Which is the best fertilizer, cow or horse manure and why?

Neither actually make very good fertilizer til they've been completely composted.

As I understand it, steer manure is pretty salty, so I'd say composted horse manure is the better of the two.

But composted chicken manure is better than both of those...

They are both equally good for the garden it is said should be composted first before putting on garden but could be applied thinly 50mm and dug in a bit.

I can remember the horses delivering bread and milk and everyone used to go and grab it to put on the garden if was some outside.their place,

I would have thought cow manure would be better for one reason feeding all grass and hay. where the horse manure might be from stabled horses and grain fed might be a difference as well as mixed with whatever bedding with urine as well. but would compost ok if had a spot.

Chicken manure is very strong and so must always be aged before using it is very good i believe.

Both are good when composted and mixed with the soil. However, cow manure will contain more weed seeds if they eat grass or hay. Horse manure is hotter then cow manure. If horse manure obtained from farms contains lots of wood chips from the stall, then I would suggest cow manure. Wood chips robs the soil of nitrogen, instead of furnishing some. I would not turn down either source of fertilizer.

it's horse poo that contains more weed seeds.... they don't have the extra chewing of cud like cows do.... that takes care of a lot of the seeds... then, once it's composted, cow really does a good job.... if horse has made it in the compost to the point where you can't tell it's horse or dirt, THEN it's about equal with cow.... as the heat from the compost will have killed many more of the seeds the horses passed thru.....

as far as NPK is concerned, tho... have a look here....

http://www.plantea.com/manure.htm