A killing frost makes plants in pastures overly sufficient in nonstructural carbohydrates. Horses that eat the plants are more prone to laminitis, obesity, Cushing’s disease and equine metabolic syndrome.
It likely okay to allow horses to graze 7 days after a killing frost if the plants or grass are green and taller than 3 to 4 inches. However, grazing should stop when the growth is down to 3 to 4 inches.” – Alexis Amarshall
Source: American Farriers Journal, October 24, 2023. Link.