11. A person, [on Shabbos], should not measure out the barley that he is preparing to set before his animal (1). Rather, he should estimate the amount necessary [to feed it] (2).
(1) That is, he should not use a measuring cup, because it looks to others as if he is measuring out an amount of barley to sell on Shabbos. This type of prohibition is called "Ma'aris Ho'Ayin" (lit:"visual appearance"), and forbids doing anything that others might construe as a transgression.
(2) On Shabbos, one is allowed to feed an animal that is dependant on humans for its food.