Seasonal and diurnal variation in forage nutritive value of cool-season grasses as predicted by near-infrared reflectance spectroscopy (NIRS)

Authors: RILEY, ANNMARIE - University Of Kentucky; Kagan, Isabelle; Davis, Brittany; LAWRENCE, LAURIE - University Of Kentucky; TEUTSCH, CHRIS - University Of Kentucky; SMITH, RAY - University Of Kentucky

Submitted to: Crop, Forage & Turfgrass Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/1/2025
Publication Date: 9/23/2025
Citation: Riley, A.C., Kagan, I., Davis, B.E., Lawrence, L.M., Teutsch, C.D., Smith, R. 2025. Seasonal and diurnal variation in forage nutritive value of cool-season grasses as predicted by near-infrared reflectance spectroscopy (NIRS). Crop, Forage & Turfgrass Management. 11(2). Article e70079. https://doi.org/10.1002/cft2.70079.
DOI: https://doi.org/10.1002/cft2.70079

Interpretive Summary: For many horses, cool-season grasses are an important part of the diet. Whether they provide adequate nutrition can be assessed by determining crude protein content, fiber content, digestibility, and the content of carbohydrates that dissolve easily in water or ethanol. These assessments were made on plots of Kentucky bluegrass, orchardgrass, tall fescue, and perennial ryegrass grown in 2015 and 2017 and managed like central Kentucky horse pastures (vegetative growth, 10 to 25 cm height). Grasses were collected in the morning and afternoon on a monthly basis, freeze-dried, and ground to a powder. The powder was evaluated for crude protein, fiber, digestibility, water-soluble carbohydrates, and ethanol-soluble carbohydrates by spectroscopic techniques. Results showed that fiber content was higher in the morning than in the afternoon, and generally highest in Kentucky bluegrass and tall fescue. Crude protein concentrations were sometimes highest in the morning and sometimes highest in the afternoon. Digestibility was generally highest in the afternoon. The amount of ethanol-soluble carbohydrates in grasses seemed closely related to digestibility. Forage quality (nutritional value and digestibility) varied with time of year, time of day, and species.

Technical Abstract: Forage quality was evaluated in plots of four cool-season grasses common to Kentucky horse pastures: orchardgrass (Dactylis glomerata L.), perennial ryegrass (Lolium perenne L.), Kentucky bluegrass (Poa pratensis L.), and tall fescue (Schedonorus arundinaceus (Schreb.) Dumort.). In 2015 and 2017, plots were maintained vegetatively with two- to four- week mowing and sampled in the morning and the afternoon every two to four weeks. Near-infrared spectroscopy (NIRS) was used to predict in vitro true dry matter digestibility (IVTDMD), as well as dry matter (DM) and concentrations of acid detergent fiber (ADF), neutral detergent fiber (NDF), crude protein (CP), and water- and ethanol-soluble carbohydrates (WSC and ESC, respectively). Concentrations of ADF and NDF were higher in the morning than in the afternoon on most harvest dates in 2015 and 2017. Species with the highest concentrations of ADF and NDF were Kentucky bluegrass and tall fescue. Concentrations of CP did not have a consistent diurnal pattern across harvest dates. IVTDMD increased in the afternoon on most harvest dates. IVTDMD was positively correlated with ESC concentrations. Forage quality varied seasonally, diurnally, and across species.