Effects of Supplemental Recombinant Bovine Somatotropin (Rbst) and Cooling with Misters and Fans on Renal Function in Relation to Regulation of Body Fluids in Different Stages of Lactation in Crossbred Holstein Cattle (Report‪)‬

INTRODUCTION The low milk production of both exotic and crossbred cattle is still the main problem in dairy farming in the tropics. The regulation of milk secretion in different types of crossbred cattle has been shown to be inherited and is thought to be among the causes of differences in bodily functions. The lower efficiency of water retention and poor adaptation in a tropical environment have been reported in 87.5% HF animals in comparison with 50% HF (Chaiyabutr et al., 1997; 2000). There is a rapid reduction of milk yield as lactation advances to mid- and late-lactation in 87.5% HF animals. The reduction of milk yield is attributed to a decrease in mammary blood flow (MBF) coinciding with the decline of plasma bovine somatotropin (bST) concentration. These changes would account for the short lactation persistency (Chaiyabutr et al., 2000). In addition to animal genetics, other factors may affect milk production in dairy cattle in the tropics, such as high environmental temperature. Animals in high ambient temperatures will suffer excessive heat load and impairment of physiological function, including body fluids (Hahn et al., 1999).