Trish Martin, who farms with her husband Andrew in Chawton, Hampshire, said she made the grim discovery when checking on the 1,300 strong-flock.
“We found a number of ewes with blood on them so we knew a dog had been round,” said Mrs Martin.
“When we walked through the field my husband spotted the dog. It was down in the stream with two ewes – one dead and one still alive which it had by the throat.”
Mrs Martin said she tried to distract the dog with a branch while her husband went to get his gun.
“All of a sudden the dog ran off and went and attacked another ewe in the field, right in front of me. I swung a branch to allow the other ewes to get away from it and run behind me.”
The Red Tractor Assured farmer said the dog, described as a Boxer crossed with either an Alsatian or ‘Husky type’, ran off and disappeared into a neighbour’s field.
“My husband came back with police officers who happened to be driving down the road,” she said.
“They helped us pull out the ewes from the water which were still alive.
“Unfortunately we had to shoot two ewes ourselves because their injuries were so horrific.”
Dozens of ewes survived the attack but were left with severe bite marks and some had to be put down.
“A lot of the injuries were to the throat because a ewe will turn and fight to the death to protect her lambs,” said Mrs Martin.
“I’ve never experienced or seen anything like it in my life.
“We’ve had dog attacks before but certainly nothing like this.”
The couple, who are supporting Farmers Guardian’s Take the Lead campaign, have reiterated industry calls for owners to keep their dogs under control in the countryside, especially around livestock.
“We have found that signs do help spread the message but this was obviously a dog which had got out and the owner probably didn’t know where it was,” added Mrs Martin.
“It must have been in the field for at least an hour to do all that damage.”