The deal with the cow horns is that you stuff them full of cow manure, bury them for six months, dig them up, stir the stuff into water according to a precise set of directions (you swirl it clockwise, and then counterclockwise, for exactly an hour), and then spray it on the crops. This is called Preparation 500; Preparation 501 involves basically the same thing, only with horn silica instead of manure.
The Guardian posed some questions about the effectiveness of these methods recently and got answers like this one:
Dr Carlo Leifert, who is heading the European Union’s largest ever
investigation into organic farming and who grew up amid biodynamic
farmers, says that "in the wider academic community the approach of
planting by the moon cycle is seen as wacky, so nobody can really find
money from the main funding bodies to look into it. My own feeling is
that the impact that the moon possibly has is probably minute compared
to other impacts, but then again, who knows?"
This seems to be about the best answer anyone can give. Maybe biodynamic compost is better than organic compost, but how much difference does that make? The crystals formed when plant juice is mixed with calcium chloride and left to dry are more complex (oh lord, there really are crystals involved), but no one knows what that might mean.
Nutty or the new normal? The farms are no doubt lovely places to be, and if the farmers want to plant by the lunar calendar, well, I guess farmers will do what farmers will do. Meanwhile, the demand for biodynamic products is out there, and Demeter continues to certify farms that engage in the practice.
But will biodynamics ever catch on for home gardeners? Heh. You won’t find me burying a cow horn in the backyard, much less remembering where I buried it six months later.