I'm truly shocked at some of the quotes on here which suggest inhumane ways of killing sheep (or any animal). If people can't afford vets' fees, then don't keep animals. It is illegal to kill animals in some of the ways suggested because of the suffering inflicted and you could be prosecuted if found out.
I would ask the vet's advice about these lambs and be guided by it. If these animals need to be put down, then only the vet or a registered person should do it.
I'm sorry, but that's a hugely unrealistic thing to suggest.
Maybe i've missed it, but the only inhuman method of dispatching a sheep that was mentioned on here, was the cruel muppet, who used plastic bags. Now, obviously that is disgusting, not to mention criminal. As far as I can see, all the other methods mentioned (maybe I've missed something) are legitimate.
The suggestion that livestock should only be put down by a vet, and that if you cannot afford to do this, you shouldn't keep stock, is just dumb.
If an animal is found to be suffering etc, and you have the legal means to dispatch it yourself, i.e the correct fire-arm, then there is absolutely no issue with doing so. In many cases, it would be far kinder, then keeping the animal in it's state of suffering whilst you got the vet out / bundled it into a trailer and took it down to the vets.
Being able to take care of your stock, and do the right thing for it, sometimes means that you need to make the decision to end it's life as quickly and humanely as possible. It is part of farming.
