It started when they realised cows in Derbyshire (etc.) thrived better with iodine supplements. I imagine the iodide was an additive, used since some time early in the twentieth century. Now dairy produce is our main source, especially in low iodine parts of the country. I think we have been feeding cows with something other than grass in winter for thousands of years. Nowadays mainly commercial feeds and root crops. See mad cow disease.