Complex sentence: Thus, the spare horses must be kept close to the cattle herd and moved along with the cattle so as to be available to riders as needed, grazing whenever possible along the way and at night in order to obtain adequate forage.
Split into simpler sentences: Thus, the spare horses must be kept close to the cattle herd and moved along with the cattle so as to be available to riders as needed. The horses graze whenever possible along the way and at night in order to obtain adequate forage.