The heatwave has hit the UK hard this summer with lovely hot sunny days, but on the other hand very hard ground, accompanied by less obvious problems. Everyone thinks about laminitis in the spring and Autumn, with fresh grass growth, warmer days and cool nights. However, in this period where a lot of the UK is seeing drought conditions there are other factors worth considering.
When a pasture becomes stressed, for example due to lack of water and hot conditions, the metabolism of the grass will slow down. Lack of water will limit respiration (glucose + water = carbon dioxide + water) before it will limit photosynthesis (CO2 + Water + light = glucose + O2), therefore the level of fructans will build up in the grass during periods of drought. Fructans are a type of water soluble carbohydrate (sugar) created during photosynthesis and used during respiration in order for the plant to grow. The problem with fructans is a horse’s digestive system does not contain the correct enzymes in order to break fructans down, which leads to the same effect as a starch overload in the hindgut.
Starch overload occurs when too much starch is present in the diet for enzymatic digestion in the foregut, so it passes through the small intestine into the hindgut. Rapid fermentation in the hindgut of starch and or fructans by gram positive and lactate producing bacteria leads to a decrease in hindgut pH and death of gram negative (fibre digesting bacteria) with formation of endotoxins, exotoxins and vasoactive amines. The lower pH makes the gut wall more permeable and the endotoxins, exotoxins and vasoactive amines enter the blood stream and damage the lamellae in the foot leading to laminitis.
Now that the weather is set to cool off a bit, and (hopefully!) we might see some rain, the grass is going to start growing again. This will be especially risky in the coming months with the cooler nights and sunny days, when again the respiration occurring in the plant will be limited by temperature, but there will be ideal conditions for photosynthesis during the day.
The safest time to graze horses is very early in the morning, as long as the overnight temperature has not dropped below about 5⁰C, as the grass will have grown overnight, so reducing the store of carbohydrate. Avoid grazing in the late afternoon/ evening time as the plant will have produced a store of carbohydrate during the day and be cautious with grass which is not growing- for example due to drought, if its already seeded or if it is heavily overgrazed.