Here's what I find on NASS also dated 1999 -

http://usda.mannlib.cornell.edu/reports/nassr/livestock/

 

http://jan.mannlib.cornell.edu/reports/nassr/livestock/equine/equi1999.txt

 

Inventory of equine in the United States as of January 1, 1999 totaled 5.32 million head, up 1.3 percent from the 5.25 million head on January 1, 1998. Equine includes horses, ponies, mules, burros, and donkeys.
Texas ranked first in equine inventory with 600,000 head followed by California, and Tennessee with 240,000 and 190,000 head, respectively.
Florida, Oklahoma, and Pennsylvania tied for fourth with an inventory of 170,000 head. Ohio ranked seventh with 160,000 head, followed by Kentucky, Minnesota, New York, and Washington with 155,000 head. An additional fifteen States had equine inventories of 100,000 head or more.