My pigs have been getting Romaine lettuce everyday since we cannot find any other lettuces in our local Tesco. However I have just read that due to calcium amount in Romaine it should not be fed every day? So now I'm worried about stones etc

.
If this is true I am going to search high and low for another lettuce...any ideas which ones are good for everyday or most days? Also how oftern should you feed Romaine then if its not okay everyday?