Net photosynthesis and respiration in growing plants cause a circadian rhythm in forage quality. Soluble sugar concentrations increase in plants during the day causing a dilution in ADF and NDF. Herbivores show a strong preference for afternoon (PM) vs morning (AM) harvested forage. Cattle, sheep, goat, horse and rabbit, when given a choice, easily identify a forage that may contain 5 g/kg or more soluble sugar. Effect of feeding PM vs AM cut hay on milk production and weight gains is being determined. Increased production may result from increased intake of higher quality forage. Small but positive changes may be expected in energy and protein utilization. Because of the increase in available carbohydrate during digestion, feeding PM vs AM hays should reduce methane, CO2 and ammonia production.