'grazing in a meadow' is a participial phrase which modifies cows, but cows that graze in a meadow seems to suggest that 'they are a special type of cows that graze at a meadow'. I guess, that's a change in meaning.

Choice A is more concise, and choice B actually changes the meaning of the sentence.

"Guernsey Hereford cows that graze in a meadow" uses a restrictive relative clause. ie. it implies that the interpreter can distinguish between Guenseys and Hereford cows that graze in a meadow, but what about cows that do not graze in a meadow? There is also a difference in meaning as relates to time: the present participle "grazing" is implied to occur at the same time as the verb "distinguish", but "cows that graze" is a general statement that could apply to the cows even while they're not visible for surveillance purposes.

Hi,
I will give an analogy which will make you understand better:
A. between Indian and American men
B. between Indians and Americans men
Clearly men in B is redundant when we have already used Indians and Americans, which will take care of this.

Thanks Everone. Point clear.

'That' modifies the word which it immediately succeeds...here 'that' will modify only Herefords cows and not Guernsey cows..Hence A is better