Complex sentence: Stallions have a high energy output during breeding season so some advise a stallion begin the breeding season at a 6 or 7 to keep him within the healthy range at the end of the breeding season.
Split into simpler sentences: Stallions have a high energy output during breeding season so it may be beneficial to have your stallion start at either a 6 or 7. By doing this, your stallion should still be within the healthy range at the end of the breeding season.