Rosh Hashanah pg 13a and Avoda Zara 23b.
The Gemara in Avoda Zara asks why were the worshipped trees forbidden? After all, no one can forbid that which belongs to someone else, and the land was owned by Israel from the time of Abraham. Answer: since Israel served the Golden Calf, it is considered as the Canaanites were acting on their behalf. The Gemara in Rosh Hashana asks "How could Israel bring the Omer when they entered into the Land of Canaanan? The grain was owned by gentiles and the verse says your grain, not the grain of gentiles."
Tosphot the Rosh Hashana gemara is saying that even though the land belongs to Israel still gentiles that buy land in Israel own the grain of their land. But still the gemara in Avoda Zara is Ok to ask how can gentiles forbid that which does not belong to them because of the trees that were there from before the time of Abraham.
So to Rav Shach, the Rambam comes out this way. The gemara in Avoda Zara is straight and simple. The trees are forbidden because Israel was OK with idolatry. Not because the Canaanites had any portion in them. [There were trees the Canaanites had a portion in --that is trees that they planted. But the trees that were growing at the time the land was given to Abraham those trees belong to Israel and the gentiles could not cause them to be forbidden. So then why was Israel commanded to destroy all the Asherot those the gentiles had no portion in. To that question the Gemara gives an answer.] Rav Shach gives a reason for saying the Rambam disagrees with Tosphot. The Rambam does not mention קצירכם ולא קציר עכו''ם.
And the Gemara in Rosh Hashanah holds like R, Elazar that יש קניין לעכו''ם להפקיע מידי תרומות ומעשרות that even though the land was given to Abraham, still a gentile can own land there and when he does, the produce is not obligated in Truma and Maasar --or as in our case the Omer. [And the other Gemara in Avoda Zara holds like Rabah that אין לעכו''ם קניין להפקיע מידי תרומות ומעשרות Even when a gentile owns land in Israel, the produce is still obligated in Trumah and maasar
