Fructans are specially adapted sugars found in certain cool-season forages grazed by horses. Simple sugars are produced in the leaves as the result of photosynthesis to supply energy to the plant. In order to store some of the sugars not being used immediately, some grasses will complex the sugars into a long chains called fructans. Fructans derive their unique properties from chemical bonds that cannot be broken by normal enzymatic mechanisms in the stomach and small intestine.
When sugars and fructans reach the hindgut, they are fermented by bacteria that produce lactic acid. Excessive amounts of lactic acid are not absorbed efficiently from the hindgut or used by the body for energy. The accumulation of lactic acid alters the microbial population in the hindgut and is one cause of colic and laminitis in some pasture-kept horses. Luckily, most horses can graze pastures without succumbing to laminitis because their digestive tracts have adapted to the diurnal and seasonal change in sugar and fructan content gradually.
Sugars and fructans are produced through photosynthesis that occurs in the leaves of plants during daylight. The sunnier the day, the more rapid the photosynthesis, and the more sugars and fructans manufactured. Sugars and fructans often reach their peak concentration in the afternoon after a day’s buildup. During the night, plants use the sugars and fructans for growth, thereby reducing concentrations.
Many cool-season grasses store fructans in the lower inch or two of the stem, just above the soil line. Nighttime temperatures are critical to fructan reservoirs, however. If the temperature is not above 40 degrees F (4-5 degrees C) at night, the plant will not grow and the fructans accumulate in the leaves, frequently in high concentrations.
When is the best time to graze in order to avoid fructans? The answer: it depends.
Fructans and sugars accumulate in plants based on temperature. When days are warm and nights are cool--below 40 degrees F (4-5 degrees C)--sensitive horses should not be grazed. If the weather is sunny during the day and warm at night, then horses should be grazed late at night or early in the morning when fructan levels are lowest. These horses should be removed from pasture by mid-morning.
To complicate this explanation, fructan and sugar content of grasses varies based on plant maturity. The first growth of grass in spring has very low levels of fructans but high levels of sugars. While fructans may be low in early pasture growth, indigestible cellulose and lignin is also low. High cellulose and lignin decrease intake. Because of this relationship, horses tend to eat more, and even though the fructan levels are low, total intake of fructans may still be high due to the volume of forage consumed.
Managing pastures so that horses do not overgraze will help reduce fructan intake. One simple step is to mow higher than usual during seasons when fructan accumulation may occur. Because plant stems contain high levels of fructan, horses should not be grazed on stubble. The condition of the entire pasture must be monitored daily, as horses tend to select high-fructan plants to graze.
Finally, stressed pasture plants can cause fructan levels to increase. For example, drought or frost can increase fructan levels by 30%, so special attention should be paid to the weather.
Kentucky Equine Research noted: Changes in the pH of the hindgut due to alterations in the microbial populations and acid profiles cause a condition known as subclinical acidosis, which can put affected horses at a higher than normal risk for colic and laminitis. High fructan intakes overwhelm the hindgut resulting in rapid fermentation, accumulation of lactic acid, and a deleterious decrease in pH. Kentucky Equine Research tested the efficacy of EquiShure against a challenge of fructan. Results showed that EquiShure-supplemented horses had less fecal lactate when compared to control horses. In control horses, both lactic acid and VFA contributed to the acidic conditions, whereas in EquiShure-treated horses, most of the lactic acid was being utilized.
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