Are you asking if you can force them to?
on what basis please?
Well, I would imagine they are saying that you should have seen them and avoided them as you would have had headlights on. They are not any different from any other obstruction.
I can't really say what their view is as we don't have their account but that seems the likely explanation.
Can I clarify anything for you?
I'm really sorry but I'm afraid that if you are travelling on a road then you are liable to avoid hazards.
The farmer might well be subject to a reduction for contributory negligence for having the cows out but he certainly isn't liable.
All the best.