ALBERT

Description: SummaryA technique is described which uses the ratio of the natural 12C and 13C isotopes of carbon to calculate the proportions of Calvin pathway (or C3) species and C4 dicarboxylic acid pathway (or C4) species in mixed samples of shoots, roots or seeds. Mean percentage of a particular component can be predicted to within ± 2% of actual values with average standard errors of less than 1% with two component mixtures under good conditions using threefold replication. The technique is particularly useful for estimating the proportion of C3 and C4 species in samples of visually indistinguishable, intermingled root systems from mixed field communities or from competition experiments. The proportion of C3 and C4 species in both oesophageal fistula and feed samples can be obtained using this technique, and it is proposed that data from fistula and pasture samples could be used to determine the extent to which animals selectively eat C3 or C4 species. The advantages and disadvantages of the technique are discussed.

Description: SummaryThe relation between the ratio of the natural 12C and 13C isotopes of carbon in the feed and resultant faeces of animals was studied to develop a technique for estimating the proportion of C3 species (tropical legumes) and C4 species (tropical grasses) selected by grazing animals.In general, theδ13C 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 δ13C values. When C4 and C3 feeds were alternated, cattle took about a week to fully achieve the new level (δ13C of – 28·7 on the C3 feed and – 13·1 on the C4 feed) in the faeces. This time lag is associated with the time taken for the feed to move through the digestive tract.When mixed C3 and C4 feeds were fed to rabbits, sheep, goats and cattle there was a negative linear relation between percentage legume (C3) in the feed and the δ13C of the faeces (P 〈 0·01). A decrease in one unit in the δ13C 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 δ13C value of the faeces and of the C3 and C4 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 δ13C 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 C3 and C4 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.