Iron in water
In many parts of the country, the water can be high in iron. You can quickly see this if the sinks and toilets have iron staining.
This is easily corrected with a water filtration system purchased at local hardware store or on Amazon. This filters all water coming into the barn. For smaller operations a smaller filter system with a water hose adapter- like you use with RV’s or campers- might be more suitable. Using a filter at the water hydrant where you water your horses from will cut down the amount of iron you are offering them tremendously.
In many barns and fields, filters are not practical, but the key is to do what you can by avoiding weeds and iron supplements that might cause your horse's insulin to elevate.