We are all acquainted with the issue of excessive sugar intake during grazing. Another compound of concern is fructan, a short-term storage sugar that plants produce to stockpile surplus sugar when they produce more than they need.
Sugar serves as the catalyst for plant growth and constitutes the primary energy source for our equine companions: Cellulose. Cellulose comprises intricately intertwined sugar molecules. It furnishes the plant with the structural support necessary for upright growth, enabling leaves to extend toward sunlight and facilitating the visibility of flowers to pollinating insects from a distance.
Three factors essentially determine the total sugar content of the grass: the weather, the nutrient supply of the plants and the composition of the turf.
The weather
For plants to synthesize sugar through photosynthesis, they require adequate warmth, sunlight, along with water, carbon dioxide, and select mineral nutrients.
The weather provides warm temperatures and sunshine, or not.
Temperatures below 5-8 C° or above 25-30 C° cause the photosynthesis rate and growth to drop sharply. In excessively cold temperatures, biochemical reactions transpire at a sluggish pace, causing growth to stall and sugar to accumulate as fructan. Conversely, excessive heat prompts the plant to seal off its leaf stomata, the small openings that facilitate water evaporation, drawing water from the soil into the leaves through capillary action. When conditions turn excessively hot, the plant loses excessive moisture, compelling stomata closure and inducing cellular biochemical processes to enter a state of “hibernation”. This, too, can lead to heightened fructan levels, serving as sugar reserves for later growth.
It’s essential to note that grass can continue growing even when the sun is not shining, provided there is enough sugar reserves and the temperature is right.
Hence, on a warm night, a substantial portion of the fructan stored during the preceding hot day undergoes conversion into cellulose, hemicellulose, pectin, and assorted structural carbohydrates. This dynamic results in the fructan levels being notably diminished the following morning compared to the preceding night.
In cases where the night is cool, potentially leading to a frosty grassy morning, the sugar or fructan content remains relatively unchanged. This is due to cold temperatures impeding the swift transformation of sugar or fructan into structural carbohydrates and, consequently, inhibiting growth. On days that succeed a cold night with abundant sunshine and agreeable temperatures, fructan levels can continue to rise, as photosynthesis generates a surplus of sugar beyond the plant’s immediate growth requirements.
When confronted with midsummer temperatures of 30°C or higher, photosynthesis grinds to a halt. Subsequent nights featuring temperatures dipping below 25°C prompt grasses to tap into their residual sugar and fructan reserves for growth. This sequence results in a drop in overall sugar and fructan content within the plants, assuming there is an adequate water supply. In instances of severely dry summer weeks when water becomes scarce, growth also ceases, yet the sugar and fructan levels remain steady.
The following table illustrates the different effects.
| Weather conditions | fructan level |
| Sunny during the day, cold at night | Very high |
| Sunny during the day, warm at night | Medium |
| Cloudy or rainy during the day, warm at night | Low |
| Cloudy or rainy during the day, cold at night | Medium |
| Persistent drought | High |
In the shelter and shade of trees, the frunctan level notably diminishes when contrasted with open pastures. This phenomenon arises from the mitigated strength of the sun’s rays beneath the tree cover (a fact familiar to those with photovoltaic systems installed on their rooftop). In many instances, during arid spells, the grass tends to attain greater length in shaded areas, a consequence of the protective shade that forestalls swift evaporation of soil moisture.
Nutrient supply of the plants
Ensuring an accurate and harmonious supply of nutrients to plants is a multifaceted endeavor. Those who have engaged in constructing a house are likely acquainted with the intricate orchestration of various interconnected components. Picture the electrician eagerly poised to proceed, yet compelled to await the bricklayer who has yet to erect the final partition wall. The electrician positions their materials at the site, poised to commence the moment the wall is in position. Many craftsmen of diverse proficiencies are requisite for the ultimate fruition of the dwelling’s construction and habitability.
Drawing a parallel between craftsmen and nutrients, an immediate revelation surfaces: analogous to the collective expertise of diverse craftsmen required for the house’s progress, plants also necessitate a comprehensive assortment of nutrients in adequate measures for their growth. Just as craftsmen leave their building materials within reach, poised for utilisation when their tasks can finally commence, plants accumulate surplus sugar in the form of short-term reserves like fructan. When a nutrient is absent, the metabolic processes essential for the plant’s growth cannot transpire. If the bricklayer is absent, no matter how many electricians and heating engineers are present on the construction site, the house will not materialise.
Hence, the cornerstone of effective pasture management inevitably lies in a fundamental soil analysis to calibrate the nutrient provision for plants. Imbalances in nutrient quantities, whether excessive or deficient, can yield dire outcomes.
Informed by the soil data, a tailored fertiliser scheme can then be devised, finely attuned to the specific locale. This approach establishes the optimal growth conditions for the desired grass varieties in our equine pastures, while ideally rendering the environment inhospitable to those we seek to exclude.
Each plant species possesses distinct prerequisites, strengths, and vulnerabilities.
Consequentially, nutrient demands depend on both the area’s vegetation and its soil composition, alongside microclimatic influences. Furthermore, variations in nutrient requirements span nutrient types. For instance, leguminous plants such as alfalfa scarcely require nitrogen in the soil to fuel their growth. This is due to the presence of specialized root-dwelling bacteria that facilitate the absorption of atmospheric nitrogen, thereby making it accessible to the legume. In contrast, perennial ryegrass experiences considerable strain in the absence of sufficient soil nitrogen, leading to sluggish growth. However, this grass variety then yields elevated sugar concentrations paired with diminished protein values.
What impact do different grass species have?
A 2018 study conducted in the US states of Utah and Colorado examined the total sugar content of 24 distinct grass species over a span of two years. This investigation unveiled a relatively consistent ranking of grass species, delineating those with the highest sugar levels.
Listed below are the notable grass species, tailored to our region, arranged in descending order of their sugar content:
At the pinnacle stands perennial ryegrass, an undisputed frontrunner (ironically classified as a toxic plant in the US due to its elevated endophyte content, which can incite laminitis and even broodmare abortions). Its supremacy is closely shadowed by tall fescue, while timothy, cock’s-foot, red fescue, meadow foxtail, Kentucky bluegrass, and Poa grasses follow at a moderate interval.
It is disheartening to observe that a vast majority of commercial seed mixtures intended for horse pastures harbour substantial quantities of perennial ryegrass. This is a recurring issue, even in mixtures marketed as low in fructan content.
It is plausible that manufacturers of such seed blends have made selections from the extensive array of ryegrass varieties, opting for those with comparably lower sugar or fructan content relative to their counterparts. Nevertheless, considering that perennial ryegrass by far exhibits the highest overall sugar content in comparison to all other grass species (measuring up to 36% in some instances), a prudent approach would be to avoid incorporating perennial ryegrass into horse pasture mixtures altogether.
Conclusion
Particularly with equines possessing delicate metabolisms, like our rotund leisure horses, those prone to laminitis, horses grappling with insulin resistance (a category encompassing many with symptoms akin to Cushing’s syndrome), and all easy keeper breeds, meticulous attention to grazing and weather conditions is imperative.
Whenever viable, it’s advisable to offer shaded pastures during challenging weather scenarios (especially prolonged drought or sunny days with chilly nights). Such shaded areas generally contain lower levels of sugar and fructan compared to sun-exposed regions. Moreover, it’s prudent to take the horses off the pasture in autumn as soon as nocturnal temperatures plummet to 8-10°C.
In the context of cultivating a healthy and low-sugar botanical milieu for equines, maintaining a well-balanced, nutritionally tailored soil provision on one’s pasture is paramount. This necessitates a comprehensive soil analysis, a relatively modest investment that frequently proves more economical than addressing acute instances of laminitis or colic stemming from mismanaged pastures.
The more sugar and fructan-sensitive the horses are, the more expansive the pasture should be. Robust grass species, such as perennial ryegrass, should not be the primary constituents of horse pastures, and should not feature whatsoever in grazing areas designated for metabolically susceptible candidates.
Remember this golden rule: a pasture’s isn’t classed as lean by being grazed down to the ground. The sugar richness of the pasture — and thus its potential hazard to horses vulnerable to laminitis and those with equine metabolic syndrome (EMS) — depends more on the plant varieties, climatic factors, and the level of stress within the pasture, rather than the height of the vegetation across the area.
More on this topic: Quick and easy method for determining the sugar content in hay
Sources
Glucose and Insulin Response of Horses Grazing Alfalfa, Perennial Cool-Season Grass, and Teff Across Seasons (Sept. 2018)
How to Maximize Energy Content in Forage Grasses (Dr. S. Ray Smith and Kelly Prince, University of Kentucky Lexington, KY)
Written by guest author Helmut Muß, die gute Pferdeweide
