Effect of scanning samples through polypropylene film on predicting nitrogen content of forage using handheld NIR

Authors: RUKUNDO, ISAAC - University Of Nebraska; DANAO, MACY-GRACE - University Of Nebraska; Mitchell, Robert - Rob; Masterson, Steven - Steve; WEHLING, RANDY - University Of Nebraska; WELLER, CURTIS - University Of Nebraska

Submitted to: AIMS Agriculture and Food
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/30/2020
Publication Date: 11/5/2020
Citation: Rukundo, I.R., Danao, M.C., Mitchell, R., Masterson, S.D., Wehling, R.L., Weller, C.L. 2020. Effects of scanning through polypropylene film on predicting nitrogen content in forage using handheld NIR. AIMS Agriculture and Food. 5(4):838-849. https://doi.org/10.3934/agrfood.2020.4.835.
DOI: https://doi.org/10.3934/agrfood.2020.4.835

Interpretive Summary: Interpretive Summary: Near infrared spectroscopy is routinely used for livestock feed quality assessment because it is simple, rapid, and inexpensive to use. Portable or handheld spectrometers are available for a fraction of the cost of benchtop instruments, making them easier to transport and be used where benchtop spectrometers are not feasible or possible. Forage samples often are stored in plastic bags or specimen vials after processing and preparation. At the time of analysis in laboratory spectrometers, the sample is placed in a cup with a glass top. Our objective was to compare the ability of handheld spectrometers and smartphone spectrometers to predict the nitrogen (N) concentration of perennial grasses taken through the sample storage container or the glass cuvette. If feasible, this would reduce sample analysis time and allow N concentration prediction in areas where laboratory scale spectrometers are not available. Our results demonstrate that scanning samples through plastic containers using handheld spectrometers affects the ability to predict N concentration of perennial warm-season grasses. However, this effect was easily corrected for by eliminating plastic absorption bands to improve instrument calibration. The same was not true for the smartphone spectrometer, where the removal of plastic absorption bands did not considerably correct for the plastic effect. Scanning through glass cups using the smartphone spectrometer led to more accurate prediction of N concentration, despite the advantages of using plastic packaging brings. Predicting N concentration based on scanning through plastic could be useful for rough screening applications or for developing rations in regions with limited access to benchtop spectrometers.

Technical Abstract: This study examined the effect of collecting near infrared (NIR) spectra of forage samples through a transparent polypropylene (PP) plastic film instead of glass cups on calibrating two handheld NIR spectrometers to nitrogen content (N). The first device was a transportable spectrometer (H1) covering 790–2500 nm at 1 nm interval, while the second device was a smartphone spectrometer (H2) covering 900–1700 nm at 4 nm interval. The spectra from each spectrometer were subjected to principal component analysis (PCA) to identify wavebands for PP packaging that would interfere in subsequent partial least squares (PLS) regression modeling to predict N. PCA results showed that the loadings of the first principal component (PC1) of the first derivative of the spectra from H1 and loadings of the second principal component (PC2) of the second derivative of the spectra from H2 were useful in identifying wavebands due to PP film. Regression models for H1 had better prediction performance when spectra were collected through glass than through PP films, in terms of coefficient of determination (r2 = 0.958), standard error of prediction (SEP = 0.96 g kg-1), and ratio of performance to deviation (RPD) = 4.93 vs. (r2 = 0.942, SEP = 1.13 g kg-1, and RPD = 4.17). Similar results were obtained for H2 using spectra collected through glass (r2 = 0.821, SEP = 1.73 g kg-1, and RPD = 2.72) than through PP (r2 = 0.499, SEP = 2.99 g kg-1, and RPD = 1.57). Removing peaks due to PP in the sample spectra improved the PLS models for H1 (r2 = 0.959, SEP = 0.94 g kg-1, and RPD = 5.02), but not for H2 (r2 = 0.521, SEP = 3.17 g kg-1, and RPD = 1.49). Hence, scanning samples through PP films can reduce the accuracy of predicting N, but for some handheld NIR spectrometers, this could be overcome by excluding wavebands due to PP.