from a purely biological view theoretically the dehorned cow should produce higher quality milk as she is not wasting energy on horn growth. theres been a bit of work done on soay sheep suggesting by using more energy growing horns the bigger horned tup lambs are more likely to die during their first winter
Hence the survival and persistence of scurred tups on Soay. When there is a population crash on the island, which happens on a roughly 7 year cycle, with worm maximums, or grass over grazed, the big horn senior tups can no longer fight for their harems, and die. This apparently is because of the extra energy needed to grow and support such large horns (Soay do have proportionally large horns). The scurred tups, which have previously been outcasts on the periphery of the flock, sneaking the odd bride when the big boy was elsewhere, suddenly find they are still alive when he's dead and thus the scurred gene is passed on.
However, this doesn't explain unspecified metabolic waste products found in the milk of dehorned cows. If large horns are a stressor, then the dehorned cow should be healthier and not have waste products in her milk. So is Mr Biodynamic saying that his cows are not dehorned and therefore the milk is better quality?
I would want to see some much better research done to prove or disprove his claim, with naturally polled cows included. Otherwise I'm with henchard - it's a load of blarney and hype. I wonder why biodynamics is so obsessed with cow horns? They do their rituals with manure in one, buried somewhere to ensure soil fertility.