When producers build rations for livestock or equines, their goal is to provide these animals with ideal nutrition. Forages are a key component of grazing-animal diets in Georgia, and accurately testing the nutritive value of your forage is important.
The nutritive value of most forages in Georgia can be evaluated through near-infrared spectroscopy (NIRS), which is rapid, affordable, and accurate. This resource focuses on common questions that producers have about NIRS. This resource and UGA Extension expert resource C 1287, Unless You Test, It’s Just a Guess: How to Take, Interpret, and Utilize a Forage Sample, can help producers better understand NIRS and how it is performed. For more information on animal nutrition, please contact your local county Extension office.
What Does NIRS Mean?
Related Content
NIRS stands for near-infrared spectroscopy. Spectroscopy is the study of the measurement and interpretation of electromagnetic radiation wavelengths. Near-infrared refers to the 780–2500 nanometer region of the electromagnetic spectrum used for analysis.
Light is filtered so that only wavelengths in the near-infrared region pass through, and the filtered light is cast onto the forage sample. The interaction of the light with the forage sample is then used to predict forage nutritive value characteristics based on specific spectral signatures (patterns) for carbon, hydrogen, and oxygen.
Why Do We Use NIRS?
Currently, two methods of forage analysis are available: NIRS and direct chemical analysis. Direct chemical analysis, often referred to as wet chemistry, involves performing a series of chemical reactions and techniques to determine a sample’s nutritional value. Direct chemical analysis may be more accurate than NIRS, but it is also more time-consuming, expensive, and labor-intensive—and should be performed in a certified laboratory.
While NIRS is faster and more cost-effective, it is a secondary method of analysis, as it relies on calibration datasets for accurate predictions of nutritive value parameters. Despite being an indirect approach, NIRS is generally preferred for forage analysis because it is faster, more affordable, and requires less work. Additionally, it is more precise—that is, it’s more reliably repeatable—than direct chemical analysis, which reduces abnormal or incorrect results.
Accuracy, cost, required nutritive analyses, and the speed at which data and results are needed will all ultimately play a role in deciding which testing method is preferred. However, NIRS is sufficient for most grazing animal scenarios in Georgia.
How Do I Collect a Forage Sample for NIRS Analyses?
The accuracy of a forage analysis is primarily limited by the quality of the sample provided. It is important to use proper sampling techniques and to compile a representative sample of the entire hay lot. Collecting samples of hay or baleage requires the use of a hay probe. This piece of equipment is extremely important, as it allows for robust forage sample collection. Most local Extension offices should have a hay probe for clients to check out or borrow.
Do not collect a sample by pulling hay from the flat side of the bale because this is not considered a representative sample. To learn more about forage sample collection, refer to UGA Extension expert resource C 1287, Unless You Test, It’s Just a Guess: How to Take, Interpret, and Utilize a Forage Sample (linked above).
How Do I Know Which Option to Pick?
When choosing a lab, select one that is certified by the National Forage Testing Association (NFTA, www.foragetesting.org). You should be able to find this organization’s logo on the laboratory’s website.
There is often some confusion over which type of forage to select on the submission form for the laboratory where you are submitting your sample. Each laboratory will have its own submission form. It is important to select the correct product: hay, baleage, or silage. Each of these products have their own near-infrared calibration.
The calibration used is also affected by forage species. Grasses, legumes, and mixed (grass and legume) stands also have their own near-infrared calibration. Although laboratories may provide options for specific species, it is unlikely that they have developed specific calibrations for them, and those designations are only for reporting purposes. For example, most laboratories use a “grass hay” equation developed to encompass all grass species commonly found in these samples.
If you need help identifying the forage species in your pasture or hayfield, please contact your local county Extension office.
What Happens to My Sample at the Lab?
After the laboratory receives a forage sample, the sample is sent through a series of steps to reduce potential variability in the results. Samples will first be dried at 130 °F in a forced-air oven until they reach the appropriate dry matter for the respective NIRS calibration. On average, this will be around 95% dry matter.
Dry samples are then processed in a forage-cutting mill to reduce particle size before being sent through a cyclonic mill to improve particle uniformity. Particle size and uniformity play important roles in forage analyses. Finer particles interact more with near-infrared light because of the increased surface area and more uniform sample surface. Finally, the processed forage sample will be transferred to a glass sample cell for NIRS analysis.
How Does NIRS Work?
Filtered light is focused onto and interacts with the forage sample. A portion of this light is absorbed, while another portion is reflected. Transmittance is the ratio of light absorbed by the sample compared to the amount of light that passes through the sample. Reflectance is the ratio of light that was reflected off the sample compared to the amount of light that was displayed on the sample.
All near-infrared instruments collect transmittance data, while certain models also collect reflectance data. These measurements are then converted to the light absorbance at a specific wavelength. The spectrum of wavelengths obtained from an individual sample is unique and is used to calculate nutritive-value parameters.
How is NIRS Used to Calculate Nutritive Values?
NIRS calibrations can successfully predict the nutritive value of the forage sample based on its spectral characteristics. Each calibration is developed to analyze a specific category of forage. The spectra are converted to the nutritive value parameters you find in the forage report. However, since this method requires calibration from direct chemical analysis, it is considered an indirect method and will always contain some small, yet acceptable level of error.
Where Do NIRS Calibrations Come From?
There are several sources of calibrations for forage samples. Some research laboratories will develop their own calibrations for research use. Most NIRS equipment companies offer calibrations for purchase with their machines.
Organizations such as the NIRS Consortium also develop and maintain calibrations for NIRS machines. The University of Georgia Feed and Environmental Water Lab is a member of the NIRS Consortium and a NFTA-certified lab.
How Do I Know My Result is Accurate?
Calibration samples are tested against an external validation set to determine their predictive accuracy before the calibration is applied to producer samples. Direct chemical analysis is performed on the validation set using the same methods used to develop the calibration. The data are then used to calculate the standard error of the laboratory reference methods.
Calibrations developed by the NIRS Consortium contain relevant data that allows for the NIRS software to determine how well the tested forage sample aligns with the calibration. These are known as the Global H and Neighborhood H distance values. Global H compares the tested forage result to the average of the calibration sample set. Neighborhood H compares the tested forage result to the next-nearest calibration sample in the set.
It is unlikely that these values will be present on a forage report, but it is possible. If you do not see these values mentioned or any other error message, then your submitted forage sample was a good fit for the calibration used, and the results accurately reflect the sample submitted.
However, abnormally large Global or Neighborhood H values may indicate an issue with the sample or how it was submitted. Some things to check are that the sample:
- is classified correctly (product and species)
- was collected and submitted properly (i.e., a hay probe was used; sample was delivered quickly).
- has an appropriate ash concentration (this will depend on the machine, but typically less than 10% to 12%).
If no issues are detected, samples with high Global or Neighborhood H values are often good candidates for addition to the calibration database and may be submitted to the NIRS Consortium for inclusion in future calibration sets.
What Can NIRS Not Do?
While NIRS serves as a swift and efficient method of feed and forage analysis, there are limitations imposed by the machine and calibrations. Failing to observe these limitations can result in inaccurate or misinterpreted results.
Most limitations can be overcome through proper sample collection and handling. However, there are no NIRS calibrations available for minor or uncommon forage species. Continuous updates to the calibrations allow forage analysis of different species, cultivars, environmental conditions, harvests, weed composition, and mixtures.
References
Baxter, L. L., Stewart, L., & Tucker, J. (2024). Unless you test, it’s just a guess: How to take, interpret, and utilize a forage sample (Publication No. C 1287). University of Georgia Cooperative Extension. https://fieldreport.caes.uga.edu/publications/C1287/
McIntosh, D., Anderson-Husmoen, B. J., Kern-Lunbery, R., Goldblatt, P., Lemus, R., Griggs, T., Bauman, L., Boone, S., Shewmaker, & G. Teutsch. (2022). Guidelines for optimal use of NIRSC forage and feed calibrations in membership laboratories (2nd ed.). University of Tennessee Press. https://voljournals.utk.edu/utk_planpubs/100/
