Definition
Growth, in the context of horses, refers to the biological process of increasing in body size, weight, and skeletal development from birth through physical maturity. The term also applies, in clinical contexts, to an abnormal tissue proliferation — a tumor, cyst, or gall — as distinct from normal developmental growth. Both uses appear in equine veterinary literature; context determines the meaning.
Normal Growth in Horses
Horses grow rapidly in the first year of life. A foal is born at roughly 10 percent of its mature body weight and reaches approximately 85 percent of its mature height by twelve months. Physical maturity — the point at which bone plates (growth plates, or physes) have fully closed — typically occurs at four to six years of age, depending on breed. Large, heavy-boned breeds like drafts mature later than lighter breeds. The withers height is the standard measurement used to track growth progress; a young horse is measured periodically to assess whether growth rate is appropriate for age and breed.
Nutrition and Growth
A filly, colt, or foal requires adequate energy, protein, and minerals — particularly calcium, phosphorus, and zinc — to support healthy skeletal growth. Overfeeding energy to rapidly growing foals can cause developmental orthopedic disease (DOD), including osteochondrosis (OC), a condition in which cartilage fails to convert properly to bone at joint surfaces. Underfeeding delays growth and compromises bone density. Body condition scoring in growing horses is done differently than in adults; a young horse should be lean but not ribby, with visible muscling and appropriate frame development for its age.
Growth Plates and Exercise
Growth plates remain open — and therefore vulnerable — until physical maturity. Hard, repetitive work on closed surfaces before growth plates have fused can cause permanent damage. This is the primary reason racehorse trainers and veterinarians monitor young horses carefully and why age-based veterinary schedules account for developmental stage. Once plates have closed, the same workload that would have damaged a young horse is appropriate conditioning for the mature animal. The farrier also plays a role in supporting healthy limb growth: balanced trimming corrects minor deviations before growth plates close, a window of intervention that closes with maturity.
Further reading: Growth on Wikipedia; Biological growth at Britannica.