The 240th mitzvah is that we are commanded to follow the laws regarding damage caused by grazing [hever].

The source of this commandment is G‑d's statement1 (exalted be He), "If a person allows his animal to graze in a field or a vineyard, [... he must pay for the damage]."

The details of this mitzvah are explained in the 2nd and 6th chapters of tractate Bava Kama and the 5th chapter of Gittin.