Heat Tolerance
Heat stress is a considerable issue in livestock production system which negatively affects the feed intake and wellbeing of the animal, apart from reduced performance. Heat tolerance is the ability of an animal to maintain its genetic potential of optimum production under hot environmental conditions.
Heat balance is a complex phenomenon affected by many factors such as climate (environmental temperature, relative humidity, wind speed, radiant heat and altitude), animal (age, genotype, type of hair coat, level of acclimatization, health status, production level) and management (housing, provision of shade and fans). The level of heat tolerance varies among species, breeds and also between animals; for example, Holsteins are less tolerant than Jersey cows, beef cattle with black hair are more prone to heat stress by direct solar radiation than those with lighter hair. The lactating animals are more sensitive to heat stress than dry cows due to high metabolic heat production. Further, heat tolerance may be described as the rate of decline in milk, fat and protein yields per unit increase of THI based on the production ability. The decline in production in heat- tolerant cows is slower with regard to increasing heat stress when compared to heat stress susceptible cows. Goats are emerging as being more tolerant to heat stress than sheep, while both small ruminant species are superior to cattle. Sheep and goat have an advantage due to their morphological and physiological traits to withstand heat stress and dissipate body heat when compared to cattle. There is genetic variation in the performance of animals under heat stress conditions which can be identified for selection of heat-tolerant animals by genomic breeding. At present, genomic best linear equitable assessment is referred to determine genetic breeding value (GEBV) for heat tolerance in association with milk production and its composition. However, GEBV represents lower heat resilient for milk production when the THI is beyond 60.28.14
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