is characterised by its own internal dynamics, and it can be affected by changing its external factors. 

In 1992, the UN Framework Convention on Climate Change (UNFCCC) defined climate change as the change resulting from long-term direct and indirect activities that induce changes in the compared time, which are much more than the natural change. On the other hand, the weather is a set of all the phenomena occurring in a given atmosphere at a given time. Climate change is an association of multi-dimensional effects on climate, including physical characteristics, causes and consequences 

The projected average increase of surface temperatures by the Intergovernmental Panel on Climate Change (IPCC) is primarily due to an increase of global atmospheric temperature and greenhouse gases – mostly carbon dioxide (CO2) concentration, precipitation variation and/or combinations of these factors.

Temperature plays a central role on livestock.  It affects rainfall, forage, production, reproduction and health. 

Forage productions are influenced by increased temperature, CO2 and/or combination of precipitation variation. However, livestock health is mainly affected by variation in precipitation and increased temperature. 

The anticipated increases in the frequency, severity and duration of temperature change in the near future as a result of global warming will directly impair animal production systems due to its effects on their health. 

The direct effect of climate change includes reduced fitness to fend off infections. These effects are compounded by thermal or heat stress conditions.  

Depending on the degree, duration and severity of heat exposure, livestock health can be affected by metabolic disorder, oxidative stress, immune suppression, decreased reproductive performance and death.

The indirect effects on livestock follow more intricate pathways and include climate influences on pathogen density, distribution and multiplication of vectors as well as vector-borne diseases, soil and food contagions as well as water-borne diseases. 

These currently impact heavily on livestock production for Zimbabwe.

Heat stress (HS) is simply defined as the point when animals cannot dissipate an adequate amount of heat from the body to balance the body thermal condition. 

To avoid this, every animal needs an ideal ambient temperature to maintain its thermo-neutral condition.  

Due to the increasing demand for animal protein, indigenous cattle have been gradually crossbred and replaced by exotic cattle. 

For instance, in Bangladesh, there are an estimated 2,3 million high-yielding crossbred cattle. The crossbreed was developed over time by crossing native cattle with Holstein–Friesian/Sahiwal/Sindhi cattle to increase productivity.  

But now it has been established that the higher-production cattle are less tolerant to heat stress (HS). 

For instance, Holstein-Friesian dairy cows are renowned for their milk production but are mostly prone to heat stress.

When the ambient temperature is over 25°C, high-yielding dairy cows become heat stressed with primary signs shown as increased body temperature and respiration rates.

As body temperature increases, feed intake and milk production decrease.

  Milk production can decrease from 22,4 to 19,2 kg/day if body temperature (rectal) increases from 38,8°C to 39,9°C.

Livestock are physiologically homoeothermic animals. 

They respond to high temperatures by increasing heat loss while concurrently decreasing the production rate of internal heat.  

This is possible by increasing respiratory and sweating rates while decreasing feed intake. 

Metabolic disorders occur during this physiological process.

Heat stress influences the lameness of farm animals contributing to ruminal acidosis or increased output of bicarbonate. 

The heat-stressed animal eats less frequently during the cooler times of day but consumes more during each feeding (reducing feed intake during the hotter part of the day, followed by increased feed intake at lower temperature).  

This is considered a major cause of ruminal acidosis, resulting in laminitis.

HS also affects an animals’ behaviour, such as lying recumbent.  Standing for long may compromise the structure of the hoofs and is associated with lameness in dairy cows.

Ketosis is a metabolic disease characterised by a relatively high concentration of ketone bodies such as acetone, β-hydroxybutyrate, and acetoacetate with a concurrent decrease of blood glucose levels. It develops when an animal in a severe state of negative energy balance suffers forceful lipomobilisation and accumulates ketone bodies, which develop from inadequate catabolism of fat.  

During HS, animals’ reduced feed intake, combined with increased energy requirement to fulfil body physiological demand, may cause negative energy balance that influences the mobilisation of adipose tissue. This consequently results in the development of ketosis. Cattle affected by ketosis lose weight and produce less milk.

Climate change also affects animal health by hampering their endocrine status, liver function, glucose, protein and lipid metabolism, saliva production, salivary HCO3 content and fitness and longevity.

In the past decades, there has been increasing research in livestock oxidative stress caused by HS. Oxidative stress in livestock may be related to a number of pathological conditions, including conditions that are relevant for livestock production and the general welfare of the animal.

Oxidative stress is caused due to an imbalance between oxidant and antioxidant molecules by increasing oxidants and/or decreasing antioxidants.   

Overall serum antioxidant levels decrease during the summer and postpartum periods in heifers and a correlation between them with HS found. It was also identified that total carotenes and Vitamin E level were reduced during summer. 

HS has been associated with an increased activity of antioxidant enzymes such as superoxide glutathione, dismutase and catalase peroxidase, which can interrupt adaptation response to increase the levels of reactive oxygen species (ROS).  

The immune system, which can be compromised by several factors, is a complex physiological defence mechanism that protects all living creatures from pathogens.  

Several studies have shown that HS may have a negative impact on the immune system in livestock. 

In poultry, chronic exposure to HS has been demonstrated to impair immune response.  

Reduced colostrum immune globulins in dairy cows impair calf immunity in lymphocyte function that hampers the efficacy of vaccinations and affects the functions of neutrophils which are important for defence against bacteria.

Dr Tony Monda BSc, DVM, DPVM, is currently conducting veterinary epidemiology, and agro-economic research in Zimbabwe. For views and comments, email: tonym.MONDA@gmail.com

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