In case a field or enclosure through which unfenced railroad track passes shall be enlarged or extended, or the owner of the land over which such unfenced track passes shall open a field so as to embrace the track of such railroad, such railroad company is required to place good and sufficient cattle guards or stops at the margin of such enclosure or fields, and keep the same in repair; provided, that such owner shall give the nearest or most accessible agent of such company thirty (30) days’ notice of such change.