Sounds like blackleg to me, does he vaccinate? If not it only cost 2 or 3 bucks per calf & is good insurance. I've heard of the acorn's making older calves sick. Seems the cow is trying to ween them & the calf can't feed with larger cows so they will eat a belly full of acorns.

He needs to contact the county extension agent. That is what they are there for. Losing 1 or 2 calves is one thing, 12 is something that needs professional investigation.
If the cows are receiving supplemental protein and minerals I would start there. It seems to me that the momma cows are passing something through their milk that is affecting nursing calves.

I've never noticed black leg make one bleed. Do they appear weak leading to death or just die with less than 24 hr notice? Has he bought any from a stock yard right before this happened?

Stockard some not many. Some from what I understand some were stumbling before death and others just turned up dead.

Look around the woods for perilla mint. Cows and calves won’t eat it most of the time but there’s times I’ve seen them eat it after a frost and it starts to wilt.

I will call him today and try and get more info, thanks guys.

I'm going to mention this because you've got a lot of sophisticated answers but sometimes in this high tech world you get to overlooking low tech answers.

Does he have a mule or some donkeys in there with them?

I'm guessing most of y'all have never seen a Mule or Donkey kill a calf.

He may be just getting started at 12.

saw a horse pick up a couple by the neck and throw them a ways lol

My first guess with 300-500 lb calves would be blackleg I got a needle kicked out of my hand and vaccinated myself in the calf on accident .so far I'm showing no signs of blackleg though!

I talked to him this AM and he said that he/vet both got hung up the other afternoon and could not get together until the next day so no blood was taken. He has done a lot of research and thinks that it is some type of acorn poisoning. Said calves if they eat to many it will be toxic to them and will cause death but the cows that eat them have no reaction to them. Double whammy is the calf nurses it gets a double dose of the toxicity and kills them. Dunno...................

I would get a vet out there asap. I would think it is a sickness (blackleg or something). If it is the feed cows would be sick also. Has been a very mild winter and dont think a calf would ingest enough acorns to kill it. Just my guess.

Me either

Sounds like to me that if it's happening in two different pastures that it's something that he has transferred between the two whether in the feed or possibly a disease carried from one pasture to the other on the farmer's shoes or his truck tires.

Had a cow vaccinate me for bruscilosis. Interesting story, but not important as dying calves.
Back to the subject matter.

Look for Mountain Laurel. It's green and sounds like the symptoms. They will eat it now.Blackleg won't make them bleed, the skin will crackle as it forms gas pockets under the skin.

Regardless of what it is I would be vaccinating all my calves with Draxxin asap. May help/may not but it sure won't hurt

I would also rule out coccidiosis. Older cows won't get it but it can be hell on calves

Aunt Bea, call the man

Also, you do understand that you are obligated by Aldeer bylaws to update this post with the vets diagnosis and a follow up of the herd health, right?

Fur, I signed a blood oath when I registered. Do you no remember doing that when you did? Oh hell, I can't remember yesterday lol.

I didn't have to do that as I went with the premium member upgrade.

Anything new on the cows?