You can have up to 2 bushels of grain in a completely empty still, but if there are a few grains left in there, the first bushel will put you over 250/500 number, which would comprise the first bushel- disallowing any more bushels to be added. (You can unbundle one if you want to add just a fraction of the grain.) So- Yes! It appears the quantity picker allows you to choose everything you have, but the good news is we won't take them all. :) We'll look over the quantity chooser on this, thanks!
ahh.. in an ideal world, you could add the bundles and it would "split it" knowing that 1 bundle equals 250 grain and just dump grains back in the inventory.