BEIJING, April 11 (Reuters) - China has imposed a nationwide grazing ban to help prevent overgrazing and erosion of its grasslands, state media said on Wednesday.

Grasslands cover nearly 40 percent of China, from the high plateaus of Tibet and Qinghai to the arid reaches of Inner Mongolia. But increased prosperity has allowed herders to raise more cattle, sheep, goats and yak, degrading pastures, while expanding cities have encroached on the best land.

Fencing of pastures and settlement of nomads has also changed migration patterns from winter to summmer grasslands, which traditionally allowed grasslands to recover.

This is the second time China has imposed such a ban, after taking similar measures in April last year. The ban will last for two months in some areas and up to a year in others, the Xinhua news agency said.

China banned grazing on nearly 90 million hectares and forbade 30 million livestock from roaming on grasslands at the end of last year, Xinhua said, citing Wang Zongli, deputy director of the agriculture ministry's animal husbandry department in cashmere producing Ordos, Inner Mongolia.

Climate change and global warming could cause Himalayan glaciers to shrink and grasslands to dry out, Chinese officials have warned.

Degraded grasslands can speed erosion and desertification, feeding the sandstorms that plague northern China in the spring.

Animals can be fed in feedlots to reduce the pressure on grazing pastures, potentially raising China's demand for feed grains and also increasing herders' expenses.

For nearly six decades since communist rule began, Beijing has tried -- sometimes forcibly -- to settle grassland nomads, who primarily belong to Mongolian, Tibetan, Kazakh and other ethnic minorities.

Settled communities of Han Chinese, Hui Muslims, Uighurs and other groups also graze animals on pastures closer to towns and villages.

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