Estimation of the proportion of C3 and C4 plant species in the diet of animals from the ratio of natural 12C and 13C isotopes in the faeces

Summary The relation between the ratio of the natural 12 C and 13 C isotopes of carbon in the feed and resultant faeces of animals was studied to develop a technique for estimating the proportion of C 3 species (tropical legumes) and C 4 species (tropical grasses) selected by grazing animals. In general, theδ 13 C values (see text for definition) of faeces from rabbits, sheep, goats and cattle were lower (more negative) than those of the corresponding feeds by from 0·4 to 2·0. This was possibly due to contamination in the gut by tissues or fluids with lower δ 13 C values. When C 4 and C 3 feeds were alternated, cattle took about a week to fully achieve the new level (δ 13 C of – 28·7 on the C 3 feed and – 13·1 on the C 4 feed) in the faeces. This time lag is associated with the time taken for the feed to move through the digestive tract. When mixed C 3 and C 4 feeds were fed to rabbits, sheep, goats and cattle there was a negative linear relation between percentage legume (C 3 ) in the feed and the δ 13 C of the faeces (P < 0·01). A decrease in one unit in the δ 13 C value was associated with an increase of 7·0–8·5% legume in the diet. Estimation of the percentage legume in the feed from the δ 13 C value of the faeces and of the C 3 and C 4 components of the diet, resulted in a consistent over estimation of the legume component because the faeces had lower values than the corresponding feeds. This bias was removed if the prediction was based on the δ 13 C of the feeds minus 1 unit; the legume percentage in the diets of the sheep, goats and cattle could then be estimated with a precision of about ± 5%. Differences in digestibility between the C 3 and C 4 components greatly bias the estimations. This bias in the diets fed to rabbits was effectively removed by using in vitro organic matter digestibility values of the two components to correct for the differences. Legume percentage in the diet could then be estimated with a RSD of ± 3%. Advantages and disadvantages compared with alternative methods of estimating the diet of grazing animals are discussed.