Rashi at the beginning of the sugya learns that the reason that bal tachsom doesn't apply to trumah & maaser is because normally one is not mafrish prior to miroach, and therefore the Torah did not include the unusual case in the prohibition.
But if thats the reason, what difference does it make later in the sugya when the Gemara differentiates between maaser rishon and sheni. Regardless of whether its mamon govoah or mamon hedyot, it still should not be included in the prohibition of bal tachsom because people are usually not mafrish prior to threshing?
Shlomo Wilamowsky, Lawrence, NY
Tosfos asks that according to the reasoning of Rashi, not only should the Terumah be Patur but also the Chulin which remains after Terumah is separated should be Patur, for one does not normally separate Terumah (and create Chulin) before Miro'ach. To answer Rashi we must explain that the fact that it is not the normally separated before Miro'ach is not the reason that this produce is exempt from Lo Sachsom. Rather it is the source in the Torah for deriving the Halachah. The Gemara learns that Matanos are Patur from Lo Sachsom because generally there are no Matanos around at the time of Dishah, and therefore the verse which says "Disho" (referring to what is normally present at the time of Dishah) is not referring to Matanos.
(It does not exclude Chulin, since there is no logical justification for exempting that from Lo Sachsom. It is more reasonable to assume that the Torah wrote b'Disho simply to exempt Matanos , because of their Kedushah, from Lo Sachsom -MK.)
If Ma'aser Sheni is Mamon Hedyot, it would be like Chulin, which is not excluded from b'Disho.
(You might wonder why Tosfos, who obviously did not accept the above answer, did not ask your question on Rashi. It would seem that Tosfos is generalizing your question by asking from all Chulin that is left after the Terumah is separated, and not just from the Ma'aser Sheni that is left -- after all, if Ma'aser Sheni is Mamon Hedyot, it is basically Chulin -MK.)