From: Rabbi Moshe Sofer
To: Rabbi Avraham Ullmann
The Chasam Sofer rules that for a definite bechor, kohanim can be compelled to accept it since refusal constitutes disgrace of priestly gifts, while the negligent Israelite owner should also be penalized by rabbinical decree. As a practical compromise, he permits entrusting the bechor to a non-Jewish shepherd with clear instructions not to allow it to mate with cows, supported by the Mishneh LaMelech's ruling that the mating prohibition applies only to the animal being mounted.
Source: Shu"t Chasam Sofer YD §302