Cargill Inc [CARG.UL], one of the world's largest privately held corporations and a top commodities trader, will likely remain private for at least another decade, the chief executive said on Tuesday as he assessed challenges facing the 150-year old company. David MacLennan, who has led Cargill for the past 13 months, also revealed that unrest in Ukraine had affected plans for the company's expansion in the Black Sea, a key grain-producing region.