What happens if a filly is bred at just year old?

It's not advisable to breed a filly at just one year old. A filly's reproductive system is not fully developed until she is at least two years old, and breeding her too early can lead to health problems and complications during pregnancy and birth. The ideal age to breed a filly is between three and five years old, when she is physically and mentally mature enough to handle the demands of pregnancy and motherhood.