The net balance of leaf photosynthesis, respiration, and carbohydrate export produces a diurnal variation in sward nonstructural carbohydrate (NC) concentrations, in temperate climates, such that minimum concentrations occur in early morning and maximum in late afternoon.  As NC content increases during daylight the concentration of neutral detergent fiber (NDF) declines slightly because of dilution which results in improved DM digestibility.  Perennial cool-season grass NC concentrations typically increase from morning to evening, by 20 to 30 g kg^–1. Across times of day and sward horizons, concentrations were greater for NC and smaller for NDF in October than in June and August.  Tall fescue cultivars varied in concentrations of carbohydrate fractions but accumulation rates were not different.  Daily mean non-structural carbohydrates concentrations for cutting dates in May, July, August and September declined from 239 to 231, 143 and 120 g NC kg^-1 adjusted dry weight (ADW), respectively.  The NC concentration increased by 22.4 (May), 16.8 (July), 21.0 (August) and 30.8 g kg^-1 ADW (September) from dawn to dusk. The NC curves for alfalfa (Medicago sativa) were sinusoidal over a 24-h period, but linear between 0900 and 1800 h.  The rate of accumulation was 2.9 and 4.6 g NC kg^–1 DM for alfalfa in May and September, respectively.  Forage samples taken to estimate preference by ruminants or for NC and digestibility analyses should be cut and preserved within 1 h to control the diurnal variation of NC proportionately within  P<0.05.  Cool season forages should generally be cut between noon and sunset for NC concentrations to be greater than the daily mean.