Research has shown that cattle are comfortable when the ambient temperature is around 25-30 degrees Celsius. It is not temperature alone, but a combination of humidity and temperature that affects the comfort of cattle, and hence the Temperature-Humidity Index (THI) is considered. Recently, another index incorporating solar radiation has also been proposed, since this has been found to adversely affect heat dissipation at the hair follicle level. With global warming becoming a reality, and many countries recording soaring ambient temperature, there has been clamour to evolve heat tolerant breeds. Excessive heat and humidity affects dry-matter intake and energy loss, resulting in losses in milk production and reproduction. In many tropical countries livestock generally have low production and fertility, and one of the reasons could be lack of comfortable ambience. Heat tolerance is negatively correlated with production (-0.3) hence higher selection pressure for production would always lead to a breed intolerant to heat. Although effects of abnormal THI can, to some extent, be reversed by manipulating the environment, but this option needs high investment and technology inputs.
There is confusion on how cattle develop heat tolerance after acclimatization over long period of time. Does heat tolerance has genetic basis? Scientists have tried to seek answers to these questions. It has been generally observed that cattle with short hair coat are more tolerant to heat as compared to long haired ones. Further work showed that short hair in cattle is due to dominant ‘Slick Gene’. Two breeds of non-Zebu cattle, Senepol and Criollo, have short hair skin coat, are heat tolerant and found to carry slick gene on chromosome 20. Another composite breed, Carora, from Venezuela (a cross between Brown Swiss breed and Creole breed), also have short hair skin coat and carry this gene. Effect of Slick gene has been shown to depend on age and lactation status. Investigations have shown that cattle carrying slick gene have, on average, body temperature lower by 0.18 to 0.4 degrees Celsius. Many breeders have considered crossing Senepol with other temperate breed cattle such as HF to introduce Slick gene into their progeny.
Indian breeds of cattle that are heat tolerant do not carry the Slick gene, hence their heat tolerance must be through a different genetic pathway, such as heat-shock genes.
Results show that Holstein-Friesian (HF) cattle carrying the Slick gene are better at regulating their body temperature. Thus breeders have shown that it is possible carefully breed cows for better heat tolerance by introducing the Slick gene.