Effects of Cooling and Exogenous Bovine Somatotropin on Hematological and Biochemical Parameters at Different Stages of Lactation of Crossbred Holstein Friesian Cow in the Tropics (Report‪)‬

INTRODUCTION Low milk yield and short persistency of lactation of dairy cattle is still the major problem for dairy practices in the tropics. Low productivity of crossbred dairy cattle is influenced by a large number of factors. A high environmental temperature is known to be one of the problems for animal production in tropical areas. Heat stress affects the performance of dairy cows by reducing their dry matter intake, feed efficiency and milk production (Shibata and Mukai, 1979; Fuquay, 1981; Collier et al., 1982; Johnson, 1987; Huber, 1994). Environmental modifications have been performed in attempts to alleviate severe heat stress in dairy cattle for example, water spray and fans (Armstrong et al., 1985; 1993), evaporative cooling, (Armstrong et al., 1985; 1988; 1993; Ryan et al., 1992; Chaiyabutr et al., 2008a). In dairy cows, milk synthesis is dependant on hormonal stimuli and the provision of nutrients from the blood to the mammary gland to sustain milk production. A previous study in crossbred cattle containing 87.5% Holstein Friesian (HF) genes found that a short persistency of lactation accompanied decreases in plasma bovine somatotropin level (bST) and blood flow to the mammary gland during the transition period from early to mid-lactation (Chaiyabutr et al., 2000). However, cows treated with rbST exhibited increased milk yield which coincided with increases in both total body water and mammary blood flow (Chaiyabutr et al., 2007). Administration of bST has been shown to be one of the management strategies that are needed to minimize the effects of heat stress and that will maintain sufficient DMI to sustain the potentially increased milk yields (West, 1994). An increase in total body water in bST-treated cows may play a central role for both lactation and heat dissipation. However, controversy exists on the use of bST due to effects on animal health. A number of studies have noted the effect of bST on bovine health in terms of risk of clinical mastitis, reduction in fertility and risk of developing clinical signs of lameness (Dohoo et al., 2003). The adverse effects of exogenous bST have also been reported in other animal species (Skarda et al., 1992). The changes in relative mass of many organs and tissue growth following bST administration have also been reported (Moallem et al., 2004), which may reflect changes in bodily functions. According to this information, blood analysis will be a useful tool and a priority for diagnosis and health monitoring of animals in veterinary medicine (Payne and Payne, 1987) in prevention of metabolic disease or other disorders during intensive milk production. Few data are available for the additive effects of supplemental rbST and cooling on both health and productivity of crossbred dairy cattle in tropical environments. Therefore, the aim of the present study was to determine effects of supplemental rbST on hematological values and variability of metabolic parameters of crossbred dairy cows housed under shade with or without misters and fans at different stages of lactation. The results of this study will provide values of hematology and blood metabolites for systematic health monitoring and nutritional status and suggest possible preventive measures to reduce the risk of adverse effects during rbST supplementation in crossbred dairy cattle under high environmental temperatures.