The possibility that Hong Kong's racehorses have been exposed to contaminated feed yesterday widened to cover more than half the population in training at Sha Tin after a second type of feed from California was found to contain the banned substance zilpaterol; a total of 17 horses have tested positive for it.

After four post-race positives for the cattle-feed supplement for trainer Ricky Yiu Poon-fai on Monday, his eight runners for yesterday's meeting failed precautionary screening and were withdrawn. Three horses from the stables of Paul O'Sullivan and Me Tsui Yu-sak were also withdrawn. The last of these, Double Glory from Tsui's stable, was withdrawn at the request of the trainer after a urine sample could not be obtained.

Drawing on common factors between the brands of feed used in the stables of Yiu and five other trainers, the Jockey Club specifically screened 27 of yesterday's proposed runners for the drug. Tsui's stable runners were not included as he uses another feed, understood to be manufactured in the same California mill.

Tsui's positives were picked up during a routine raceday drug testing which targetted zilpaterol and was accented
towards clearing as many runners as possible to race. Some runners were permitted to run with miniscule traces of zilpaterol evident.

The Tsui finding called into question whether the feed he uses - identified as LMF by the Jockey Club's director of racing operations, John Ridley - might also contain zilpaterol, but the club's laboratory was still working last night to establish that fact.

If that is found to be the case, the episode raises the possibility that "more than 50 per cent" of the horses at Sha Tin have been eating contaminated feed.

"The club will be contacting the manufacturer concerned to advise them of the developments, of course, but the club has to prioritise and right now our priority is that horses in Hong Kong don't have this substance in their system," chief steward Kim Kelly said. "As far as who is to blame, they are matters to be addressed as the investigation unfolds."

Declarations will take place as normal today for Sunday's meeting at Sha Tin. Horses will be tested for zilpaterol and club officials will meet all trainers this morning to discuss the situation.

"It is necessary to have a consultation process with the trainers - if 50 per cent of the feed is contaminated then we have to look at some alternatives. Then trainers have to decide 'is this what we want to feed or not?'," chief executive Winfried Engelbrecht-Bresges said.

"Then we will look at other logistics and maybe we have to fly things in. We know the dimension of the problem, now we have to look at the scenario planning."

If the club has to fly feed in as an emergency measure, it will be costly. Eight container loads of 40 different kinds of feeds arrive at Sha Tin each month, all by sea, with a five-week lag between order and delivery. One container load of feed is equal to 15 of the pallets used in air transport.

"We need to work out if this is a batch problem or a total feed problem at the production line to understand the ramifications," Ridley said. "Logistically, if we have to change feed for more than 50 per cent of the horse population, it becomes very difficult to do overnight."

The club laboratory and vet department are instituting a "significant urine sampling process" of horses known to have had zilpaterol in their systems.

"Zilpaterol is a new substance, it's a substance which has no place in equine feeds, so the excretion times are not known," Kelly said. "The impact, if any, on our fields for Sunday will rely on excretion times. All use of the first feed, used by Ricky Yiu, stopped on Tuesday. But as we haven't, as of this moment, definitively established the second feed is also contaminated, horses will have gone home after racing today to be given that feed."