Definitely keep the dog on a lead. My GSD chased a sheep years ago & caught it, & I have never forgot it. It was my fault, he went through a stile in front of me & a flock of sheep were the other side of the stile. It was a walk I have done many times & never ever had sheep been in there before. There was no sign up either, the stile was very high up & I couldn't see the other side, because of the drop the other side. The sheep scattered & Zak went after them. He targeted 1 & actually got hold of it's fleece, luckily he did not injure it, luckier still that the farmer was not about He is normally obedient, but instinct took over, the thrill of the chase etc. Only a couple of years ago my terrier took off over the North Yorkshire Moors. We were being sensible, the sheep were approx a mile away. She doesn't bother about my chickens or goats, but the sheep she decided to chase were literally a dot on the horizon. She didn't even know what she was chasing, but did she run. I thought that we would be coming home without her! It was only the very thick heather that stopped her going any further. I think that I have been lucky on the 2 occasions that mine have chased sheep, you might not be so lucky.

I have also heard the ram theory, might be an idea if the farmer can be persuaded & the ram isn't too big