The Hong Kong Jockey Club is exploring the possibility of importing new batches of horse feed after a spate of positive swabs.

Seventeen horses have tested positive to zilpaterol, a cattle-feed supplement, but there are concerns more than half of Hong Kong's racehorse population could be affected by the contaminated feed.

While declarations for Sunday's Sha Tin meeting were taken as usual on Thursday, horses will be swabbed for zilpaterol before they are allowed to start.

Feed for Hong Kong's horse population is shipped by sea from California and the prospect of new batches being flown in as an emergency measure presents logistical and financial issues for the club.

"We need to work out if this is a batch problem or a total feed problem at the production line to understand the ramifications," HKJC's director of racing operations John Ridley told the South China Morning Post.

"Logistically, if we have to change feed for more than 50 per cent of the horse population, it becomes very difficult to do overnight."

Twelve of the horses which tested positive are trained by Ricky Yiu.

Horses from Paul O'Sullivan and Me Tsui Yu-sak stables also returned positive results.