As I mentioned, I'm on 80 acres and have only killed the hots up near the house. It'd be impossible to go and kill off all the hots. While animals might not be dumb, they are overly curious. I've had many animals get injured due to how curious they are. Additionally, cattle are much different than goats. Size, for one example.
The average weight of a cow is 1400. The average weight of a goat is say..120. The cow is about 12 times heavier than the goat, meaning the venom will be diluted. I once had a 1200 pound horse get bit by a snake. Other than localized swelling, she was fine. As I mentioned earlier in the thread, I had a 60 pound goat get bit who ended up losing some muscle function after being bit.
I'd also hazard a guess that you're uncle doesn't have five ponds near/on his property and a large network of creeks like I do (I have three ponds on the property, two that are about 100 meters apart with a third that's about a quarter mile away. On either side of the property, you can hop a fence and find two more ponds. Crisscrossed over the east and south part of the property are numerous creeks..In short: prime cottonmouth territory).