Farm Animal Metabolism and Nutrition

region (Murray and Williams, 1987). Since
NIR absorptions are weaker, longer path-
lengths can be used for liquids (particularly
where water is involved) and for solids;
unlike the MIDIR, dilution with KBr or
preparation of KBr pellets is unnecessary.
The NIR spectral region, in various
spectroscopic forms (diffuse reflectance,
transmission, etc.) over the last decade has
been used increasingly to determine the
composition and quality of many products
and to monitor the progress of various
biological or chemical processes (Marten et
al., 1989; Kemeny, 1992). With solids, NIRS
in the reflectance mode has been used
extensively to determine the composition/
quality of materials such as hays (Marten et
al., 1989), silages (Reeves et al., 1989;
Reeves and Blosser, 1991), grains (Osborne
et al., 1987; Tkachuk, 1987) and food
products (Osborne and Fearn, 1986). In the
pharmaceutical industry, NIRS has also
being used to monitor the identity of the
raw materials arriving at loading docks
(Ciurczak, 1992b). Transmission NIRS has
been used extensively to monitor biological
processes, such as fermentations and
chemical reactions (Kemeny, 1992).
The successful application of NIRS to
problems in such diverse areas is due to the
nature of the absorptions in the spectral
region, and the variety of instrumentation
available. Near infrared spectrometers based
on the use of filters, gratings, acoustics,
interferometers and crystals are in use
(McClure, 1987; Williams, 1987; Workman
and Burns, 1992). Sampling devices include
rotating and stationary sample cups and
fibre optic probes (Kemeny, 1992).
Despite the widespread acceptance and
use of NIRS, there is still a great deal of mis-
understanding about its use and a need for
more research on the basis of NIR spectra.
On the spectroscopic side, decades of work
have been carried out to identify spectral
bands in the MIDIR (Colthup et al., 1990)
and programs are even available which
automate the process (Sadtler IR Mentor,
Bio-Rad Sadtler Division, Philadelphia,
Pennsylvania). Conversely, NIR spectral
interpretation can be described as prelimin-
ary at best (Murray and Williams, 1987;

Ciurczak, 1992a). This can be largely traced

to the fact that most of the work in NIR has

been done by non-spectroscopists who were

and are more interested in the final results

than in theory.

In Fig. 9.1, a diagram of a diffuse

reflectance accessory for a NIR spectrometer

is shown. For animal feed samples, this is

one of the primary methods used to obtain

spectral information. Although work is per-

formed using transmission spectroscopy, the

majority of feed analysis is performed using

this diffuse reflectance method. As shown in

Fig. 9.1, several types of events occur when

light is shone on a sample contained in a

sealed sample cell. Only the NIR radiation

which penetrates fragments of the sample, is

scattered randomly from particle to particle

(diffuse reflectance) and eventually leaves

the sample cell in various directions is of

interest. Depending on the composition of

the sample, energy at various wavelengths

will be absorbed to various degrees, and it is

this absorption which provides information

on the composition of the sample. Unlike

the determinations previously discussed, to

determine the composition of feedstuffs by

NIR, a half dozen to hundreds of wave-

lengths are used, depending on the samples,

instrument used and constituent in question.

Thus, instead of a data set of absorbances at

a specific wavelength, one obtains an array

of absorbances for each sample, where each

element of the array consists of an

absorbance at a given wavelength. When

data are collected over a range of wave-

lengths at specific intervals and the

absorbances are plotted against the range of

wavelengths, the result is a spectrum, as in

Fig. 9.2 for lucerne hay and wheat straw.

While not all NIR determinations require a

complete spectrum as shown in Fig. 9.2, this

is the place where calibration development

begins.

Calibration Development

General aspects

Just as a calibration curve is necessary for

determining the protein or P content of a

186 J.B. Reeves III