As dairy farms move toward more data-driven nutrient management, near-infrared spectroscopy (NIRS) is emerging as a practical tool. It enables rapid, non-destructive analysis of slurry nutrients directly on-farm, reducing the need for frequent laboratory testing.
However, one key challenge is often underestimated, temperature. Slurry in barns, tanks, and lagoons rarely stays at a constant temperature. It changes with seasons, time of day, and position in the manure management chain. These fluctuations can distort NIR spectra and, if not properly understood, lead to unreliable nutrient predictions.
A recent article in Microchemical Journal by Wang et al. (2025) addresses this issue using advanced spectral methods to examine how temperature reshapes the NIR signature of dairy slurry and what this means for real-time sensing in practice.
What the study investigated
The central question was:
How do temperature variations between 0 and 40 °C affect the near-infrared spectra of dairy farm slurry, and what does this imply for NIRS-based nutrient analysis?
While NIRS has proven effective for quantifying nutrients in manure and slurry, temperature effects are often handled through simple calibration adjustments rather than a deeper understanding of molecular-level changes. This study set out to:
- Characterize temperature-induced spectral changes in dairy slurry
- Examine how functional groups and hydrogen bonding respond to heating
- Provide a scientific basis for more robust, temperature-resilient NIRS models
How the research was done
Dairy slurry samples were collected from several stages of the manure management chain, including the manure gutter, regulating tank, sedimentation tank, and storage lagoon. Using a portable NIRS device under field-relevant conditions, the team recorded near-infrared diffuse reflectance spectra at temperatures from 0 to 40 °C, generating 2,880 spectra in total.
To analyse these data, the authors applied:
- Principal Component Analysis (PCA) to quantify how much of the spectral variation is driven by temperature and how spectra cluster across the temperature range.
- Two-dimensional correlation spectroscopy (2D-COS) to reveal how different absorption bands, and the functional groups behind them, respond to temperature in a defined sequence that cannot be easily observed in conventional 1D spectra.
This combination made it possible not only to observe that spectra change, but also to understand how and in what order they change.
Key insights on temperature effects
The study reports several important findings for anyone using or developing NIRS tools for slurry:
-
Temperature leaves a strong, structured fingerprint.
PCA showed that the first two principal components explain over 99% of the spectral variance, with the main component closely aligned with temperature. As temperature rises, spectral scores shift systematically, demonstrating that temperature alone can drive large changes in the NIR signal. -
Water structure and hydrogen bonding are central.
Spectral regions associated with O–H, N–H, and C–H vibrations, representing water, proteins, ammonium, and organic matter, change as temperature increases. The weakening of hydrogen bonds and shifts in water structure lead to intensity and position changes in key absorption bands, explaining why models calibrated at one temperature may perform poorly at another. -
Process stage and anaerobic fermentation matter.
Comparing spectra from different stages, for example manure gutter versus storage lagoon, showed that anaerobic processes alter the spectral baseline, with some peaks disappearing and others shifting. Temperature effects are therefore layered on top of microbial and compositional changes along the slurry chain. -
2D-COS reveals ordered molecular responses.
The 2D-COS analysis demonstrates that functional groups do not respond randomly to heating, they follow a predictable sequence of change. This ordered behaviour provides a framework for designing targeted temperature correction strategies and for selecting more temperature-stable spectral regions.
Why this matters for digital nutrient management
For on-farm sensing and precision agriculture, the implications are clear:
- Temperature is not just measurement noise, it systematically reshapes NIR spectra and can bias nutrient predictions if ignored.
- Portable NIRS systems for slurry need explicit temperature strategies, not only generic calibrations.
- By identifying temperature-sensitive and temperature-stable spectral regions, this work supports the development of more robust temperature correction models, reliable real-time nutrient monitoring, and better-informed, site-specific slurry application.
In the longer term, integrating these insights into smart farming systems can help reduce dependence on synthetic fertilizers, lower nutrient losses to the environment, and improve alignment with circular economy and environmental policy goals.
Full reference (APA)
Wang, P., Che, J., Cordeiro, C. M., Liu, S., Aernouts, B., Li, M., Sindhøj, E., Ma, H., Siesler, H. W., Zhang, K., Yang, Z., & Zhao, R. (2025). Analysis of the near-infrared spectral characteristics of dairy farm slurry under temperature variations using two-dimensional correlation spectroscopy. Microchemical Journal, 219, 116165. https://doi.org/10.1016/j.microc.2025.116165