The aim of the experiment was to study diurnal variation in water-soluble carbohydrate (WSC) concentrations in leaf and stem of tall fescue and lucerne in autumn and to determine the impact of these changes on the rate and the extent of in vitro fermentation. Vegetative herbage of lucerne and tall fescue were sampled at 0900, 1300 and 1700 h, following three consecutive regrowths between April and May. Whole plants, leaf and stem were analysed for chemical composition and in vitro gas production. There was a linear increase in WSC throughout the day for all plants and plant components (P < 0.001). In both fescue and lucerne, this rise resulted in an increased rate of gas production, although this relationship was more pronounced in fescue. Use of time of day to govern grazing management decisions to potentially improve the rumen supply of WSC and nitrogen in autumn can be achieved from tall fescue or lucerne pastures.

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The content of water-soluble carbohydrates (WSC) in plants is variable, and depends on plant species and environment conditions. This content may be the limitation for the fermentation during silage making and also for microbial synthesis in rumen. The objective of this study was to measure the variations of the WSC in temperate forages, during autumn, in different moments of the day and in different parts of the plant.