Kautilya's Arthashastra 2.32.22

2.32.22

dantamūlaparīṇāhadviguṇaṃ projjhya kalpayet /
abde dvyardhe nadījānāṃ pañcābde parvataukasām //E

Leaving as much as is equal to twice the circumference of the tusk near its root, the rest of the tusks shall be cut off once in 2½ years in the case of elephants born in countries irrigated by rivers (nadija), and once in 5 years in the case of mountain elephants.