Twenty-four steers, comprising British (Herefords and Hereford x Shorthorn crosses), Zebu (Africander), and Zebu cross (British x Brahman or Africander) breeds, were maintained either on pasture or yarded, and fed on diets of a low and a high nutritional value. Blood volumes were determined on five occasions at intervals of 3 months by the 51Cr labelling technique, plasma and red cell volumes being then derived from the venous haematocrit. The blood plasma, and red cell volumes are all very significantly, correlated with, and represent respectively 4.97, 3.27, and 1.70% of, the fasting body weight. To avoid the confounding effect of body weight, the parameters are expressed as "contents", i.e, in terms of volume per kilogram fasting body weight. In the grazing group breed differences were generally absent. This may in part be due to the fact that the Zebu crossbreds belonged mostly to the F2 and partly to the F3 generation. In the yarded group, where F1 crossbreds were compared with British steers, breed differences were more frequent. British steers tended to have higher plasma contents, but lower red cell contents and haematocrits. Nutrition had no effect on plasma contents, but good nutrition was generally associated with higher haematocrits, as well as with elevated blood and red cell contents. Seasonal differences were in evidence, and all parameters generally reached minimal values in winter or spring. The significance of these findings in relation to adaptation to a tropical environment is discussed. The haematocrit does not necessarily reflect changes in the red cell volume. There is also some indication that the water and plasma contents may be related. The possible usefulness of the red cell volume for predicting the body composition is discussed.
The effect of carbon dioxide on relative red cell volume
