Responsa for Bava Batra 247:1

אף מוחכרת ומושכרת שבחא דממילא קא אתי דלא חסרי בה מזוני

so [in the case of] one rented out or given on hire, the appreciation [must be] such as comes naturally and they do not lose thereby [the cost of its] food.<span class="x" onmousemove="('comment',' I.e., when the renter or hirer provides the fodder, otherwise the firstborn would not take in the appreciation a double portion. ');">¹</span> In accordance with [whose view is the law<span class="x" onmousemove="('comment',' That a firstborn son takes a double portion in the natural appreciation of a bequeathed estate. ');">²</span> quoted]? — It is [in accordance with that of] Rabbi. For it was taught: a firstborn son is not [entitled] to take a double portion in the appreciation of the estate, which accrued after the death of their father. Rabbi said: I say, A firstborn son does take a double portion in the [natural] appreciation of an estate which accrued after the death of their father,<span class="x" onmousemove="('comment',' The law quoted is in agreement with this statement of Rabbi. ');">³</span>

Teshuvot Maharam