Wow, I'm surprised they'ed eat 19 cups. I feed the same amount of birds in 1 of my pens about 5 cups per day and they usually make it last all day with a bit left over the next morning.
That looks like a huge scoop and 6 scoops from that should be enough to feed a horse! Sure you/re not feeding the local squirrels, rats and wild birds as well? If you feed layer mash you don't really need extra oyster shells because it has enough calcium.
X2 what everybody else said about all...