The best square bale feeder I ever had was a retired metal hospital gurney that our local hospital wanted to get rid of. With the sides raised, it held the bales well, the cows had no trouble getting to the hay, and it had large wheels so it could be rolled to wherever the cows were. They also didn't rust. And since they were free for the asking, the price was right, too.
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The best hay feeder for cattle you already have. Put the hay in a single row on the ground and use a different area each day. A timid cow can starve triying to get into a hay feeder where you have a strong boss cow. When you feed on the ground as I mentioned above the timid cow only has to move to the place vacated by the boss cows and eat there. Feeding on the ground has other advantages also. You can feed in the pasture where the ground is needing fertilizer and the cows will accomodate you. You will not have mud up the the cows belly in wet weather since they do not trample the earth in one spot. Parasite problems are reduced since the cattle are not picking up scraps of hay from manure. Additionally you do not have to wade through the manure to get to the hay feeder. Try the above, I think you will like it.
Build it right into your fenceline, 2-3' deep--depends on size of bale your using-- with 6" wood posts for corners, 3-2x12's high in the back [as you face the fence] and on each side; make it long enough for seven cows to stand side by side. Leave your posts high enough to frame in angled feeding slats of 2x4's on top of the backside to discourage them from pulling the hay out of the feeder. Make a way to put a roof on it if you get a lot of rain or snow. You could also box in the front of it if you wanted to, to keep them from scattering the hay to far away from themselves, which is what my cow did. [That would go right where the fence would have been.] Good luck! I can attach a drawing if you like, but need to scan that in first.