Fertility fluctuates based on condition. Ideally, retired horses should be above 40% condition. If they fall below this, and they can lose fertility. If you don't want to breed your mares every year, just make sure you are exercising them enough to keep condition up. The fertility fluctuation also impacts stallions.
In both mares and stallions, fertility can go up or down. Raising fertility by having optimum condition is a low chance, but is possible. Lowering fertility due to poor condition is much more common and becomes a higher risk the lower fertility drops.
Fertility also now has the chance to drop to 'Barren' status. This only happens to mares - stallions cannot drop below low fertility. If this happens it will show up in bold red text on the genetics table. A mare who is barren will never get in foal. However, if her condition improves there is a chance she could eventually become fertile again.
Condition and energy must both be high at the end of a month for a horse to have a chance at improving their fertility.
The use of the item Vigor helps mares regain fertility!