What breed of cows? There is a guy near here who had a bunch of cows die due to copper poisoning(he believes), too much copper in the feed. He has Jerseys, apparently they can't take or use as much copper as Holsteins.

We had a problem with toxins in our silage a couple years back. Have you had your feed tested for toxins? Our cows went off feed, then went downhill from there-it affected older cows more.