I know that sounds like a silly question, but I really need an advise. Recently a herd appeared at my local trail. Often they block the trail, and I (being a city boy) don't know how to behave toward them. I don't feel comfortable to shout at someone who can easily brake half of my bones with a single shot, especially when there are little ones with moms. What is the right way to ask them politely to free the way?
To answer your question title in very short: You don't have to fear them but some healthy portion of respect won't be wrong.
The longer version: normally cow herds aren't really aggressive so it is quite safe to just go around the herd (if it blocks the trail) or pass them if they're close to the trail. Going right through the herd isn't a good idea in any case unless you know what you're doing.
But there are two situations where some more caution is appropriate: mother cows that want to protect their calves as well as bulls can be somewhat aggressive. If those are present in a herd you should better keep a good distance and maybe avoid the herd by some more amount to prevent coming too close to them. Depending on the size of their meadow it may be the easiest to just avoid their meadow at all or at least forgo them by some 50 to 100 meters.
Something that has not been mentioned so far. Be aware of the herding dog. In the area where I bike (South Italy), herds are usually left with a couple of herding dogs by the shepherd. The only bad experience I had with a herd was not with cows itself, but with a dog. Since then, I pass through if there is no dog, or I wait/turn around if there is a herding dog.
When I read your question I had a vague recollection of reading something not so long ago about this. Have a look at this article from a UK newspaper (check out also the related articles on the page):
So you are absolutely right to be cautious - people have been killed by cows.
Personally, I live in a rural area (the New Forest in southern UK - ponies, cattle, donkeys and even pigs roam pretty much freely) and have done a bit of walking myself. I've never seen what you might call aggression, but cows can get boisterous especially, for example, if they think you're bringing them food.
Bearing in mind that a cow could weigh several hundred kilos or more, if one were to bump into you, even if not particularly intentionally, you'd certainly know about it. The article is right in that when we see them at a distance we think nothing of it, but meet them close up - they are enormous and you can quite easily imagine how people get trampled and crushed.
Now, as regards what you should do, I think you probably need to play every scenario by ear, but most of the time they won't be interested in you so as long as you proceed with caution and give them as wide a berth as possible you should be fine. Don't necessarily expect them to move for you, it'll probably be you who needs to take action to avoid them, but you should be fine.