Sheikh Mohammed and Godolphin have beaten off Coolmore to buy a Frankel half-brother to Golden Horn at the Tattersalls October Sale at Newmarket.

The Dubai-based operation beat off Coolmore to secure the most expensive yearling colt sold anywhere in the world on Tuesday - going to 3.6 million guineas ($A6.8 million) for a Dubawi half-brother to St James's Palace Stakes winner Barney Roy.

The two powerhouse owners did battle again 24 hours later for a son of Frankel and a half-brother to the brilliant Golden Horn, who won the Derby, the Coral-Eclipse, the Irish Champion Stakes and the Prix de l'Arc de Triomphe in 2015.

The bidding developed into another intriguing head-to-head between MV Magnier of Coolmore and Anthony Stroud, representing Sheikh Mohammed.

Others who were involved during the early exchanges - Chad Brown, Alastair Donald and Juddmonte - dropped away.

The colt was ultimately knocked down to Stroud for 3.1 million guineas.

The sale was another excellent result for Irish-based consignors Norelands Stud, with a Siyouni half-brother to 2000 Guineas winner Magna Grecia knocked down to Magnier for 1.3million guineas earlier in the day.

"I am not often lost for words, but I think I am now. This is easily the best price we have had in the sales ring, " Harry McCalmont of Norelands Stud said.

"There has been a lot of interest in him, there were some serious people who liked him, however you never know what is going to happen until you get to the ring."

McCalmont bought the mare of the Frankel colt, Fleche D'Or, at Tattersalls in December 2012 for 62,000 guineas after she had foaled Golden Horn.