In Central Kentucky, many horses graze pastures that consist mainly of cool-season grasses, such as Kentucky bluegrass, orchardgrass, tall fescue and perennial ryegrass. During growth, the plants produce carbohydrates via the process of photosynthesis. In recent years, there has been increased interest in the capacity of different cool-season grasses to produce different types and amounts of carbohydrates.

Researchers at the United States Department of Agriculture-Agricultural Research Service (USDA-ARS) Forage Animal Production Research Unit and at the University of Kentucky Martin-Gatton College of Agriculture, Food and Environment have conducted several studies on carbohydrate accumulation in cool-season grasses. Their main focus has been on a group of carbohydrates referred to as “water-soluble” carbohydrates (WSCs). These types of carbohydrates are comprised of sugars like glucose, fructose and sucrose; as well as fructans (chains of fructose).

WSCs are of particular interest as high concentrations have been associated with exacerbation of insulin resistance in horses. Various research studies suggest that some management strategies can be taken to lower the concentrations of WSCs to which horses are exposed when grazing. Some of these strategies have been reviewed (Watts and Chatterton 2004). This article is a summary of results found when fertilization and different varieties of cool-season grasses were employed, with morning and afternoon sampling. Results were published in the Journal of Equine Veterinary Science.

Figure 1. WSC concentrations in tall fescue (TF), perennial ryegrass (PRG), orchardgrass (OG, and Kentucky bluegrass (KBG), harvested from field plots on May 13 and 27, June 10 and 24, July 8 and 22, August 19, September 15, October 13, and November 3 of 2015. Plots were mowed after each harvest. Only the results from afternoon harvests of nitrogen-fertilized plots are shown.

A forage WSC concentration below 10% (on a dry matter basis) has been recommended for horses at risk for insulin resistance and some other metabolic conditions (Frank et al. 2010). In a study of plots of cool-season grasses (perennial ryegrass, orchardgrass, tall fescue or Kentucky bluegrass) mowed every two to four weeks (Kramer et al. 2020), most samples (harvested at 2” above the soil to mimic grazing) had WSC concentrations below 10% (Figure 1). Orchardgrass had lower WSC concentrations than perennial ryegrass, tall fescue or Kentucky bluegrass on most harvest dates, based on three orchardgrass varieties. Orchardgrass WSC concentrations were consistently below 10% from May to November.

However, in a study done in the Virginia Piedmont (Kagan et al. 2011), one orchardgrass variety had WSC concentrations above 10% when allowed to grow in experimental plots to a hay cutting stage (late April). While the difference between measurements in the two studies may partly reflect differences in the timing of harvest, management or methods used to measure WSCs, it also suggests the value of having forages tested for WSC concentrations, especially when using a different variety.

Time of grazing is frequently mentioned as a means of controlling the concentrations of WSCs available to horses. The WSCs produced through photosynthesis are constantly utilized through respiration, but during the night, when photosynthesis does not occur, respiration depletes WSCs. The result is a lower WSC concentration in the morning. WSC concentrations have been found to increase from noon to early evening (Lechtenburg et al. 1972). Grazing in the morning may present the least risk to horses prone to metabolic issues. However, in grasses sampled in early to mid-afternoon, WSC concentrations were not always higher than in the morning (Kramer et al. 2020), suggesting that not enough time had elapsed for WSCs to accumulate appreciably. These results were variable, suggesting that while early afternoon grazing may present a smaller risk than late afternoon grazing, morning grazing is the safest option for horses at risk for metabolic issues.

The extent to which nitrogen status, or fertilization, affects WSC concentrations in pastures is unclear from the abovementioned studies. When grasses are fertilized, the products of photosynthesis are expected to be utilized for growth instead of accumulating as WSCs. Hence, WSC concentrations are expected to be lower in fertilized than in unfertilized pastures. However, in the Kentucky study mentioned previously, WSC concentrations of about 80% of samples were unaffected by nitrogen (50, 35, and 50 lb/acre in March, May and August). Repeated mowing of plots may have affected the trends observed, as Lechtenburg and colleagues (1972) found that only the first cut of tall fescue exhibited an inverse relationship between fertilizer and fructans.

Monoculture field plots for the study by Kramer et al. (2020). Caution should be used when extrapolating from small monoculture plots to pastures.

In conclusion, testing of varieties for WSC concentrations, and morning grazing may help to manage the amount of WSCs to which grazing horses are exposed. Additional management strategies, such as consideration of weather patterns, may be helpful as well and are described elsewhere (Watts and Chatterton 2004).

 Acknowledgments: Authors’ joint research was funded by the U.S. Department of Agriculture. USDA is an equal opportunity provider and employer.

References:

Frank N, Geor RJ, Bailey SR, Durham AE, Johnson PJ. Equine metabolic syndrome. J Vet Intern Med 2010; 24:467-75.

Kagan, I.A., Kirch, B.H., Thatcher, C.D., Strickland, J.R., Elvinger, F., Teutsch, C.D., and Pleasant, S.R. Seasonal and diurnal variation in simple sugar and fructan composition of orchardgrass pasture and hay in the Piedmont region of the United States. J Equine Vet Sci 2011; 31:488-497.

Kramer KJ, Kagan IA, Lawrence LM, Goff BM, Smith SR. Water-soluble carbohydrates of cool-season grasses: prediction of concentrations by near-infrared reflectance spectroscopy and evaluation of effects of genetics, management, and environment. J Equine Vet Sci 2020; 90:103014.

Lechtenburg VL, Holt DA, Youngberg HW. Diurnal variation in nonstructural carbohydrates of Festuca arundinacea (Schreb.) with and without N fertilizer. Agron J 1972; 64: 302-305.

Watts KA, Chatterton NJ. A review of factors affecting carbohydrate levels in forage. J Equine Vet Sci 2004; 24: 84-86.

Isabelle A. Kagan, PhD, plant pathologist with USDA-ARS and adjunct instructor within UK’s Plant and Soil Sciences Department; Laurie M. Lawrence, PhD, professor within UK’s Animal and Food Sciences Department; Kelly J. Kramer, graduate student within UK’s Plant and Soil Sciences Department at the time of this research; and S. Ray Smith, extension professor within UK’s Plant and Soil Sciences Department