potrero para pastar.

a field covered with grass or herbage and suitable for grazing by livestock


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a field is any area of farmland enclosed by a fence or natural boundaries (rivers, etc.), regardless of whether is is used for grazing or crops

regarding other answers already given:

'pasture' can be confusing in English since it is not always clear whether it means 'potrero' or 'talaje' ('talaje' = grazing for animals)

'paddock' is usually reserved for a space enclosed with wooden fences (as distinct from barbed wire or wire mesh) and used for horses; the equivalent for cattle may be a 'corral' (but this is quite different to a 'potrero' ...