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Here’s Why We Don’t Use Brix to Measure Forage Quality

By   /  September 11, 2017  /  6 Comments

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If you want to find out whether a change you’ve made is making a difference on the ground, you want to use a  measurement technique that will give you reliable results every time. Brix is not that kind of measurement technique. Because Brix measurements are so susceptible to change based on weather, time of day, barometric pressure, handling of the sample, and more, it’s very difficult to get an accurate baseline. What you get instead is a picture of the sugars in a plant at a very specific instant in time under very specific circumstances. That means it’s not such a useful way of figuring out how things are going in your pasture.

What is “Brix?”

Brix is a measurement of sugars in a plant. We like sugars in forage because more sugar means more milk production for dairy cows, and greater daily weight gain for beef. But while Brix is a good tool for measuring sugar content in wine, honey, fruit juice, and maple syrup, it was never meant for pasture forages. Here’s why:

Photosynthesis Means Sugars Change Over the Course of a Day

Photosynthesis adds sugar to a plant, so brix values start increasing a few hours after sunrise and go up by several percentage points as the day progresses. Readings peak in the evening at about 6 pm. If it’s cloudy, less photosynthesis occurs and readings will be lower. In addition to time of day, shade or direct sunlight can affect a plant’s sugars. And all of this can affect your measurements.

Plants Move Sugars Around Due to Barometric Pressure

If the weather is about to turn and barometer readings are dipping, plants send carbohydrates to the roots, to preserve its sugars and protect itself should an impending storm damage the above ground plant. This movement of carbohydrates to the root can cause a lower brix reading, much like sampling on a cloudy day, or early in the day.

Temperature Changes Mean Changes in Results

If the sample and the prism in the refractometer are different temperatures, your reading will be inaccurate. Digital refractometers can compensate for this, but it may take 40 seconds to a minute for the compensation to be made, and during that time sugar concentrations can change too. In one study, the brix measurement dropped 6% when the temperature dropped less than 5 degrees.

Plant Maturity, Different Plant Parts or Species Will All Change Brix Results  

Plants change over the course of the growing season, and no two growing seasons are quite alike. A good example of seasonal variation due to plant maturity comes from measurement from four New Zealand Dairy farms where digestibility ranged from 65-80%, and crude protein from 13-32%. Those variations also would be reflected in brix readings.

Readings can also change depending on which part of the plant you sample. Leaves have more sugars than stems so you have to use the same part every time. Sugar concentrations can also vary widely among types of plants. If you have multiple species growing in your pasture, will you just look at one species or will you sample multiple species? Mixing clover and ryegrass can be mixing apples and kiwis.

Sample Preparation and Reading Time Can Change the Results

Brix reading goes up the longer you roll the plant between your hands

Part of the sampling process includes “pre-juicing” – rolling the grass around in your hands as a way to get the sample ready. This rolling time can change the reading a lot. Rolling the sample for just under a minute increased the reading by 250 to 400%

During the time it takes to get a sample and read the result, the sugar content can change. In as little as 2 minutes, the brix reading can change by about 10%. This could be because of enzymatic changes, but no one is quite sure.

Finally, contaminants can affect brix readings. Dirt and water, like dew or the dust on a blade of grass, can contaminate the sample and affect the reading. Your sample can also include bits of plant cell walls, minerals and proteins, and lipids, along with the sugars, all of which can make a big difference in the results.

What Does All This Mean?

It’s really hard to take measurements at the exact same time, temperature, and barometric pressure, and then ensure that you choose the exact same part of the same type of plant, and that the rolling and reading of the plant is the same every time. It’s even nearly impossible to do multiple samples in the same pasture on the same day because something will have changed by the time you make it from one end to the other.

All forms of measurement are imprecise to some degree, but brix measurement has so much going against it that we don’t recommend it as a pasture management tool. While it has the appearance of science and data collection, the reality is that the science just doesn’t back it up. For this reason we are also cautious about products sold based on their ability to increase brix measurements.

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About the author

editor and contributor

Rachel’s interest in sustainable agriculture and grazing has deep roots in the soil. She’s been following that passion around the world, working on an ancient Nabatean farm in the Negev, and with farmers in West Africa’s Niger. After returning to the US, Rachel received her M.S. and Ph.D. in agronomy and soil science from the University of Maryland. For her doctoral research, Rachel spent 3 years working with Maryland dairy farmers using management intensive grazing. She then began her work with grass farmers, a source of joy and a journey of discovery.

6 Comments

  1. Victor Shelton says:

    Good article. I knew brix readings tended to be precarious, especially due to photosynthesis or lack of, but it is still a tool for the toolbox that can provide a snapshot of present conditions… all things considered. I’ve found that several samples taken at the same time, of the same species, and like parts, tend to provide very similar results. Comparing those results with another time frame is where it starts getting precarious.

  2. Charlie Taplin says:

    When I was doing Fish & Seaweed (foliar) I kept track of a couple Alfalfa fields on a farm in Cabot, Vt and it does fluctuate quite a bit with weather, time of day and time of year but one thing I did find was if I averaged everything at the end of the season that over a period of years the avg brix got higher. Just a “study” for myself after other businesses were using brix to sell their product and I had to prove how much it could change.

  3. Nicole says:

    A very well thought out article, which lays out why you need to understand the limitations of a tool like a refractometer. Its important to understand these issues, and not throw the baby out with the bathwater.
    We have used Brix for nearly 20 years as one guide among many to show changes over time, compare management and inputs. We share those limitations with ranchers and farmers so they are also aware it has limitations. Its not a tool to compare other operations with due to those constraints, which is why taking your own measurements and using the same sampling method is key. After working with more operations than I can count taking them from low Brix to high Brix systems, we stand by it. I often see operators who can’t lift their brix argue it has no value. Come see ranchers who are doubling brix and measuring quality improvements in soil and on forage tests too. New Zealand dairy farmers record a direct influence between high brix pastures to high milk production compared to low brix fields.
    As one note, when humus and biological activities are low I do find there is massive instability and variation in all the sap measures we take.

  4. Jim Cropper says:

    Great job of explaining the pitfalls of trying to measure brix in standing forages. Since brix is so changeable in the first place due to weather conditions and the time of day, just think how that impacts the grazing animal’s forage intake of sugars. The level you might measure has little relationship to the average sugar intake of the pastured animal unless you are measuring it when the herd or flock is actively grazing at different times during day and night.

  5. richard parrott says:

    wow what a detailed article.
    I had not gone down the trail of brix measuring grass but would have if you said too!!
    I will have to use my brix tube thingy to brag on our sweet corn sugar.. or just taste it … so could we taste the grass???

  6. Bill Fosher says:

    I’m just wrapping up a two year study of brix as a stand-in for forage testing to allow real-time assessment of forage quality in grazing systems.

    It appears that brix is a fairly good horseshoes and hand grenades stand-in for either RFV or TDN, but as you’d expect there’s very little correlation to protein content. In well-managed pastures, protein is seldom a limiting factor to production, but I think my conclusion is going to be that it’s a bit wanting, even as a snapshot-in-time measurement of forage quality.

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