I don't know cows, just sheep and goats, and really, even after 25 years, not even them that well (ignorance, not modesty)...
I forgot to include the picture of her sitting several hours before I found her
I never got a look at her poop or teeth. All I know is that they were “dewormed” before arrival.
I drug her out to a far pasture and closed the gate to it. Cows can’t access her body
The vomit was very watery consistency and of course green. No blood in it
I’m new to owning cattle. I’ll keep that penicillin comment in mind and do some research on that
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If a ruminant has a really heavy worm load, it may eat like all get out trying to make up for lost nutrients, until near the end. If it's then wormed, all the dead worms can clog 'em up.
Even if that doesn't happen, they can be so anemic that they just can't get enough oxygen, so they pant.
More often than not, whatever they die from, they die on their sides, kicking dirt.
Hopefully, someone can give you a better answer than this, but my bet is infection, either with a blood born or intestinal parasite, and the cause of death anemia or associated pneumonia.