Cobalt could have accumulated in three horses trained by Lee and Shannon Hope through their long-term feed and supplements regime, a tribunal has heard.

The father and son training partnership are fighting charges that could potentially result in a lengthy disqualification after three horses returned cobalt readings exceeding the threshold allowed under the racing rules.

Racing Victoria stewards and their experts argue the cobalt must have been administered in a high dose the day before their races or in a lower dose on race day.

But an expert called by the Hopes' legal team has told the Racing Appeals and Disciplinary Board there are several possible causes.

Monash University Professor Colin Chapman said it was possible the long-term program of nutritional supplementation and injectable medication containing cobalt could result in it accumulating in a horse's body.

That could result in its unpredictable excretion including occasions where the level could exceed the declared raceday threshold of 200 micrograms per litre of urine, he told the tribunal.

Prof Chapman said the cobalt readings of Best Suggestion (510 micrograms per litre), Choose (450) and Windy Citi Bear (290) could have occurred from long-term use, short-term within the rules of racing or short-term use outside the rules.

Under cross-examination by Jeff Gleeson QC for Racing Victoria stewards, Prof Chapman said he could not say which was more likely.

Mr Gleeson asked: "You don't advance the innocent administration as disclosed as being more likely?" Prof Chapman replied: "My wording is that it is possible."

Mr Gleeson has questioned why thousands of other horses did not produce frequently high readings, if the accumulation explanation held.

Prof Chapman, from Monash's Faculty of Pharmacy and Pharmaceutical Sciences, repeatedly said there were many factors involved.

"If there's accumulation and then other factors come into play then of course what you've seen happen can happen," he said.

The RAD board heard the supplementation regime was basically the same for all of the Hopes' 25 horses and had been given for many weeks before the high readings recorded for the three in June to September last year.

The Hopes changed the supplements and medication regime after their vet died and after being told about an elevated cobalt reading, with the new vet advising them to remove the intravenous administration of supplements.

The Hopes have pleaded not guilty to the administration, or causing to be administered, the prohibited substance as well as the presentation of the horses to race with a prohibited substance in their system.

Prof Chapman said the type of supplements used by the Hopes were like people taking a multivitamin.

"We take it to optimise our health. It's that sort of thing.

"You're optimising performance."

The Hopes' hearing, the first cobalt inquiry in Victorian thoroughbred racing, will continue on Tuesday.