Sounds like a very exciting project! 
First of all, personally I would never run this kind of operation on smartthings assuming that the health of the animals might be affected. It’s just not reliable enough. In fact, the official product usage guidelines warn about the same thing:
https://www.smartthings.com/guidelines
Data accuracy and consistency from SmartThings sensors, including those provided by SmartThings directly, resold by SmartThings, or supported by SmartThings, is not guaranteed. Therefore, you should not rely on that data for any use that impacts health, safety, security, property or financial interests. For example, because temperature readings may vary significantly from reading to reading on an individual device, between devices, or over time, those readings should not be used to control heating and cooling in environments where food spoilage, health risks, or damage to physical goods could occur.
Smartthings has had at least one outage each of the last 20 months except for one, and more than one outage in some months. Again, speaking just for myself, I wouldn’t take the risk.
OK, with that out-of-the-way, as far as protocols go, your best bet is probably going to be zigbee, that’s what most commercial operations, including a number of dairies, use. Although Z wave plus has a longer range per hop, the signals do not travel as well through rain or snow, and you are limited to four total hops and 232 total devices. In contrast, zigbee allows four 15 hops into the hub and 15 hops out and can handle thousands of devices. It is the usual choice for large sensornets. But again, even if you choose zigbee, I personally would not use smartthings because of the reliability issues.
So I won’t go into a lot of the other potential technical issues right now. I suggest you start investigating monitoring packages for farms which will probably match up to your needs pretty closely and are likely to end up as much less expensive as well as more reliable in the long run.