If the idea takes off, I don's why a DIY approach can't be viable. Produce your own collars (you'd need many thousands, a factory would be happy), plant your own towers with your own transmitters (likely bog-standard LoRa, or similar), write your own open-source firmware for all that, and an open-source control app.

It's pretty doable, given enough will.

That's relatively straightforward for more technical people, but I _really_ can't picture farmers going through all that trouble unless someone gets the foundation ready to go.