Weed killer is used in the cultivation of barley to eliminate 2 of the 3 kinds of weed that hamper barley growth. It is created in a Kettle from 1 Toad Skin mushroom, 5 wood, and 5 water. These ingredients will produce a batch of 50 weed killer.
Weed killer is not required to grow barley, but its use tends to increase barley yield per planting, and dramatically improves the chance of the crop reaching "Ready to Harvest" status. However, there are successful cultivation strategies which ignore weedkilling altogether. See the barley guide for different approaches to planting.