Commons and all other unfenced grazing land in East Sussex have been put off-limits to help stop the spread of foot-and-mouth disease.
Wasteland where animals graze is also included in the wider set of restrictions.
Footpaths, bridleways and large tracts of council-owned land in the countryside remain closed.
No cases of foot-and-mouth disease have so far been reported in Sussex but 230 have been reported nationally.
County council rights of way manager Nigel Kitchener said: "We are taking every precaution to keep the disease out of the county and local people are doing what they can to help.
"But it is important to keep repeating the message - please don't walk or ride horses in the countryside."
The National Farmers' Union appealed for walkers and horse riders to continue to obey the countryside ban.
A spokeswoman said: "Because there is no infection in Sussex, to the public it seems a bit unreal. But we would urge the public to bear with us on this and please, please stick to the rules."
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