The Celtic wedding tradition of throwing rice on the newlyweds is extremely old and predates Christianity. The Celts were not only warriors, but also agriculturalists. Celts were farming various forms of wheat, barleycorn, and even oats, rye and millet which seem to have been introduced during the Iron Age. All of these may have been used as blessing foods for such ceremonial uses.
This tradition symbolized growth, health, and fertility as well as overall prosperity. Also, the throwing of grain was a pragmatic form of protection. In ancient times, Celts often made offering to various spirits of place for appeasement, for bounty and for favor. There was widely held belief that malevolent spirits would attend weddings to sap off the energies of the people present; and to try and jinx the future of the happy couple out of greed, jealousy or envy.
To throw rice or at least some kind of grain at the happy couple helps to appease the spirits by feeding them. Satiated and duly distracted, the spirits allow the happy couple to make their vows and getaway. The spirits themselves, even after they feasted, did not ruin the grain. Often after such ceremony the grain would be collected, cleaned and donated to a local family who were struggling in order to help bring prosperity back into their lives as well. This would help propigate the cycle of prosperity for the entire village, camp or area where the newlyweds were to live.
It would also help create a new, fresh beginning as they start their new household together. For many Celtic peoples, it was essential that the ancestors were honored with dignity. Starting fresh with a clean slate, a good dowry, a good contract and a good home were fundamental. Often times, the man would have to have already proven himself in whatever class he was in before he would even be allowed to marry.
For the woman, she too had to earn her place in the household. So, though they both had their distinctive roles with the household, they approached marriage as equals. These days, the tossing of rice is still performed at some ceremonies. One concern is that such a practice is harmful to wildlife.
It is utterly false that throwing uncooked rice at a wedding is harmful to birds. Rice will not expands in their stomachs or cause them to internally rupture and expire. Birds routinely eat such grains as part of their everyday diet. However, many venues still don’t allow this time honored custom, because of humans slipping on the grains and the insurance liability. While measures have been taken to mitigate this occurrence, it has still been known to happen.