Unless there is a `Public footpath` across your fields or lambing areas, and it`s private land, then no-one has any right to be there at all.
ALL dogs should be ON LEADS on any public path that crosses over a field with livestock in it, no matter how well trained the dog is.
In the UK many farmers install notices which state. " Your dog will be shot if found among sheep".
You don`t give any clues as to if you are in a partitioned off field doing the lambing, and if its private land or not. So its hard to give you any real positive answer.
The laws on the `right to roam` may be a lot different where `you` live, to where I live in the UK. So again, hard to answer more positively.