Gemara

The Beraisa should say 'if a cow gored, was sentenced, became pregnant and gave birth, the calf is forbidden.'

Question: This is like the opinion that forbids Zeh v'Zeh Gorem (something that results from two (or more) causes, and one of the causes is forbidden. Here, the calf's mother is forbidden, and its father is permitted). According to the opinion that permits Zeh v'Zeh Gorem, how can we answer?

Answer (Ravina): The Beraisa should say that if it gored, became pregnant, was sentenced and then gave birth, the calf is forbidden, for a fetus is considered part of its mother (so it was sentenced with its mother).

Avodah Zarah 48b (Mishnah): One may plant vegetables under an Asherah (a tree forbidden due to idolatry) in winter (when its shade is bad for them), but not in summer. One may not plant lettuce under it in summer or winter (shade is always good for lettuce).

R. Yosi forbids even vegetables in winter, for the leaves fall and fertilize them.

Question: R. Yosi forbids Zeh v'Zeh Gorem and Chachamim permit. Elsewhere, each holds just the opposite!

(Mishnah - R. Yosi): One grinds up (idolatry) and throws the dust to the wind, or casts it to the sea.

Chachamim: No, this will fertilize the ground (on which it rests), and we may not get any benefit from idolatry - "Lo Yidbak b'Yadcha Me'umah Min ha'Cherem."

Answers: The opinions must be switched. Alternatively, R. Yosi permits throwing dust of idolatry to the wind, for it will be scattered and not fertilize anything. Chachamim permit due to Rav Mari's reasoning;

(Rav Mari brei d'Rav Kahana): One may not flay a blemished Korban from the legs. Even though this is good for the hide, there is a greater loss to the meat.

Here also, the gain due to the leaves (fertilizer) is offset by the loss due to the shade.

49a - Conclusion: R. Yosi permits Zeh v'Zeh Gorem even regarding idolatry. He addresses Chachamim according to their reasoning. I permit Zeh v'Zeh Gorem. Since you forbid it, you should forbid vegetables even in winter. Chachamim permit, like Rav Mari taught.

(Rav Yehudah): The Halachah follows R. Yosi.

Question (Rav Amram): A certain garden is fertilized by (blood of Korbanos to) idolatry. What is the law?

Answer (Rav Yosef): Rav Yehudah taught that the Halachah follows R. Yosi (the Peros are permitted).

Temurah 29a (Beha citing R. Yochanan): Muktzeh is forbidden only after it is fed vetch of idolatry.

30b (Mishnah - R. Chanina ben Antigionus): If a Kosher calf nursed from a Tereifah, it is may not be offered for a Korban,

Suggestion: This is because it was fattened by the Tereifah's milk.

Rejection: If so, we should forbid an animal that ate vetch of idolatry!

Answer (R. Chanina Trita'ah): The case is, it nursed (from a Tereifah) every morning, for this is enough to sustain it.

Rishonim

Rif and Rosh (Avodah Zarah 21b and 3:8): R. Yosi forbids to plant Orlah nuts. If he planted, it is permitted, for he permits Zeh v'Zeh Gorem. The Halachah follows R. Yosi. A garden fertilized by manure of idolatry is permitted.

Ran (DH v'Asikna): At first, we thought that Chachamim permit Zeh v'Zeh Gorem. It was not difficult why they forbid planting vegetables under an Asherah in summer, for we initially understood that Zeh v'Zeh Gorem is permitted when the causes mix, like land and falling leaves, but not when they are separate, e.g. shade and land. We conclude that the lenient opinion permits Zeh v'Zeh Gorem even when they are separate. Also the Rambam says so. However, the Ramban says that also the conclusion forbids when they are separate. Perhaps he forbids even b'Di'eved.

Gra (YD 142:29): The Ramban explains that in the conclusion, R. Yosi does not address Chachamim who argue about shade. That does not depend on Zeh v'Zeh Gorem, for it does not mix with the Heter. Rather, he addresses the Chachamim who forbid to grind up idolatry and throw the dust to the wind.

Ran (21a DH v'Hivrich): Why did the Gemara infer that R. Yosi (who forbids vegetables even in winter) forbids Zeh v'Zeh Gorem? Even the lenient opinion permits Zeh v'Zeh Gorem only b'Di'eved! Rather, all forbid an overt act, e.g. planting a nut of Orlah, for he is Mevatel Isur l'Chatchilah. The Hana'ah of shade comes automatically to vegetables. This is not like Mevatel Isur l'Chatchilah, and one who permits Zeh v'Zeh Gorem permits it l'Chatchilah.

Hagahos Ashri: It is permitted even regarding Avodah Zarah, and even if there is more Isur than Heter, like the Rashbam says.

Rambam (Hilchos Avodah Zarah 7:13): One may plant vegetables under an Asherah in summer when they need shade, or in winter, for the forbidden shade and the permitted ground make the vegetables sprout. Zeh v'Zeh Gorem is permitted in every case. Therefore, if a field was fertilized with manure of idols, one may seed it. One may eat a cow fattened with vetch of idolatry. The same applies to all similar cases.

Tosfos (Temurah 31a DH she'Yonkah): Likewise, if an animal ate vetch of idolatry it is forbidden, for it grew primarily from Isur. It seems that it is forbidden even to people.

Poskim

Shulchan Aruch (YD 142:11): One may plant vegetables under an Asherah in summer (when they need shade) or winter, for the forbidden shade and the permitted ground make the vegetables sprout. Zeh v'Zeh Gorem is permitted in every case. Therefore, if a field was fertilized with manure of idols, one may seed it. One may eat a cow fattened with vetch of idolatry. The same applies to all similar cases.

Beis Yosef (DH u'Mah she'Chosav): The Tur says that the Rosh holds like the Rambam, for the Rosh did not bring the Sugya. He merely said that R. Yosi permits Zeh v'Zeh Gorem, and he addresses Chachamim according to their reasoning. This connotes that he totally permits, even in summer. If he forbade, he would have specified! The same applies to the Rif. Those who permit, permit even lettuce. The Rambam did not need to specify, for lettuce is included in vegetables.

Beis Yosef (DH Sadeh): The Tur permits a field fertilized with manure of idols or a cow fattened with vetch of idolatry only b'Di'eved. Even R. Yosi permits only b'Di'eved (when one does an overt act), like the Ran said.

Taz (16): The Shulchan Aruch permits the latter cases only b'Di'eved. He permits planting vegetables l'Chatchilah, for then the (forbidden) Hana'ah comes only later, automatically.

Rema (60:1): If a (Kosher) animal was fattened with Isur, it is permitted, unless it was fed only Isur its entire life.

Shach (5): Isur v'Heter forbids l'Chatchilah to buy an animal that nursed from a Tamei animal. It seems that he forbids even if it did not nurse Tamei milk its entire life. If so, all the more so one may not feed it forbidden food. This is like 142:11, in which we permit a cow fattened with vetch of idolatry only b'Di'eved, due to Zeh v'Zeh Gorem. However, the Mordechai (Yevamos 66) permits feeding it Isur l'Chatchilah. It seems that he permits this its entire life. This is unlike a cow fattened with vetch of idolatry, for one may not benefit from idolatry, but one may feed it Isur that is Mutar b'Hana'ah, for one does not eat the Isur itself. Tosfos (Avodah Zarah 49a DH she'Im) connotes like this. In Klal 47 also Isur v'Heter permits, for digestion is like burning. The same applies even if it ate Isur its entire life. We do not distinguish fattening from basic food needs. Surely, 'its entire life' is not precise. It means most of what it ate.

Gra (1): In Temurah, we brought a source to disqualify for a Korban a cow that ate vetch of idolatry. This connotes that one may eat it. Tosfos was unsure. Perhaps this is when it ate also other things, but if not, one may not eat it, like it says in Avodah Zarah. The Gemara said that it nursed every morning, i.e. but it also ate Heter. However, the Sifri permits a calf of a Tereifah (to people) only if it nursed also from a Kosher animal. (If it nursed only Isur, it is forbidden to people.)