The cobalt threshold in Australia will be halved to bring it into line with major international jurisdictions.

From September 1, the threshold will be 100mcg per litre of urine, Racing Australia announced on Thursday.

RA will also introduce a cobalt plasma threshold for the first time of 25g/L.

It says plasma sampling is a more effective detection of cobalt as plasma cobalt levels remain elevated longer than urine levels after high doses.

In a notice to trainers, RA said a normal racing diet was more than sufficient to meet a horse's nutritional requirements for cobalt and vitamin B12.

It said trainers should consult their veterinarians to ensure that their oral supplementation regimen provides only the amount of cobalt necessary to meet the scientifically established nutritional requirements of the horse for cobalt.

Several high-profile trainers have fallen foul of the 200mg limit, most notably Black Caviar's trainer Peter Moody who closed his business after being handed a six-month suspension.

NSW trainers Sam Kavanagh and Darren Smith are serving lengthy disqualifications.

Kavanagh's father, Melbourne trainer Mark Kavanagh, and Danny O'Brien will have their appeals against cobalt bans heard next month.