Refractometers have been used for years for quality control purposes in the food-and beverage-processing industries. Today, there are hand-held models that can be used out in the field to measure sugar content in plant juices as a way to determine the best time to harvest produce for the fresh food market or further processing into preserves, wine and fruit juices.
Beef producers across the U.S., and now in Canada, have picked up on this technology as another tool to help manage pasture rotations and determine the optimum time to cut crops for silage and hay.
Refractometers measure the concentration of total dissolved solids in a liquid. The bulk of the dissolved solids in the juice squeezed from growing plants is carbohydrates in the form of sucrose sugar. A small portion would be simple sugars, such as glucose and fructose that combine to make sucrose, as well as minerals.
When a drop or two of plant juice is placed on the prism surface of the refractometer, light passing through it refracts onto a scale to give the measurement in degrees Brix. Each degree Brix translates into 1 gram of sucrose per 100 grams of plant juice. So, there would be one gram of sucrose and 99 grams of water in every 100 grams of liquid.
The Brix scale was developed in the 1800s by a A.F.W. Brix, who used a hydrometer to determine the weight of dissolved sugar per volume of solution based on its specifigravity (density of a liquid relative to that of pure water). Even though the Brix scale was developed to measure sugar content and refractometers measure total solids, the refractometer reading is considered to be an accurate measure of sucrose content.
For example, the ideal Brix reading for grapes to be harvested for winemaking is 20 to 25. During the winemaking process, about 55 per cent of the sugar is converted to alcohol, so grapes with 20 degrees Brix would yield about 11 per cent alcohol.
The science isn’t that exact when it comes to using refractometers to gauge forage quality because there haven’t yet been any peer-reviewed studies published on this topic. However, the basic principle that plant sugars (as simple sugars, starches, and carbohydrates) provide energy in cattle diets is well founded. Therefore, forages with higher Brix readings indicating higher sugar content should provide better nutrition than those with lower Brix readings.
There is a wide range of refractometers designed for various purposes. The general-purpose models give Brix readings from zero to at least 32 degrees. The manual models show the reading on a scale viewed through an eyepiece and the latest models have digital readouts. U.S. prices range from around $150 and up for the manual models to $300 and up for the digital models.
BRIX LEVELS IN FORAGES AND WHAT THEY MEAN
“Definitely, a refractometer reading can tell you more than the naked eye,” says Terry Gompert, extension educator with University Nebraska-Lincoln. He uses a refractometer as a teaching tool and to monitor forage quality on his home place. In fact, he never leaves on a pasture walkabout without his
refractometer and garlic press in his hip pocket.
The Brix level of alfalfa is typically between 10 and 15. On the same field, he finds that the grasses will have Brix levels in the six to 12 range and it varies from species to species in the mix. Regardless of the species, the highest energy levels will always be in the lower stems and leaves, he adds. The very highest readings will be from the juice of the blossoms.
A Brix level of 22 is considered excellent for alfalfa, while eight is average. For cereals, 18 is said to be excellent and 10 is average. Another general observation is that forages with Brix levels greater than 12 seem to have improved resistance to insects and disease.
Dr. Anibal Pordomingo is familiar to producers who attended the Manitoba Forage Council’s 2006 grazing school. He is a well-known researcher and extension specialist with the National Institute of Farming Technology near Santa Rosa, Argentina. On his own ranch, he uses a chain of perennials and annuals for grass finishing beef cattle.
He has found that the best ration for finishing cattle on grass is when soluble carbohydrates (directly measured by the presence of sugar) and crude protein are balanced with the protein being 14 to 18 per cent and the soluble carbohydrates being at least 15 per cent. Young, vegetative plants tend to be too high in protein and too low in energy to produce adequate gains and marbling in a grass-finishing program. As the plants approach the flowering stage and dry matter content accumulates, the protein and energy will come into balance. (www.mbforagecouncil.mb.ca/eventinfoproceedings/event-proceedings/grazingschoolproceedings/default.aspx).
The Brix level fluctuates with the time of day and kind of day, Gompert explains. Photosynthesis is most rapid on sunny days. The product of photosynthesis is oxygen and simple sugars, which the plant uses for its own growth or energy reserves. On a sunny day, the Brix reading will be higher later in the day than first thing in the morning. You won’t see this variance on cloudy days or in shady areas.
“There is research to show that the energy content of hay harvested in the middle of the afternoon on a sunny day can be as much as two per cent higher than hay harvested early in the day or on a cloudy day,” he adds.
In Idaho, researchers with the USDA Agricultural Research Service found that sugars begin to accumulate a couple of hours after sunrise, peak about 6 p.m. and drop after sunset as photosynthesis ceases and the sugars migrate to the roots. Cattle, sheep and goats strongly preferred hay cut at sundown compared with hay cut at sun-up.
At Utah State University, researchers fed a ration of 40 per cent alfalfa hay to two groups of dairy cows. One group received a ration with hay cut in the morning and the other was fed a ration with hay cut in the late afternoon. The cows ate six pounds more and produced 7.5 pounds more milk per day on the ration containing the late-afternoon-cut hay.