The use of faecal Δ13C values to improve the reliability of estimates of diet quality when sampling tropical pastures with oesophageal fistulated cattle

Abstract

Summary. The nutritive value of extrusa samples collected from oesophageal fistulated cattle can be an unreliable indicator of the quality of the diet of resident cattle grazing the same pasture. This is largely because the botanical composition of the extrusa from 15–30 min of grazing may differ from that of the whole diet of the resident cattle. A methodology is described whereby the reliability of some attributes of diet quality can be improved by correcting for differences between the C3/C4 proportions of extrusa samples and the dietary C3/C4 proportions of resident cattle derived from their faecal δ13C values. Examples from 23 paddock samplings covering 2 sites, 3 experiments and 6 sampling occasions, are used to illustrate the application of the methodology. Carbon ratios (δ13C) of the naturally occurring isotopes 12C and 13C were used as an index of legume (C3)/grass (C4) proportions in extrusa samples and in the diet of resident cattle assuming dietary δ13C = faecal δ13C + 1. Regression relationships were developed relating the nitrogen, mineral and digestibility levels of extrusa samples to extrusa δ13C values. Where appropriate, these relationships were then used to estimate the level of these attributes in the diet of resident cattle from their estimated dietary δ13C values. Nitrogen and calcium concentrations in extrusa were usually closely related to legume content (δ13C) so that adjustments were considered very advantageous with average RSDs of 0.08 and 0.09% for calcium and nitrogen respectively. Boron and sulfur concentrations were also related to legume content but only at 1 of 2 sites. There was no apparent relationship between legume content and the concentrations of aluminium, copper, magnesium, manganese, selenium, titanium and zinc at either site. In general, in vitro digestibility of extrusa samples was poorly related to legume content. The use of alkane technology is suggested as a more versatile alternative to the carbon ratio technique and the benefits and limitations of the proposed methodology are discussed briefly.