DEL MAR — Thoroughbred racing and its regulators have enacted rule changes in recent years to limit jockeys’ use of whips, capping the number, timing and ferocity of strikes in an apparently productive effort to protect horses from pain, welts and cuts.

Now it’s the enforcement of those rules that is leaving a mark.

As sport’s finest gather at Del Mar this week for the Breeders’ Cup races Friday and Saturday, it is split over the validity of the decision by the Horseracing Integrity and Safety Authority to suspend national wins leader Paco Lopez for six months for repeated violations of whip rules.

The debate extends to broader questions about the usefulness of restricting riders’ use of whips on the track, where that sight and sound was part of the drama of stretch duels for well over

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