Heat Stress - The Often Neglected Factor

In my honest opinion, many cattle and goat breeders in Malaysia do not know the concept of heat stress or severely underestimate the threat of heat stress.

The fact is heat is a major constraint on animal productivity in the tropical belt and arid areas (Silanikove,1992). Growth, milk production and reproduction are impaired under heat stress as a result of drastic changes in biological functions caused by the stress (Habeeb,1992; Silanikove,1992).

How?

Let's begin with the research by Dowling done in 1955, before our Merdeka year. Dowling studied the difference between temperate cattle (British Shorthorn or bos taurus) and tropical zebu cattle (Indian brahman or bos indicus). He focused on thermoregulation, or how animals regulate their body temperature and what happened when animals are put in different condition.

Heat is produced by the body, it is a fact. Feed is turned to energy by a complex chemical combustion process, almost similar to car engine, but much more complex. This combustion process releases heat. Excess heat must be dissipated or removed, otherwise heat stress begins and the organs start failing. Whereas car engine uses radiator to dissipate heat and prevent engine overheating, animals have sweat glands under the skin to remove excess heat from their bodies.

Heat is also produced in the environment, mainly by solar radiation (the sun). Environmental heat does affect the mechanism of thermoregulation by the body. The body can absorb and lose heat from the surrounding environment through radiation, convection, conduction, evaporation of water and expired air. Humidity affect how fast or how slow the body releases heat into the environment. Low humidity means it is easier for the body to release excess heat. High humidity means it is harder for the body to release excess heat, unless there is some form of adaptation. In arid weather, relative humidity stands between 20% - 40%, in temperate weather, relative humidity stands between 40% - 60, and in tropical weather, relative humidity stands between 60% - 80%.

For example arid region where it is hot and dry is like South Africa, central and southern Australia, and the desert countries of Arabia.

Temperate region where it is not too hot, not too cold: characterized by four seasons (its comfortable, that's why it's temperate) is like countries around the Mediterranean, European, New Zealand, Japan and Canada etc.

Tropical region where it is hot and humid is like Southeast Asia, northern Australia, Brazil, Argentina, and West Africa

Usually, heat by body metabolism account for 30-40% of total heat load.

Warm blooded mammals will always try to maintain steady body temperature at 36⁰ Celsius. (Heat balance or homeothermy)

Now back to Dowling. He took sample of skin from the Shorthorn and the Brahman. He wanted to know the number of hair follicles and sweat glands and see if there is any difference between them. Our skin has lots and lots of hair and pores. We just could not see with the naked eyes. Dowling used microscope. The root of the hair is called a follicle. Next to each follicle is a pore and each pore has one sweat gland. Has I make myself clear? Ok, after some studying, Dowling found out that the skin of Brahman has higher density of hair follicles and sweat glands (1698/cm²) compared to Shorthorn (1064/cm²). Dowling also found out that sweat gland of the Brahman is much closer to the skin compared to the Shorthorn.

It means the Brahman is able to release much more excess heat and release it faster compared to the Shorthorn. How? Through elevated sweating rate. More sweat glands mean more sweats and more sweats means the faster the time for the body to cool down. This characteristic is very important for mammals that live in the tropics. It is our form of adaptation to the climate. Slower rate of heat loss will cause overheating in our bodies, endangering our bodily functions.

The Shorthorn on the other hand does not have the need for faster heat loss rate since it live in cooler climate. In fact, for temperate breeds, slower rate of heat loss is much more useful function, in order to preserve body heat during very cold nights and during winter. Slower rate of heat loss prevents temperate breeds from experiencing cold stress.

When temperate or arid breeds move into tropical climate, they experience heat stress, due to slower heat loss and when tropical breeds move into temperate or arid climate, they experience cold stress. Humidity is the determining factor here.

What are the effects of heat stress?

When the body is experiencing heat stress, the pituitary gland in the brain instructs the thyroid gland to produce/secrete less of hormones called thyroxin (T₄) and triodothyronine (T₃)(Habeeb, 1992). The concentrations of T₄ and T₃ in blood plasma were found to decline under heat stress conditions by up to 25% (Magdub 1982, Beede and Collier, 1986). Lower level of these hormones lead to decreasing metabolic rate, feed intake, growth and milk production.

Ever enter a sauna room? How does it feel? Stuffy, right? A sauna room is hot and very humid. After a few minutes, we feel uncomfortable. We sweat a lot, our breathing feel shallow, and we want to make a quick exit. ( I do not think sauna is appropriate for male anyway, since the heat kills off the sperms and suspend production of sperms, but for female maybe it is fine). A temperate or arid breed which enter our tropical zone feel exactly like entering the sauna. They could not stand the high humidity. Their body starts overheating and experiencing heat stress. That explains why boer, feral, toggenburg, saanen etc loses body condition, appetite and sexual urge when imported to our tropical country. In acute cases of heat stress, deaths can be expected. Just like what happened to so many breeders here when they receive their shipment of boers and ferals.

One more thing I would like to highlight. There is a relationship between temperature and breeding efficiency of ruminants. High environmental temperature which exceed comfortable level for ruminants, drastically reduce conception rate and increase embryonic loss (Cavestany 1985, Holmes 1986, Biggers 1987, Dollah 1990, Adballa 1993). Heat stress also affects ovum and sperm in the reproductive tract and alters the hormonal balance of the dam (Ingraham 1979, Ocfemia 1993, Stott 1972, Thatcher 1974).

Breeds of ruminants indigenous(local) to tropical and subtropical environment generally perform better than their counterpart from more temperate zones in terms of survival, reproduction and expression of their genetic potential for growth and milk yield (Finch 1984). These characteristic relate to their capacity to maintain their appetite under heat stress (Silanikove 1992) and also to the way their bodies maintain heat balance efficiently, reducing heat stress greatly.

The conclusion I can make is our tropical breeds has been designed beautifully to cope with our weather and the challenges it brings. We need to pay more attention to our own indigenous genetic resources, research more and protect them. There is so much potential in tropical breeds that have not been discovered. I am very sad when I see many in the industry glorify foreign breeds and ignore or give little value to our own local breeds. I see so much confusion in our small livestock industry that the future looks cloudy. Many breeders are threatened by misguided policies, stemming from lack of knowledge and ignorance. I hope some one with a clear head and a noble heart will arise, spearhead the industry and replace ignorance and falsehood with truth and facts.