Assembly characteristics of cows (and possibly horses)

All cows in prime number groups or alone

After adjusting the moving the cucumbers up on to their new growing frame (our old shoe rack) and taking another crop from the peas, I drove home from the allotment past some cows.  Last week there were horses running about in the field next door.  After watching both I have come up with an observation:

when cows are milling about in a field, if they are not alone, they form in to groups with a prime number of members. It seems to work with horses too. Did you ever see 4 cows in a group in a field?