**4. Effect of environmental modification for dairy cows under tropical conditions**

Reducing heat stress in dairy cattle requires a multi-disciplinary approach. It involves breeding for improved heat tolerance, improved nutrition and improved reproductive management for the animals, and improved the structural design and the environmental control of their housing.

312 Milk Production – An Up-to-Date Overview of Animal Nutrition, Management and Health

which affects the reproductive function and embryonic development.

[36,37]. Heat stress delays follicle selection and lengthens the follicular wave having

Heat stress cause infertility and represented a major source of economic loss in dairy cows under tropical conditions. In recent study, the conception rate of dairy cows in Thailand decrease 20-30 % in hot season. The conception rate was lower in April and May when compared to other month, which lowest in summer and highest in winter (Figure 1). The effects of heat stress can be directly related to the increase in body temperature of dairy cow,

**Figure 1.** Conception rates of dairy cows in commercial farm located in the central part of Thailand.

**4. Effect of environmental modification for dairy cows under tropical** 

Reducing heat stress in dairy cattle requires a multi-disciplinary approach. It involves breeding for improved heat tolerance, improved nutrition and improved reproductive

enhance pregnancy rates significantly [24,41].

**conditions** 

The detrimental effects of heat stress on the reproductive performance of dairy cows have been well documented. These include a suppressed intensity of oestrus, a reduced preovulatory LH surge and decreased secretion of luteal progesterone [25], altered ovarian follicular development [33], decreased embryo development [38] and lower fertility [39]. In an attempt to minimize these effects, modifications to dairy cattle housing environments have been implemented to alleviate thermal stressors and improve cow comfort, through the use of shade, fans, sprinklers, and evaporative cooling [11,37,40]. These methods can

potentially adverse effects on oocytes quality [34] and follicular steroidogenesis [35].

Heat stress reduces milk production and reproductive efficiency. In an attempt to minimize these effects, modifications to dairy cattle housing environments have been implemented to alleviate thermal stressors and improve cow comfort [11,37,40,42]. The major objective of any cooling system is to keep the cow's body temperature as close to normal for as much of the day as possible. An acceptable range in rectal temperature is 38.5-39.3 C. There are two general approaches to cooling dairy cows. One is to modify the environment to prevent heat stress or to utilize methods that increase heat dissipation from the skin surface of cattle. The easiest and most obvious way to help heat-stressed cows is to provide shade. Direct sunlight adds a tremendous heat load to the cow and can be blocked by shades, but shade alone is inadequate to reduce the effect of heat stress. A more economical method to reduce the effect of heat stress is by evaporative cooling. Evaporative cooling can be accomplished by two approaches; 1) direct evaporation from the skin surface of the cows (fan and sprinkler combinations) and 2) indirect evaporation which involve cooling the micro-environment of the cows, with cooling pads and fans, in an enclosed barn. When water evaporates it absorbs heat, there by reducing the temperature and increasing heat dissipation from the skin of cattle. When water evaporates it also increases the relative humidity, due to the increased level of water vapor present. In hot and humid regions, evaporative cooling always requires the use of forced ventilation.

A number of studies have shown that housing systems in hot climates can be modified by the use of evaporative cooling to improve both milk production and reproductive efficiency of dairy cows [43-46]. There is a great potential to reduce temperature and THI. However, as relative humidity increases and or temperature decreases, the potential for evaporative cooling to modify the environment decreases. In hot and humid climates, high relative humidity reduces the potential of evaporative cooling. Therefore, there are questions regarding the effectiveness of evaporative systems in climates with high relative humidity. This chapter showed the impact of heat stress on postpartum cow performances and evaluated the effects of utilizing an evaporative cooling system for improved cow comfort and cow performance of early lactating dairy cows in a hot and humid climate.
