In general, pasture content of storage carbohydrates including non-structural carbohydrates is highest in the spring, lowest in midsummer, and intermediate in the fall. Most nutritionists refer to the storage carbohydrates as water-soluble carbohydrates which include glucose, fructose, sucrose, and fructans. Fructans have been implicated as a cause of laminitis. The theory is that fructans, which can only be digested by fermentation in the hind gut of the horse, when at high enough levels, create the same problem as undigested starch reaching the hind gut. This, however, is only a theory and has not been scientifically documented. For example, there is limited information on the quantities of pasture fructan or other water-soluble carbohydrates required to cause significant changes in hind gut function that may increase the risk of laminitis. To specifically answer your question, if you are grazing a cool-season grass such as fescue, bluegrass, orchardgrass, ryegrass, etc., the most active period of growth (most photosynthesis) will produce higher levels of water-soluble carbohydrates. As you have pointed out, weather/temperature (environmental) conditions can change the most active periods of growth. There are also daily fluctuations that coincide with patterns of energy storage and utilization. Pasture water-soluble carbohydrate tends to rise during the morning, reach a maximum in the afternoon, and decline overnight. If you have horses that are prone to laminitis, there is some rationale for restricted access to pasture, particularly during the portion of the growing season when you would see the greatest rate of growth of the particular pasture grass.

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