If the height of your pasture grasses is less than three inches, that pasture is overgrazed and needs immediate attention for both your horse’s and your pasture’s sakes.
The plant stores most of the sugars it uses during times of heat, cold or drought stress in its bottom three inches. Once the horses have grazed the plants down below that, the plants are less resistant to stress and may die. You’re better off beginning to supplement hay earlier in the fall than to risk damaging and/or losing the pasture grasses.
Because the plant sugars are concentrated in its bottom three inches, horses prefer the shorter grasses, making matters worse. This is not only bad for the grasses, it also could be dangerous for horses prone to grass founder (see July 2002).
If the pasture is both overgrazed and stunted because of weather conditions, it’s doubly dangerous because of the risk of high nitrates in the grasses. Plants pick up nitrogen from the soil in the form of nitrites and nitrates, which they then convert to protein. Anything that stops or slows growth, such as prolonged periods of cloudy weather, drought, extreme heat or cold nights, slows or stops this conversion. And, like the sugars, the nitrate accumulates in the lower portions of the plant.
In the worst cases, severe nitrate poisoning can cause abortion and interfere with oxygen delivery to the horse’s blood. Less-dramatic poisoning can cause exaggerated inflammatory responses and interfere with thyroid function. Foals born of mares grazing high-nitrate pastures or eating high-nitrate hays can also have thyroid problems. Your state university or commercial forage testing labs can check nitrate levels for you.
Proper management will preserve your pastures and reduce the risks. Pay attention to the height of the grasses/plants your horse actually eats — not the overall height of the plants in the field. When the height of the grass gets around three inches or growth stops at this level because of weather or drought, grazing should be stopped or sharply limited. Substitute hay and keep the horses off of the field until it is able to regrow and strengthen itself.
Also With This Article
Click here to view ”Safe Nitrate Levels For Horses (dry matter).”