Peggsue...yes I totally agree with you.
You can't compare a quart of beetpulp it wont weight the same as a quart of oats, yep totally agree.
Now that being said the OP didn't say they fed DIFFERENT feed then she was feeding just that they fed by quarts, instead of pounds.
So going by the theory that the horse was getting for example 2Lbs of complete feed, she can say I want my horse on 1 quart complete feed....get my drift
It just means using the formula, of 320z makes 1 quart....you can weigh your feed and convert it to volume.
But as you say you cannot COMPARE different types of feed this way as the balance and nutrient will of course be different...one must treat each feed separately.