Water-soluble carbohydrates of cool-season grasses: prediction of concentrations by near-infrared reflectance spectroscopy and evaluation of effects of genetics, management and environment

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

Submitted to: Journal of Equine Veterinary Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/28/2020
Publication Date: 4/8/2020
Citation: Kramer, K.J., Kagan, I., Lawrence, L.M., Goff, B.M., Smith, S. 2020. Water-soluble carbohydrates of cool-season grasses: prediction of concentrations by near-infrared reflectance spectroscopy and evaluation of effects of genetics, management and environment. Journal of Equine Veterinary Science. 90:103014. https://doi.org/10.1016/j.jevs.2020.103014.
DOI: https://doi.org/10.1016/j.jevs.2020.103014

Interpretive Summary: Cool-season grasses can be rich in water-soluble carbohydrates (WSC). These compounds provide energy for grazing animals, but excesses may contribute to metabolic issues, such as pasture-associated laminitis, for horses. Amounts of WSC in grasses vary with geographic location, time of day, and type of grass. WSC measurement can be time-consuming and costly if done chemically. One goal of this study was to expedite measuring WSC through the use of near-infrared reflectance spectroscopy (NIRS), which can be done without wet chemistry after a subset of samples is analyzed chemically. Another goal was to use NIRS to measure WSC in different grasses managed in different ways. A total of ten varieties representing tall fescue, Kentucky bluegrass, orchardgrass, and perennial ryegrass were sampled every two to four weeks from plots with or without added nitrogen, in the morning and afternoon of each harvest date. Plots were kept at 10 to 25 cm height, in keeping with standard management recommendations for horse pastures in central Kentucky. A subset of samples was selected from each harvest, and WSC were measured chemically. These chemical measurements were used as the basis for predicting WSC amounts by the more rapid NIRS measurements. The NIRS method predicted WSC concentrations with greater than 90% accuracy. Grass species, presence or absence of nitrogen fertilizer, time of day, and harvest date influenced the amount of WSC in grasses collected from May to November. Most samples had more WSC in the afternoon than in the morning. For the majority of samples, nitrogen fertilizer did not affect the amount of WSC in the grasses. On nine of ten harvest dates, perennial ryegrass or tall fescue had the most WSC. Among the ten varieties, Aberzest perennial ryegrass had the most WSC on six harvest dates. Grasses were generally low in WSC, possibly due to the type of chemical analysis used, the choice of sampling time, or the regular mowing. These results suggest that minimizing WSC intake for horses at risk for laminitis (or some other metabolic issues) may be possible by the choice of variety and methods of pasture management.

Technical Abstract: Water-soluble carbohydrates (WSC) provide energy for grazing animals, but an excess of WSC can present a risk for horses prone to pasture-associated laminitis or some other metabolic conditions. Determining WSC concentrations in commonly grazed cool-season grasses, at different times of day and under different fertilization treatments, can provide a framework for determining optimal grazing times and management strategies. However, colorimetric or chromatographic quantification of WSC in pastures can be time-consuming and costly. The goals of this study were to develop a calibration curve enabling quantification of cool-season grass WSC by near-infrared reflectance spectroscopy (NIRS) and to apply it to ten cool-season grass cultivars representing four species. Tall fescue (Lolium arundinaceum), Kentucky bluegrass (Poa pratensis), orchardgrass (Dactylis glomerata), and perennial ryegrass (Lolium perenne) were sampled every two to four weeks from plots with or without added nitrogen, in the morning and afternoon of each harvest date. Plot heights were maintained at 10 to 25 cm. A subset of samples was selected from each harvest, randomly or with NIRS software, and WSC were quantified by a phenol-sulfuric acid assay. The NIRS calibration equation predicted WSC concentrations with greater than 90% accuracy. An interaction of species, nitrogen treatment, time of day, and harvest date influenced WSC concentrations in ten harvests (P = 0.040). Of the 80 treatment pairs in this interaction (paired by sampling time or nitrogen treatment), 69 had higher WSC concentrations in the afternoon than in the morning, and 63 exhibited no effect of nitrogen. A modest positive relationship was observed between photosynthetically active radiation (PAR) and WSC concentration (R2 = 0.5033; P = 0.0237). No effect of PAR was observed on change in daytime WSC accumulation (r = 0.13). On nine of ten harvest dates, perennial ryegrass or tall fescue had the highest WSC concentrations. Among cultivars, Aberzest perennial ryegrass had the highest WSC concentrations on six harvest dates. WSC concentrations did not exceed 150 g/kg freeze-dried weight, possibly due to assay method, sampling times, or repeated defoliation. These results suggest that minimizing WSC intake for horses at risk for laminitis or some other metabolic conditions may be possible by cultivar choice, grazing time, or mowing frequency.