I grab a couple handfuls of grass when I go to feed, unless I see the rare dandelion. Honestly, whatever is handy on the way from the feed shed to the hutch ... they don't seem to mind weeds. If you want some uber-nutritious plants, parsley and kale pack a punch. If you haven't been feeding fresh green stuff, be sure to start them off ***SLOWLY*** or you risk upsetting their system which can lead to diarrhea, which can be deadly this time of year. Also, it doesn't take a whole lot of BOSS - I usually mix it in with pellets twice a week. BOSS can add weight, being high in fat/oil. Great for nursing does, but too much adds fat which is also a factor in fertility for both sexes.
On judging weight, I usually feel along the spine and upper rib cage. You should be able to feel the ribs faintly, but not actually see them.