1) THE STAGE AT WHICH GRAIN BECOMES SUBJECT TO "MA'ASER"
QUESTION: The Gemara quotes two Beraisos which state that one does not transgress the Isur of "Lo Sachsom" when he muzzles cows that are "Merachsos" -- stamping soaked barley grains to remove the chaff. RASHI (DH ha'Merachsos) explains that the Isur of "Lo Sachsom" does not apply to grain which has already become obligated to have Ma'aser separated, and in this case the threshing ("Dishah") of the grain, which already took place, is considered the final stage of processing ("Gemar Melachah") with regard to Ma'aser.
Why does Rashi say that the grain becomes obligated in Ma'aser at the time of threshing? All other produce becomes obligated in Ma'aser from the time of "Miru'ach" -- the leveling of the pile of produce. (YAD DAVID)
ANSWERS:
(a) The YAD DAVID answers that the Beraisa here refers to grains of barley that were soaked in water and then oven-dried, and which are now being tread on by the cows in order to remove the chaff, as Rashi explains (89b, DH ha'Merachsos ba'Tevu'ah). This process is done only with a small quantity of grain, and thus there is no "Miru'ach" since there is not enough grain to make a pile. Consequently, the "Gemar Melachah" is the time of threshing.
(b) The RITVA (89b, DH Tanu Rabanan Paros) cites Rashi's explanation of the process of "Merachsos." He then writes that the reason one does not transgress the Isur of "Lo Sachsom" is that "the grain has already been leveled in a pile and thus has become obligated in Ma'aser." According to the Ritva, it indeed is the "Miru'ach" and not the "Dishah" which is the "Gemar Melachah" that makes the grain obligated in Ma'aser, in contrast to Rashi's explanation. (I. Alsheich)
2) FOOD FOR THE COW
OPINIONS: The Beraisa states that one does not transgress the Isur of "Lo Sachsom" when he muzzles a cow that is threshing produce of Terumah and Ma'aser. However, he should suspect from the cow's neck a feedbag with the same type of produce with which the cow is working so that people will not suspect him of muzzling the cow unlawfully. Raban Shimon ben Yochai rules that one should hang Karshinim (a type of bean) around the cow's neck, "because Karshinim are the healthiest of all foods" for the cow.
Is Raban Shimon ben Yochai arguing with the Tana Kama, or is he adding to the words of the Tana Kama?
(a) The RITVA in the name of his Rebbi (the RE'AH) writes that Raban Shimon ben Yochai's statement is an addition to the words of the Tana Kama. Even though the Tana Kama requires the owner to feed the cow the same type of produce with which it is working, one may nevertheless feed the cow Karshinim even if it is not working with Karshinim, because of the superior health benefits of Karshinim.
The Ritva proves from the Gemara later that Raban Shimon ben Yochai cannot be arguing with the Tana Kama. The Gemara later asks whether one is permitted to muzzle a cow which experiences digestive problems when it eats. If the Torah prohibits muzzling the animal because it is detrimental to the animal's health, one would be permitted to muzzle it in a case in which eating is detrimental to its health. On the other hand, if the reason for the prohibition is that the animal experiences distress when it cannot eat food that it sees, one would not be allowed to muzzle an animal even in such a case, because the animal will experience distress when it sees the food. Rav Sheshes answers this question from the words of Raban Shimon ben Yochai in the Beraisa, who says that one should feed his cow Karshinim because it is the healthiest food. Rav Sheshes infers from this that the Isur of "Lo Sachsom" is because of concern for the animal's health.
If Raban Shimon ben Yochai is arguing with the Tana Kama, how does Rav Sheshes answer the Gemara's question based on the view of Raban Shimon ben Yochai? His view is a minority opinion with which the Chachamim argue, and the Gemara's question would therefore be the subject of a Machlokes Tana'im between the Tana Kama and Raban Shimon ben Yochai. The Ritva therefore infers from the statement of Rav Sheshes that Raban Shimon ben Yochai agrees with the view of the Tana Kama and merely adds to his words.
(b) The ROSH (7:6) writes that the Tana Kama and Raban Shimon ben Yochai disagree. Raban Shimon ben Yochai maintains that one should hang Karshinim around the cow's neck because of their superior nutritional value. While the Tana Kama agrees with Raban Shimon ben Yochai regarding the nature of the Isur of "Lo Sachsom" (i.e. that it is for the sake of the animal's health), the Tana Kama argues and maintains that one may not feed the cow Karshinim when it is working with another type of produce, because others will suspect that he is transgressing the Isur of "Lo Sachsom."
This also seems to be the view of TOSFOS (DH Amar) who writes that the Rabanan (the Tana Kama) do not argue with Raban Shimon ben Yochai with regard to the nature of the Isur of "Lo Sachsom." This implies that they do argue with regard to another issue: they maintain that the owner may not feed the cow Karshinim and must feed it only the type of produce with which it is working. The TOSFOS SHANTZ cited by the Shitah Mekubetzes writes this explicitly. (See, however, the MINCHAS CHINUCH, Mitzvah 596.) (I. Alsheich)

90b----------------------------------------90b

3) MUZZLING WITHOUT AN ACTION
QUESTION: Rebbi Yochanan and Reish Lakish disagree about the law in a case in which one yells at a cow to prevent it from eating but does not do an action, such as muzzling it, to prevent it from eating. RASHI (DH Chasmah b'Kol) explains that their dispute is in a case in which the animal bends down to eat and the owner yells at it and stops it from eating. Rebbi Yochanan considers the movement of the owner's mouth to be an action (and thus it is a "Lav she'Yesh Bo Ma'aseh"), while Reish Lakish exempts the owner from Malkus because he maintains that yelling is not considered an action.
RAV ELCHANAN WASSERMAN Hy'd (in Kovetz Shemu'os) asks why this act is considered a "Lav she'Ein Bo Ma'aseh" according to Reish Lakish. The Isur of "Lo Sachsom" is a prohibition against working with a muzzled animal, and once the owner yells at his animal to stop eating and then uses it to thresh grain, he is working (threshing) with a "muzzled" animal. The threshing is an action, and thus the owner should be liable for Malkus. (Rav Elchanan Wasserman proves this from the fact that when an owner muzzles his animal and someone else works with the animal while it is muzzled, the second person is the one who transgresses the Isur. It is clear that the prohibition is not on the act of muzzling itself but on working with a muzzled animal. See TOSFOS RID (90a).)
ANSWER: Rav Elchanan Wasserman answers that the Gemara refers to a case in which someone else is threshing with the animal and the owner yells at it and prevents it from eating. Just as one may not work with an animal that is muzzled, one may not muzzle an animal while it is working. (The law that the owner does not transgress the Isur when someone else threshes with the animal refers to a case in which the owner muzzles the animal before the other person works with it. If, however, the owner muzzles his animal while someone else is working with it, he does transgress the Isur.) Accordingly, the owner transgresses the Isur of "Lo Sachsom" when he merely yells at his animal and thereby prevents it from eating. (See also DARCHEI DAVID.) (I. Alsheich)