How many people actually lift the lips of horses to see if they are registered in some way, shape or form? Even in the early 2000’s? Was that addressed in the original paper? It appears it was self-reported by the feedlots but I don’t know that to be the case.
I found this (2012) by AVMA. It doesn’t give a breed breakdown but has some numbers at least.
Oh, a bit more info from within that link:
Horse breed/type. The results suggest that Quarter horses sold at a
premium, relative to grade horses, which do not have a declared breed
registry. Ponies also tend to sell at a premium relative to grade horses,
for those ponies sold in the higher categories (i.e., quantiles). An
unexpected result was that other breed types, Paint horses,
Appaloosas, and Thoroughbred horses sold at either a discount or did
not show statistically significant difference in price, relative to grade
horses. This could have been due to the small number of observations
compared to other breeds and that for certain breeds, such as
Appaloosas, there could be a lack of buyers for these types of horses.
Then Appendix II has a chart and here is a footnote:
Notes: The estimates in the table that are statistically significant at the 0.05, 0.01, and 0.001 percent
levels are noted by one, two, or three asterisks, respectively.
Although the sales data included 27 breeds, the primary breed types were Quarter horses, 73.9
percent of the horses; grades (low-valued horses without breed designation), 12.1 percent; and Paint
horses, 11.9 percent; with a small number of observations for breeds such as ponies, 0.57 percent;
Appaloosas, 0.45 percent; and Thoroughbreds, 0.25 percent.
a
The upper bounds for the quantiles correspond to horses priced at $600 (20th), $1,000 (40th),
$1,400 (50th or median), $1,750 (60th), and $3,000 (80th).