There's no easy answer because so many factors are involved, including genetic pre-disposition.
In cases of metabolic disorders, supplementing deficient vitamins/minerals may help.
In cases of inefficient absorption of nutrients (in some cases,relating to age), supplements that are easily metabolized are veryhelpful (geriatric feeds).
Stress management is also an important issue. Some we cancontrol -- avoiding overcrowding, controlling predators, eliminatingdrafts, parasite control, providing fresh water and feed, goodsanitation, pain management). Other factors are out of ourcontrol, such as seasonal weather changes and molt.
In horses, we used products such asred cell, DMG,and digestive supplements to boost immunity and stamina.
Pam