Trainer Ron Quinton has escaped a penalty over illegal levels of cobalt found in two of his horses with incorrectly labelled feed accepted as the likely culprit.

Quinton fronted a Racing NSW stewards inquiry on Thursday after pre-race urine samples taken from Boss Lane and Imanui in August revealed higher than permitted levels of cobalt.

Stewards carried out tests on the Barastoc Phar Lap feed Quinton was using with the analysis finding cobalt levels 40 to 50 times that listed in the ingredients.

Quinton said the Barastoc Phar Lap feed was the only explanation.

"Obviously it's come from the feed. I have no suspicion of any of my staff. There's no chance. I'm not into that stuff," Quinton said.

"All we ever do is aminoplex (a vitamin and amino acid injection) on a Monday morning and the only treatment they have is if they've got an infection or a cut on their leg.

"I had been feeding this feed for 10 to 12 years and have never had a problem and all of sudden in the period of one month I've got all sorts of problems.

"I have total respect for the Rules of Racing but this was totally out of my control."

RacingNSW chief investigator Nathan Heywood visited Quinton's Randwick stables on August 3 after Duchess Pedrille, The Gavel, Boss Lane, Chiquita and Imanui returned high, but not excessive, readings from tests taken in July.

Chief steward Marc Van Gestel said the National Measurement Institute found the cobalt levels in the feed used by Quinton were 40 to 50 times the 0.5 milligrams of cobalt per kilogram listed on bags taken from the stables.

Racing NSW senior veterinary surgeon Dr Craig Suann said if Boss Lane and Imanui were fed three kilograms of the pre-mixed feed daily for several weeks it was "conceivable the elevated urine samples could be attributed to the feed".

Stewards tested Dixie Blossoms, Daysee Doom and Imanui after Quinton stopped using the feed and their cobalt levels were within the permitted range.

Imanui finished fourth in a Benchmark 80 Handicap (1500m) at Rosehill on August 12, and returned to racing last month.

Boss Lane was also fourth in the Group Two Missile Stakes (1200m) at Randwick on August 5 and has not raced since.

Quinton was found guilty under the rule governing presenting horses to race with prohibited substances in their systems and disqualified both horses from their respective races.

Boss Lane returned a reading 118 micrograms per litre of urine and Imanui 157 mcg/L.

The allowable threshold is 100mcg/L.

Once Quinton's case emerged stewards warned trainers about the particular batches of feed and also told them to be careful when ordering custom-made horse feeds, feed additives, and/or supplements as some manufacturers were not listing all the active constituents, and/or the concentration of all active constituents on the labels.