Review: Near Infrared Spectroscopy of Faeces to Evaluate the Nutrition and Physiology of Herbivores

Abstract

Near infrared (NIR) spectroscopy, usually in reflectance mode, has been applied to the analysis of faeces to measure the concentrations of constituents such as total N, fibre, tannins and d 13 C. In addition, an unusual and exciting application of faecal NIR (F.NIR) analyses is to directly predict attributes of the diet of herbivores such as crude protein and fibre contents, proportions of plant species and morphological components, diet digestibility and voluntary DM intake. This is an unusual application of NIR spectroscopy insofar as the spectral measurements are made, not on the material of interest (i.e. the diet), but on a derived material (i.e. faeces). Predictions of diet attributes from faecal spectra clearly depend on there being sufficient NIR spectral information in the diet residues present in faeces to describe the diet, although endogenous components of faeces such as undigested debris of microorganisms from the rumen and large intestine and secretions into the gastrointestinal tract will also contribute spectral information. Spectra of forage and of faeces derived from the forage are generally similar and the observed differences are principally in the spectral regions associated with constituents of forages known to be of low, or of high, digestibility. Some diet components (for example, urea) which are likely to be entirely digested apparently cannot be predicted from faecal NIR spectra because they cannot contribute to faecal spectra except through modifying the microbial and endogenous components. The errors and robustness of F.NIR calibrations to predict the crude protein concentration and digestibility of the diet of herbivores are generally comparable with those to directly predict the same attributes in forage from NIR spectra of the forage. Some attributes of the animal, such as species, gender, pregnancy status and parasite burden have been successfully discriminated into classes based on their faecal NIR spectra. Such discrimination was likely associated with differences in the diet selected and/or differences in the metabolites excreted in the faeces. NIR spectroscopy of faeces has usually involved scanning dried and ground samples in monochromators in the 400-2500 nm or 1100-2500 nm ranges. Results satisfactory for the purpose have also been reported for dried and ground faeces scanned using a diode array instrument in the 800-1700 nm range and for wet faeces and slurries of excreta scanned with monochromators. Chemometric analysis of faecal spectra has generally used the approaches established for forage analysis. The capacity to predict many attributes of the diet, and some aspects of animal physiology, from NIR spectra of faeces is particularly useful to study the quality and quantity of the diet selected by both domestic and feral grazing herbivores and to enhance production and management of both herbivores and their grazing environment.

NIRS prediction of the feed value of temperate forages: efficacy of four calibration strategies

“…The statistical values associated to OMD and VI were similar to those reviewed by Dixon and Coates (2009) using NIR spectra of faeces, although for VI, there are reports of R 2 C values .0.95.…”

“…According to Dixon and Coates (2009), the NIR spectra of faeces closely resemble the NIR spectra of the forage from which the faeces are derived.…”

Faecal near-IR spectroscopy to determine the nutritional value of diets consumed by beef cattle in east Mediterranean rangelands

“…A rapid evaluation of ash content is important in order to assess if faecal samples are contaminated with soil. The R 2 value for the calibration of faecal ash (0.94, Table 2) is in the top range of values compilated from six studies by Dixon and Coates (2009). Our result for SEcv (13 g/kg DM), that is, the accuracy of prediction, is better than all values reported in the above review.…”

“…However, due to welfare regulations, alternatives have been used: only 4 of 32 FNIRS studies reviewed by Dixon and Coates (2009) relied on fistulated animals; all others were based on confined animals fed hay or mown pasture. The application of calibrations established in confinement to assess diets in free-grazing animals is probably justified for mono-specific artificial pastures (Boval et al, 2004) and has been tested in Australia under various conditions (Coates and Dixon, 2010), but lacks supporting evidence for highly heterogeneous pastures.…”

“…A first estimation of the accuracy of calibrations was by aid of cross-validation (with SEcv as estimate of quality), in which onesixth of the calibration samples were randomly sampled and used to validate calibrations calculated with five-sixths of the sample: SEcv was calculated as the average SE of partial validations. A criterion of quality for NIRS calibrations is that the ratio of performance to deviation (RPD, calculated as the ratio of SD to standard error of prediction (SEP) or, in the case of crossvalidation, to SEcv) be >2.5 (Williams, 2001;Coates, 2004; see also Dixon and Coates, 2009;Coates and Dixon, 2010).…”