6 cows can graze a field in 28 days.

Let x be the number of days. Then, we have 

Clearly, more number of cows will take less number of days to graze the field.

So, it is a case of inverse proportion.

Now, $28 \times 6=x \times 14$

$\Rightarrow x=\frac{28 \times 6}{14}$

$\Rightarrow x=12$

Therefore, 14 cows will take 12 days to graze the field.