AI assume that the millet you are referring to is pearl or horse millet (Pennisetum glaucum) and not red millet that is a Sorghum species. The pearl or white millet is often grown as birdseed and is not likely to be a problem to horses unless there was a heavy seed crop and the horses over-ate the seed. Some species of millet cause problems when grazed as lush pastures because they contain significant levels of oxalates. The oxalates interfere with calcium absorption and horses develop bone malformation and lameness. This has been reported as a problem in Australia, but this has not been the case in the United States.

If the plant is a Sorghum species of millet, hay from some species of Sorghum (e.g., Sudan grass/Johnson grass hybrids) has been associated with nerve degeneration of the hind legs and urinary bladder in horses that are fed the hay for long