Letting animals feed continually and intensively (集中地) in the same grazing areas can require costly replanting. Animals eat the most desirable growth first. When that keeps happening, the roots do not have enough time to recover. As a result, less desirable plants may replace them. Intensively used grasslands are also harmed as the soil is continually crushed under the weight of heavy animals.

　　While rotational grazing can save money over time, it also requires planning. And that starts with a good map to mark fences, water supplies and grazing areas.

　　Changing methods of grazing also requires time. Farmers may want to put up electric fences to enclose (圈起) grazing areas, called paddocks. The paddocks will need water. Some farmers design a path for animals from different paddocks to drink from a common watering place.

　　Farmers can start rotational grazing by removing animals from a pasture when the grass is eaten to less that five centimeters. The pasture is then kept empty until the grass grows to more than fifteen centimeters high.

　　Sheep and goats may require special preparations. They may need stronger fences than other animals. And while they eat the grass, they may need guard animals to protect them from animals that would like to eat them.

　　67. Grasslands won't depend on chemical fertilizers when __________________.

　　A. the soil of grasslands is naturally rich

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