The tradition of showering newlyweds with rice, seeds, and grains predates Christianity. The Celts, mighty warriors, were also an agronomist culture who tossed rice, millet, and other grains to appease spirits and ask for blessing and fertility for the couple. The rice toss is a symbolic wish to the just-married couple for a life of prosperity and fruitfulness, which to the ancients meant many children. As a blessing, guests shower the couple with rice as they exit the ceremony.

Source: Brides