The summer months in South Africa are characterised by high temperatures. This article examines the impact of heat stress on animal production and emphasises the importance of clean, fresh drinking water.

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Climate change is a reality in large parts of Southern Africa and other parts of the world. It leads to higher temperatures, sporadic rainfall and frequent droughts. Heat stress in high-producing animals can have a dramatic impact on production, with a drop of up to 35% in milk production.

It is essential that one reconsiders nutritional strategies and investigates other routes to minimise the impact of heat stress. Heat stress is defined as any environmental condition that exceeds the animal’s thermal comfort zone. This usually ranges between 5 and 20°C in dairy cattle (Armstrong et al., 1993).

Temperature and humidity

Temperature must be considered in relation to the relative humidity. This is expressed as the temperature humidity index (THI). A THI of 68 is considered sufficient to cause heat stress in high-producing animals.

An animal can use two routes to regulate its body temperature. The first route is increased heat output by, among other things, panting (shallow, rapid breathing) and increased subcutaneous blood flow. This increases the animal’s energy requirements for maintenance purposes. The second route serves to limit the animal’s internal heat production by adjusting its activities and feeding habits. This route tends to drastically reduce dry matter intake (DMI).

Metabolic heat production

Specific nutrients are involved in metabolic heat production. The first nutrient that affects heat stress is fibre. It has been well documented that fibre digestibility and fibre levels have a drastic impact on DMI, chewing and rumination habits, and digestive heat increments.

The energy density of feed must be adjusted during warmer periods, while the protein and amino acid supply must also be optimised. This is particularly important in intensive production, especially in dairy herds that tend to select against poor-quality roughage. This leads to a reduction in rumen pH.

A reduction in rumen pH causes decreased fibre digestion and a lower bypass rate. The animal’s metabolic buffer system is also affected by the panting action and increased saliva evaporation. It is therefore important to assess the energy density of the ration, which can be successfully manipulated using, among other things, rumen-inert fats.

Mineral nutrition is also essential as heat stress can alter mineral turnover. Furthermore, mineral nutrition must accommodate metabolic buffer requirements, as panting causes more CO2 to be expelled. Heat stress also leads to the increased production of free radicals. Minerals such as selenium are therefore important to support the animal’s antioxidant system.

Importance of water

Water is the most important nutrient during heat stress, as it is involved in thermoregulation. The ability of animals to adapt to water restrictions or heat stress is influenced by the evolutionary adaptation of the species and breed.

For example, well-adapted sheep can survive in extensive conditions by taking in adequate amounts of water every second or third day, without limiting their production. However, the availability of fresh, clean drinking water is non-negotiable in intensive conditions, such as feedlots or dairies. Water temperature can also have a significant impact on an animal’s thermoregulation.

Regulation of body temperature

The good news is that various feed additive technologies can limit the impact of heat stress. The use of rumen-inert fats is already an established practice in several production systems. Plant extracts and yeast cultures also have a positive impact on the regulation of body temperature.

Yeasts generally have a positive impact on nutrient digestibility and feed efficiency. They play a role in controlling the rumen pH, where a decrease in digestibility can occur. DMI suppression can thus be reversed in heat stress conditions by including a good, live yeast.

Using certain plant extracts can potentially support the thermoregulatory system. This can be attributed to its impact on vasodilation and the animal’s subsequent increased capacity to expel body heat.

The latter is a field of research that is receiving more attention. Early results indicate a positive impact on, among other things, rectal temperature, increased plasma activity of glutathione peroxidase (antioxidant status), decreased respiratory rate and an improvement in nutrient digestibility.

Important management strategies

There are three broad management strategies to limit the impact of heat stress in animals:

  • Manipulating the environment by providing shade, adequate air circulation, cool drinking water and environmentally controlled housing.
  • Developing the right breed for the environment. This should be a long-term management strategy.
  • Considering the nutrients involved. This can minimise the impact of heat stress.

For more information, phone Dr Francois van de Vyver on 018 011 8888 or send an email to Francois van de Vyver, national technical manager: ruminants, Nutri Feeds