In Robert Chapman's book, "Woodturning - A Fresh Approach" he advocates immersing the wood on liquid garden fertiliser for 24hrs, then seasoning, which is very similar to Terry's recipe link, but without the shite.
Phil,
The fertiliser produces the nitrogen which encourages the bacteria which causes the spalting pattern, but fertiliser alone wont do anything. The black spalting lines on the beech or birch spalting patterns are caused not by the infection itself but by different bacteria establishing themelves in the wood and then confronting another bacteria, the lines are zonal lines and are the demarcation between the two different bacteria.
Now I'm not an expert on Horse digestion systems but would think that what goes through a horses gut is probably not the best source of bacteria that would produce spalting, but I could well be wrong. I have been wrong before, I may well be wrong now and I will be wrong in the future! Feeding Horses actimel before they shit on your newly felled tree could be expensive!
I feel a lot better, and my wife says smell a lot more acceptable, having smeared decomposing leaf and woodland vegetation on the tree than horse shit. It certainly makes picking your nose less gruesome! The beer issue needs some clarification as the fizzy stuff is sterile as far as yeast and bacteria are concerned. Real Ale is alledgedly beneficial as it is a live beer. I can recommend a beer that will destroy anything, McMullens, a local brew to Hertford. It must be good for helping to spalt wood as its got no other known use to man.
Back to the fertiliser, Ammonia chemical formula, NH4 contains the nitrogen in the miracle grow and is cheap, first you drink the beer, wait an hour and then point in the direction of the log masquerading as a garden ornament behind the wifes geranium bed. Just make sure the mother in law has gone home.