Can Brix predict forage quality?

Soder is a research animal scientist with the USDA Agricultural Research Service based in State College, Pa.

Contributing authors were Eric Billman, USDA-ARS research agronomist, and Jeff Horst and Kristi Balk with AgriKing Inc. 

Farmers and farm consultants often seek rapid and inexpensive methods to assess the feeding value of forages in the field to make timely harvest or grazing decisions. However, current methods of forage nutritive value analyses can be expensive, time consuming, and technically difficult without sending samples to a testing lab.

Brix readings can be taken rapidly in the field using handheld refractometers and have been used anecdotally by farmers as a proxy for energy value in fresh forage, but data validating Brix as an estimation of sugar and thus energy content of forage is lacking.

Brix is a unit of measure that has been used in wine, honey, and fruit industries to estimate sugar (sucrose, fructose, or glucose) content. It has been thought that Brix could also be used to estimate the sugar content of forages; however, it was never meant for pasture forages.

Forages aren’t fruit

The liquid extracted from fruits, such as grapes, is primarily made up of sugars, but this is not the case with forages. Plant liquid can also contain starches, free amino acids, salt, proteins, and pectins.

Brix reads all dissolved solids — not just sugars — in extracted liquids. This is why Brix is used in a number of other industries. For example, the salinity of saltwater in marine aquariums can be monitored using Brix, where there are no sugars, but plenty of salt. A little closer to our agricultural roots, Brix can be used to evaluate colostrum quality by estimating antibodies. High-quality bovine colostrum will have a similar Brix reading to high-quality wine grapes, which demonstrates how Brix can read different components of a solution.

In a solution of dissolved sucrose, a Brix value of 25% means that there are 25 grams of sucrose and 75 grams of water in a 100-gram solution. This is a true Brix value. However, the same Brix reading in a solution extracted from grass means that there are 25 grams of soluble solids, which may include dissolved solids other than sucrose and may provide an overinflated estimation of the energy value of a forage.

Brix has been touted to provide an idea of nutrient density of a forage, given that the refractometer measures total dissolved solids and not just sugars. However, some of those dissolved solids may not be nutrients, per say, but may provide noise that inflates the reading on the refractometer.

All those other things — pectins, proteins, free amino acids, and more — in the liquid extracted from forages may give a false high Brix reading as the sugars cannot be sorted out from the rest. The sugar content of most fruits will be six to 10 times higher than the sugar content of forages. Therefore, a Brix reading from fruit is more likely to reflect the actual sugar content of the fruit compared to forages.

Forages are complex

Forages constantly react to the environment around them. While it is true that sugars accumulate in forages during the daylight hours due to photosynthesis and are utilized by the plants at night, the process isn’t so simple. Not all sugars accumulate in the leaves — some are stored in the stems and roots. In addition, plant maturity, plant functional group, forage species and variety, and different plant parts can affect sugar accumulation, which can also affect Brix readings.

Environmental factors can also impact plant sugars. For example, if the barometric pressure drops due to an impending storm, plants may send energy to the roots to protect themselves should there be storm damage to the leaves. If the day is cloudy, photosynthesis slows down. Moreover, most of our pastures have many different forage species in them. How will you take a forage sample for Brix testing that is representative of the mixture?

On the other hand, many factors can also impact Brix readings. Human variability is one major issue. As part of the sampling process, the plant part is prejuiced when it is rolled around in your hands. The longer the rolling time, the higher the Brix reading can climb, inflating two- to four-fold. The time between when the liquid is extracted and when the result is read on the refractometer can also alter the sugar content by over 10% due to enzymatic changes. How hard plant liquid is squeezed from forages can impact results as well.

We cannot prepare forage and take Brix readings at the exact same time. Environmental conditions can change from one field to the next — the sun may go behind a cloud and the temperature may change. Dew, soil, and other contaminants on a blade of grass can also affect the reading.

Wet chemistry comparison

USDA Agricultural Research Service (ARS) researchers in State College, Pa., conducted a study to determine how well Brix readings correlated with actual sugar analysis in orchardgrass and alfalfa via wet chemistry. Samples of each forage were collected monthly from May to August for two years on sunny days with temperatures at 75°F to 80°F.

To minimize human error, plants were not rolled before taking a Brix reading. Plant tissue was crushed with a garlic press and read on a Brix refractometer. The remaining uncrushed forage was frozen in liquid nitrogen to stop all cellular activity and sent to a laboratory for individual sugar analyses.

Results showed that the relationship between Brix and forage sugar content was not consistent across species or harvest (Figure 1). We analyzed total sugars as well as several of the major individual sugars, including fructose, glucose, and sucrose.

Bars above the red line in the graph show a positive correlation, which means when sugars went up, Brix readings did, too. The taller the bar, the stronger the correlation. Bars below the red line show a negative correlation, meaning when sugar content rose, Brix readings declined and predicted the opposite of actual sugar values.

Alfalfa showed moderate to strong correlations in May, June, and August. In July, there was a moderate to strong negative correlation, meaning Brix predicted the opposite of the actual forage sugar content.

Orchardgrass was a different story. Positive correlations, when present, were weak to moderate at best in May. In June and July, there were moderate to strong negative correlations, meaning that Brix predicted the opposite of what was true for total and individual sugars — if sugars were higher, the Brix reading was lower.

Don’t rely on it

Taking Brix readings may seem like a practical solution to analyze forage nutritive value in the field, but the reality is that the science does not support this premise. Even if we could control human and environmental variables that can affect Brix readings, it is still not a reliable method to estimate energy content of forages.

Laboratory forage analysis via wet chemistry or near infrared reflectance spectroscopy (NIRS) is still the gold standard for making forage decisions as they have high repeatability and fewer sources of error. The downside is that these methods take more time and have a higher cost. However, progress is being made in developing technologies that may allow real-time analyses of forages in the field, such as the portable NIRS spectrometers. Science-backed technologies such as these will provide more accurate results than unproven methods like Brix.

This article appeared in the February 2025 issue of Hay & Forage Grower on pages 16-17.

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