I might think that even honey and milk (are to be tithed); it is, therefore written "which goes forth from the field year by year." In sum, the criteria (for tithing) are: going forth from the field and being edible.
From (Ibid. 23) "And you shall eat before the L-rd your G-d … the tithe of your corn, your wine, and your oil," I might think that only these are to be tithed. Whence do I derive (the same for) other fruits? From "the produce of, etc." Let it then be written (just) "produce."
If so, I would say: Just as "produce" is distinct in that it is stored for preservation, so all such growths are to be included (for tithing). Whence do I derive the same for rice, millet, poppy, and sesame? Whence would I derive pulse? It is, therefore, written "of your seed."