Apparently they use it on dairy cows as well, its to combat the drastic increase in corn prices. And they only mix it 3% to the rest as hay or other cheaper grain feed. They use ice cream sprinkles as well. They said it increases milk production by upto 3 lbs per cow per day. Though they don't mention any effect on the sugar content of the milk.
https://www.snopes.com/fact-check/do-farmers-feed-cows-skittles/
https://www.snopes.com/fact-check/do-farmers-feed-cows-skittles/