Date: Purim 5592
From: Rabbi Moshe Sofer
To: Rabbi Yosef Shmuel Segal
The Chasam Sofer rules leniently, permitting a Jew to allow his bechor to graze among non-Jewish cattle where it will naturally mate with them. He concludes that mating constitutes at most a rabbinic-level concern, and passively allowing the bechor to graze among other cattle falls outside even that scope. The non-Jews' willingness to feed the bechor in exchange does not constitute genuine financial benefit to the Jewish owner.
Source: Shu"t Chasam Sofer YD §304