May 2017 Newsletter - #64
In This Issue
About Us
The Dairy One Forage Lab excels in providing high quality analyses  
and customer service. Our goal is to provide analytical services designed to meet the expanding demands of modern agriculture.  
New technology and traditional methods are combined to deliver fast, accurate results.
What you always wanted to know about NIR analysis - Part 3     
By  Paul Sirois, MS, PAS - Lab Manager

 This is the third article in a series to enhance your understanding of the function and use of NIR technology.
How are samples prepared for NIR analysis?
Whether analyzing samples via wet chemistry or NIR, a uniform, homogenous, representative sample needs to be presented for analysis. Samples are blended, oven dried, then ground through a 1mm screen (see Fig 1). This essentially reduces the sample to a powder and helps insure that the subsample taken for each of the different procedures is uniformly the same.

 Figure 1. Alfalfa hay ground through a 1mm screen next
to corn kernels for comparison.

NIR calibrations are sensitive to particle size. Figure 2. shows the NIR spectra for grains of sugar of varying particle size. Closer to home, Figure 3. illustrates the results for the same sample of alfalfa hay ground at two different particle sizes. The samples were analyzed in triplicate at each grind size and the resultant averages are displayed in Table 1.. Note the NIR will predict different results for the different grind sizes. In this example, the 1mm grind size results are correct as the original calibration was based on a 1mm grind. The farther the particle size is from the grind size used for calibration development, the more disparate the results.

 Figure 2. NIR spectra of table sugar at various particle sizes.

Figure 3. NIR spectra of an alfalfa hay sample ground at 1 or 2mm. The samples were scanned three times at each grind size.

Grind size 

Table 1. Average results for an alfalfa hay sample
scanned at 2 grind sizes.

What about moisture?
There are two very strong moisture bands in the NIR Spectra (1450 & 1940 nm). Figure 4. is the NIR spectra of water. Having two definitive peaks makes NIR a potent tool for moisture determination.

 Figure 4. NIR spectra of water

What about the analysis of wet samples?
The cleanest calibrations are based on dried, ground samples that are relatively uniform in both moisture content and particle size. Beyond this, NIR technology is being employed to analyze intact samples in their natural state at the farm. Forage chopper mounted NIR units are used to measure the moisture content of forages as they're being harvested. Some handheld units are also available and a few systems are moving beyond moisture to predict other nutrients. This technology is being used with varying degrees of success. As described in Newsletters 62 & 63, the robustness of the reference database and reference methods employed will directly affect the usefulness of these calibrations.
A good example of the challenge of calibrating forages in their natural state is corn silage. Think about it - corn silage is made up of the stalk, leaves, husks, cob and grain - all of which vary in particle size and moisture content (Figure 5.). Corn silage is about as non-homogenous as you can get and this presents a real challenge when trying to develop "wet" calibrations.

Figure 5. Corn silage and diversity of particle sizes.
To compensate for this, several strategies can be employed:
  1. As with all sampling, it is imperative that a good, representative sample is presented for analysis. 
  2. The sample size must be large enough to capture the diversity of particles sizes in the intact sample.
  3. Collecting multiple spectra and compounding the data for predicting results.
  4. In conjunction with 3, scanning multiple locations across the sample.
For comparison, the analysis of a dried and ground sample is typically conducted in a spinning cup (Figure 6.). During the one minute it takes for analysis, the NIR collects 32 individual readings (spectra) from different sites as the spinning cup passes by the detector. These data are merged together to generate the final results. Note that this is done to smooth the spectra of particle sizes 1mm or less. Thus, you can appreciate the challenge of trying to capture the picture of a sample with much larger and diverse particle sizes. Strategies 1. - 4. must be employed to a greater degree to capture good information from the intact sample. In combination with this, the use of NIR as an on-farm tool will continue to improve with advancements in calibration software and expansion of sample databases.

Figure 6. NIR spinning sample cup packed with  
1mm ground alfalfa. 
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Dairy One - Forage Laboratory
730 Warren Road ~ Ithaca, NY ~ 14850
Phone:  1-800-344-2697 Ext. 9962