That 20% is just what fraction of the shipped alfalfa is water. It doesn't tell you anything about how much of the water used to grow the alfalfa stays.

If you used a 1000000 kg of water to grow alfalfa, you'd get around 25000 kg of alfalfa (which would contain 5000 kg of water and 20000 kg of other stuff).

So for every 5000 kg of water that is in that alfalfa when shipped around 995000 kg of water stays.