Some Thoughts on an Old Wedding Custom
The Received Tradition knows three rites of grain-throwing, and each is implicated in the others.
Grain-Throw the First: the actual Sowing of Seed.
The symbolism of this gesture, both practical and ritual, needs little explication, beyond the observation that virtually every agricultural society sees sexual symbolism here.
Grain-Throw the Second: showering the newly-married with Barley.
Barley is the oldest cultivated grain known to humanity: we've been raising it for maybe 12,000 years, since the end of the last Ice Age. Though it would be impossible to prove, it's my guess that we've been tossing it at newlyweds since the end of the last Ice Age, as well. The symbolism of this playful, immemorial act can hardly be lost on anyone. Speaking as a (naturalized) Midwesterner, you've really got to love the custom's implied micro-aggression as well.