From another gardening forum:
"I mixed a large sample of manure with distilled water, I inserted my pH meter into the water and got a reading of 10.1. Thinking that the meter must be out of whack I checked my tap water and then some vinegar for calibration reference. The meter was working properly.
After a little research I learned that stables frequently add lime to horse manure for odor control. This will result in manure that is so alkaline that it could be disastrous to add to a vegetable garden, especially here in the west where soil is frequently quite alkaline to begin with. Adding limed manure could raise the soil pH significantly for many years, causing severe nutrient deficiencies in the plants."
And also:
"I've seen horse manure composts that contain as much as 43 lbs. of lime per cu. yd."