Understanding water-soluble carbohydrates in Central Kentucky pastures

Authors: Kagan, Isabelle; LAWRENCE, LAURIE - University Of Kentucky; KRAMER, KELLY - University Of Kentucky; SMITH, S. - University Of Kentucky

Submitted to: Trade Journal Publication
Publication Type: Trade Journal
Publication Acceptance Date: 9/12/2022
Publication Date: N/A
Citation: N/A

Interpretive Summary: In Central Kentucky, many horses graze pastures that consist mainly of cool-season grasses, such as perennial ryegrass, tall fescue, orchardgrass, and Kentucky bluegrass. Among the carbohydrates (sugars) in grasses are water-soluble carbohydrates, or WSCs (glucose, fructose, sucrose, and chains of fructose called fructans). Large amounts of WSCs have been associated with metabolic disorders in horses, and forages with less than 10% WSCs on a dry matter basis have been recommended to mitigate the risk of equine metabolic disorders. Some management strategies may have the potential to mitigate high concentrations of WSCs in grasses, based on studies in central Kentucky with field plots managed like Kentucky horse pastures. In plots mowed every 2 to 4 weeks (to 10-12 cm) from May to November, WSC levels were below 10% in most samples of tall fescue, perennial ryegrass, orchardgrass, and Kentucky bluegrass on most harvest dates. The orchardgrass varieties in that study consistently had WSC concentrations below 10%, suggesting that some orchardgrass varieties may be low in WSCs. However, because a different orchardgrass variety had higher WSC concentrations in another study, forage testing of different varieties may be helpful. WSC concentrations were generally lowest in the morning in the abovementioned studies. In the first study mentioned, some plots decreased in WSCs following nitrogen application, while others increased in WSCs or remained unchanged. Effects of nitrogen may depend on how often grasses are harvested. In summary, it appears that forage variety choice, and morning grazing may help to manage the amounts of WSCs to which grazing horses are exposed. Other management strategies, like consideration of weather patterns, are described by other authors.

Technical Abstract: In Central Kentucky, many horses graze pastures that consist mainly of cool-season grasses. Grass carbohydrate composition includes water-soluble carbohydrates (WSCs), such as glucose, fructose, sucrose, and fructans (fructose chains of varying lengths). High concentrations of WSCs have been associated with exacerbation of insulin resistance in horses. Forage WSC concentrations below 10% (on a dry matter basis) have been recommended for horses at risk for insulin resistance and some other metabolic conditions. Some management strategies may mitigate WSC concentrations in grasses, based on studies in central Kentucky with field plots managed like Kentucky horse pastures (kept at 10 to 25 cm height). In one study, WSC concentrations were below 10% (freeze-dried matter basis) in most samples of tall fescue, perennial ryegrass, orchardgrass, and Kentucky bluegrass collected from May to November of one year. Orchardgrass varieties in that study consistently had WSC concentrations below 10%. Other studies have revealed higher WSC concentrations in a different variety of orchardgrass. While different testing methods influence results, these differences may demonstrate the value of forage testing for different varieties. WSC concentrations were generally lowest in the morning across all species and varieties. Nitrogen fertilization had variable effects on WSC concentrations, possibly due to the repeated harvesting . Generally, forage variety testing, and limiting grazing to the morning hours may help to manage the amounts of WSCs to which grazing horses are exposed. Other management strategies, like consideration of weather patterns, are described by other authors.