CHAPTER ONE

                                                                                                        INTRODUCTION

Heat stress plays a significant role in cattle performance and likely will be of even greater importance in the future as climate change continues. Cattle are produced in a wide range of environments, some of which present thermal challenges to productive performance, even survival in extreme cases. High temperatures can have negative consequences for milk production, and for reproduction, welfare and health, in dairy cattle. Especially high yielding dairy cattle would be susceptible because their thermo-neutral zone is rather limited as compared to low yielding cows (Kadzere et al., 2002). As a result of heat stress, losses of 600 to 900 kg milk per cow per lactation has been reported with regard to milk production (West, 2003).Selection of cattle adapted to warm environments represents one strategy to mitigate the effects of heat stress. There are a number of environmental factors that contribute to heat stress; these include high temperature, high humidity and solar radiation. In tropical regions, animals must be able to balance heat production and heat gain from their environments with dissipation of heat through the skin and respiratory surfaces; simultaneously, they must avoid excessive thermal energy incoming from the environment (Da-Silva et al.,2003).

The productivity and health of these animals are being affected by adverse meteorological conditions prevailing in the tropical Africa, predisposing them to hyperthermia (heat stress) and hypothermia (cold stress) (Da-Silva et al.,2003). In the tropical conditions of Nigeria, heat stress is common during dry season, occurring between November and May and with a mean monthly rainfall of less than 51mm (Igono et al., 1982; Walter, 1969). The harmattan season is characterized by marked fluctuations in ambient temperature (AT) with high AT in the afternoon

hours of the day and relatively low temperature of about 10 ^OC in the evening and early morning hours of the day. The season is associated with a dry cold and dust-laden wind that blows from Sahara desert and low relative humidity (RH) (Ayo et al., 1998a). The hot-dry season is also characterized by high ambient temperature, relative humidity and long duration of sunshine. Of all the stress factors adversely affecting dairy production in the tropical environment, ambient temperature manifesting in hypothermia and hyperthermia and humidity changes are the most crucial. It has been shown that high ambient temperature and high relative humidity with wide fluctuations in the values result in heat stress which may alter many physiological parameters in livestock (Ayo et al., 1998b; Sinkalu et al., 2009). These may impair homeostatic mechanisms resulting in pathological changes and alteration in body homeostasis (Teeter et al., 2005).

The general homeostatic responses to thermal stress in mammals include a decrease in fecal and urinary water losses, a reduction in feed intake and production, and an increase in sweating, respiratory and heart rates. Most of the adjustments made by the cow involve dissipating heat to the environment and reducing the production of metabolic heat (Kadzereet al., 2002). As milk production increases in dairy cattle, the metabolic heat production rises with the metabolizing of large amounts of nutrients, which makes the high producing cow more vulnerable to high environmental temperatures and humidity than animals that are metabolically less active (Kadzere et al., 2002). High producing dairy cows must dissipate large amounts of heat produced during the metabolism of high dietary energy used for body maintenance and milk synthesis. Several studies reported that heat stress in dairy cattle affects production and reproduction (Garcia-Ispierto et al., 2007; Morton et al., 2007; Bryant et al., 2007). Despite the moderate effects of THI on milk production, some blood parameters related to energy balance and enzyme activities had significant alterations and cows in the middle of lactation had the highest changes in these parameters (Dikmen et al., 2008). These authors have indicated that the thermoregulatory characteristic of animals associated with this phenotype is probably due to a lower metabolic rate, increased sensible heat or evaporative heat loss, more efficient transfer of heat to the surface, or a combination of these adaptations. They further reported that dry bulb temperature is nearly as good a predictor of rectal temperatures of lactating Holsteins in a subtropical environment as THI. Hormones known to be homeorhetic regulators are also implicated in acclimatory responses to thermal stress. These include thyroid hormones, prolactin, somatotropin, glucocorticoids and mineralcorticoids. Triiodothyronine (T₃) and thyroxine (T₄) are hormones associated with metabolic homeostasis and susceptible to climatic changes (Perera et al., 1986). Johnson et al. (1988) also showed decline in thyroid hormones T₃ and T₄ in response to heat stress which is an attempt to reduce metabolic heat production in the cow. Stress response hormones (glucocorticoids) are elevated during initial heat stress exposure and then become depressed with prolonged periods of thermal stress. Wheelock et al. (2006) demonstrated that heat stress elevates plasma insulin concentrations in lactating dairy cows and this may be important for glucose disposal in peripheral tissues. The blood biochemical profiles are considered important in evaluating the health status of animals. The estimates of biochemical constituents are the prerequisites to diagnose several pathophysiological and metabolic disorders in cattle (McDowell et al., 1992). Numerous authors reported close relationship between blood levels of calcium, phosphate, total protein, aspartate aminotransferase (AST), alanine aminotransferase (ALT) and reproductive traits in dairy cows (Malik et al.,2003).