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Is grazing giving your horse a Sugar High?

illustration, RPeterson, used with permission,

There are many factors (including seasonal and regional differences) that affect growth of pasture grass, but the amount of sugar that it supplies your grazing horse within a 24 hour period under ideal growing conditions is generally highest during or just after peak periods of photosynthesis (around 4 pm in the summer months). Sugar content is lowest just before daylight because the sugars have been used up in the process of plant growth. The hay you feed is also affected by this cycle and its sugar content will vary depending on the time of day it was harvested. Weather influences this basic cycle too. Sugar content might remain high in the morning if the grass has been stressed overnight. Grass may not grow as much after a frost and so its sugar levels stay high. In drought conditions (our normal summer situation) or in excessive heat the grass will have photosynthesized more sugar than it can use up in growth through the night, so it will be relatively high in sugar the next morning. Over time, with the sun still shining and without enough water, the pasture grass may not grow at all but it still accumulates sugar. Although it seems a bit counter intuitive, short, over grazed, pasture grass may contain significantly higher sugar levels than pasture that is vigorously growing. You can apply your local weather forecast and this growth information as a general guide for grazing management. However, forage analysis is the best way to determine actual sugar content of pasture grass or hay. The advantage of hay is that, once cut, the sugar levels will not increase and remain relatively stable if properly stored. If you have horses with metabolic conditions that predispose them to founder, (Cushings, metabolic disease, obesity) knowing how to avoid a “sugar high” is vitally important for their well being. Thoughtful grazing management is both art and science. Understanding the science of grass growth and sugar accumulation improves your ability to craft the art of grazing time. Discuss your objectives with your RBE veterinarian to come up with a healthy plan for your animals.

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