In ruminants, feeding diets high in grain and other highly fermentable carbohydrates increases the risk of ruminal and blood acidosis. Although, ruminaI pH varies considerably within a d, cows possess a highly developedsystem to maintain ruminalpH within a physiological range. However, if the acid production from fermentation is more than the system which can be buffered, ruminal pH compensation fails and it may drop drastically (Krause and Oetzel, 2005). An arterial and/or venous blood gas (ABG and VBG, respectively) is a clinical tool for determining pulmonary and metabolic status in animals. However, most blood tests are done on a sample of blood taken trom a vein due to the following: (i) Collecting blood from an artery is more painful than collecting it trom a vein because the arteries are deeper and have more nerves, (ii) Artery may be inaccessible due to periarterial tissues (overlying muscle, connectivetissue, or fat). The importance of venous blood gas measurements in the ruminant is useful to predictsome health problems such as ruminal parakeratosis, erosionad ulceration ofthe ruminal epithelium(Garry, 2002), laminitis,sole abscesses and ulcer (Nocek, 1997). Therefore, the determination of ABG or VBG in ruminal acidosis can prevent some health problems (Garry, 2002). ABG and VBG provide a direct measurement of partial blood pressures of carbon dioxide (PCOz) and oxygen (P02), hydrogen ion activity (PH), total hemoglobin (Hb total), oxyhemoglobin saturation(HbOz)and bicarbonate ion concentration (HC03-). The objective of the present experiment was to investigate the effect of diets providing different concentrate to alfalfa hay ratios on partial bloodpressures of carbon dioxide (PaC02) and oxygen (POz), hydrogen ion activity (pH), oxyhemoglobinsaturation (HbOz) and bicarbonate ion concentration (HC03)in Holstein steers.

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