an animal needs more land for grazing in a dry climate than in a moist one. An average of between one and of land per horse will provide adequate forage in much of the world, though hay or other feed may have to be supplemented in winter or during periods of drought. To lower the risk of laminitis, horses also may need to be removed from lush, rapidly changing grass for short periods in the spring and fall (autumn), when the grass is particularly high in non-structural carbohydrates such as fructans. Horses turned out to pasture full-time still need to be checked frequently for evidence of injury, parasites, sickness or weight loss. If the terrain does not provide natural shelter in the form of heavy trees or other windbreaks, an artificial shelter must be provided; a horse's insulating hair coat works less efficiently when wet or when subjected to wind, horses that cannot get away from wind and precipitation put unnecessary energy into maintaining core body warmth and may become susceptible to illness. Horses cannot live for more than a few days without water. Therefore, even in a natural, semi-feral setting, a check every day is recommended; a stream or irrigation source can dry up, ponds may become stagnant or develop toxic blue-green algae, a fence can break and allow escape, poisonous plants can take root and grow; windstorms, precipitation, or even human vandalism can create unsafe conditions. Pastures should be rotated when plants are grazed down to avoid overgrazing or deterioration of pasture quality. Manure management is also improved by pasture rotation; horses will not eat grass that contains too much of their own manure and such areas are a breeding ground for parasites. Decomposition of the manure needs to be allowed while the horses are kept in an alternative paddock. Fences and pens Horses evolved to live on prairie grasslands and to cover long distances unfettered by artificial barriers. Therefore, when fenced in, accident potential must be considered. Horses will put their heads and legs through fences in an attempt to reach forage on the other side. They may run into fences if chased by another animal, or even when running at play if the fence (such as a wire fence) is not particularly visible. The smaller the area, the more visible and substantial a fence needs to be. For exercise alone, a pen, run, corral or "dry