public int heightmapMinimumLODSimplification { get; set; }

Sets the lowest level of simplification for the Terrain. Also affects areas that are outside of the camera frustum. Use this property to correct a situation where Terrain that is outside of the camera's view casts an overly simplified shadow inside the camera's view. A value of 0 means there's no limit on reducing the Terrain's level of detail. Each increment of the value quadruples the minimum number of triangles used to render the Terrain. A high value can reduce performance because of high culling time.

using UnityEngine;public class Example : MonoBehaviour{    void Start()    {        Terrain.activeTerrain.heightmapMinimumLODSimplification = 2;    }}