The Access Code says:
When grass has just been sown, treat it like any other crop and follow the appropriate guidance (see fields of growing crops). When on land in which grass is being grown for hay or silage you can exercise access rights unless it is at such a late stage of growth that it might be damaged. Such grass will be grown in enclosed fields and have no animals grazing on it. A "late stage of growth" is considered to be when the grass is above ankle height (about 8 inches or 20cm). In such cases, use paths or tracks where they exist or go along the margins of the field. Grass can also be grown for turf, usually on relatively flat ground and in large fields. In these cases, use paths or tracks where they exist or go along the margins of the field, when the turf is at an early stage of establishment or if you are cycling or horse riding.