The cattle will be able to smell the dog on your clothes, and be suspicious of where the animal is hiding.
If the farmer carries a bag to the field to supplement their diet with extra feed, any bag carrying human is going to be of intense interest.
If you can't get pictures any other way, and don't mind the expense, take a piece of salt lick for the cows, approach them confidently at a tangent to where you'de like them to be, and drop the salt lick there for them to fight over.
If they seem to be coming at you too fast, open your arms wide with coat unzipped so you seem bigger, and sing them a song. No, not just winding you up to get pictures of that, though it would be funny. Singing seems to perplex cows long enough they think twice about approaching, unless you smell of dog, in which case you will just seem twice as threatening.
I really think animals are psychic. Or at least, hypersentive to things we miss. Not in a very precise way, but if you are concentrating on an objective or aim you are there to fulfill, like bagging a great photo for instance, it seems to make your whole body language becomes like that of predator stalking prey, and they sense this in a general way, the same way they can sense when you are afraid. If you give off predator vibes, they aren't going to like you, hence, pretending you are there to sing.
I had a BF once who took a pair of socks he's worn a few days and would go up to the cows and drop them there, the cows would spend ages sniffing them. (don't know how they didn't faint from the smell).
Did anyone see the video (The Wild Horses of Newbury?) of the horses that stopped a road building company from proceeding with plans to cut down 400 year old oaks? It was quite odd, they really looked like they were protecting the trees. Each time a road worker approached a tree they chased them off. They even brought in police horses to try and get them away in the end. The police horse and one of the "wild" horses stood nose to nose for the longest time, like they were sharing a sad moment. Daft humanising of animal behaviour, I know, but the film was oddly moving.
