Differences in natural carbon isotope ratios of milk and hair from cattle grazing tropical and temperate pastures

ABOUT 99% of all carbon is the 12 C isotope while 1% is 13 C. The precise ratio of the isotopes will vary depending on the material analysed. In plants, fractionation of carbon is brought about primarily by carbon dioxide assimilation in photosynthesis and is due to preferential utilisation of 12 C and exclusion of 13 C. Curiously enough, it has been found recently 1,2 that higher plants which fix carbon dioxide by way of the Calvin C 3 cycle pathway differ in 13 C/ 12 C ratios from plants which fix carbon dioxide through the C 4 -dicarboxylic acid pathway. Temperate pasture species fix carbon by way of the Calvin pathway and have 13 C/ 12 C ratios of approximately −28‰ (ref. 3 and M.M.L., J.H.T., and R. J. Jones, unpublished), whereas tropical pasture grasses fix carbon through the C 4 -dicarboxylic acid pathway 4 and have 13 C/ 12 C ratios of approximately −12‰ (ref. 2 and M.M.L., J.H.T., and R.J.J., unpublished). 13 C/ 12 C ratios are expressed relative to a carbonate standard 5 .