Kale stems make for great vegetable stock or you can put them in your compost heap. Put them in your countertop compost bin and then feed your plants with it or transfer to a larger compost pile. Kale stems contain all the nutrition that is in the leaves. Since it is the transportation system for nutrition, it retains all the necessary food for the plants. Throwing them away is like throwing away nutrition. Find some way to use it up.