Glucosamine and MSM (methylsulfonylmethane) are muscle and joint supplements that are usually fed to performance horses. When fed according to manufacturers’ recommendations, it is not harmful to horses. So feeding it to pregnant or lactating mares is not harmful, but there is probably no reason to feed it to those types of horses. They probably have no nutritional need. However, if the mares are older and have joint problems, then glucosamine may help them in terms of being comfortable moving around.