farm animals are not bred specifically to be less "sentient". (you mean sapient, by the way, though the mistake's a common one.)
farm animals are bred specifically to be more efficient producers of whatever they're supposed to produce, and in some cases, to better withstand the conditions of factory farming. occasionally, this might result in reduced or altered intelligence levels as a side effect. i am not aware of any breeding efforts aimed specifically at reducing their intelligence, however; it's generally low enough for them to be manageable anyway. and that's all factory farming cares about, that the production means be manageable on the cheap.
(this even extends to plant varieties. growing your own tomatoes, for instance, only makes sense if you go for tomato varieties that factory farms can't economically use, because they don't keep or transport well enough, or similar. growing the usual store-bought tomatoes saves you nothing, and gets you fruit that taste just as cardboard-like as store-bought tomatoes do. less used varieties can taste MUCH better, but can't survive the mechanized shipping and handling, or the long waits between vine and checkout counter.)
if you want ethically raised meat, eggs, and milk, you almost have to raise the animals yourself. second best is to support local-to-you, small-scale farmers; they might use ethical animal husbandry as a selling point, and perhaps even allow enough insight into their farms to verify that claim. large-scale agribusinesses pretty much always compete on price and only on price, selling out ethics and animal welfare to lower that price.
if you have the ability and legal right to keep a couple of backyard chickens for the eggs, i'd almost recommend it just for the experience. might teach you a thing or two about what farm animals are like --- or used to be like, because the most common chicken breeds in battery farms don't make good breeds for small-scale backyard coops anymore; urban chicken keepers tend to use somewhat old-fashioned breeds.