That is really a good question.
I have read and "heard" that Melatonin is a no-no on females. It will supposedly effect heat cycles and potentially could keep them from cycling. I have also heard from (first hand) people that have fed it to their heifers for years and never had a problem. I have never talked to anyone nor do I know of anyone that has implanted heifers.
I read a study that was done by the University of Wisconsin (I believe), regarding Melatonin therapy in Dairy herds. They statistically proved that Melatonin therapy actually IMPROVED fertility in Dairy cows. I don't know if it was fed or implanted.
With that said, we have a heifer on it right now. So far, she cycled and is bred while on Melatonin. We do not implant.
I will give you the recipe if you send me a PM. It will grow hair on one like nothing I have ever seen.