If in doubt walk along the perimeter of the field so you can dive into a bush or something. I think the best policy if you have nowhere to dive to immediately is to avoid any interaction, act like your presence is the most natural thing in the world, don't suddenly freeze or run or stare the beasties out, only shout if they are within feet and haven't yet slowed down :)
I used to be completely unafraid of cows except when there was horns of any length or description, they freaked me out totally. Until I was ambushed by portugese cattle with 2ft horns and came very close to being overrun, now I treat all bovines with respectful indifference and avoid mixing with the male of the species where possible. Kind of hard with creatures this curious though.
I've been 'escorted off the premises' a few times by frisky bullocks that only charge once you are within touching distance of the fence, mostly they are all bluster but above all they are extemely curious animals. If you have to resort to shouting at them then just shout, dont go all nutty jumping and stamping, they will usually retreat and respect an authorative voice but an upredictable nutcase might make them feel a more 'proactive' response might be called for to eliminate the danger!