Had this happen last winter. Happened with a 250 lb bull and a 400 lb steer. The larger calf i found while alive, but nothing I did helped. He wouldn't stand. Wouldnt eat. Appeared bloated. Never did ****. Died 2 days later.
The smaller calf I found dead. Had flopped around on ground and gone under a fence and down a hill about 20 feet. I saw him that morning problem free. No **** on his rear.
Both in same pasture. Months apart. No clue what it was. Thought maybe blackleg on the smaller calf. But I'm assuming. And that never works out right.