Data from: Racehorses are getting faster

Previous studies have concluded that thoroughbred racehorse speed is
improving very slowly, if at all, despite heritable variation for
performance and putatively intensive selective breeding. This has led to
the suggestion that racehorses have reached a selection limit. However,
previous studies have been limited, focusing only on the winning times of
a few elite races run over middle and long distances, and failing to
account for potentially confounding factors. Using a much larger dataset
covering the full range of race distances and accounting for variation in
factors such as ground softness, we show that improvement is, in fact,
ongoing for the population as a whole, but driven largely by increasing
speed in sprint races. In contrast, speed over middle and long distances,
at least at the elite level, appears to be reaching an asymptote. Whether
this reflects a selection limit to speed over middle and long distances or
a shift in breeding practices to target sprint performances remains to be
determined.