UPDATE 2-ICE to take on CBOT in grains trading

* ICE plans to introduce agricultural futures contracts * Proposal needs CFTC approval * Contracts challenge cornerstone Chicago Board of Trade By Tom Polansek and John McCrank April 12 (Reuters) - IntercontinentalExchange Inc mounted the biggest challenge yet to archrival CME Group Inc's cornerstone U.S. grain futures business on Thursday, launching five lookalike wheat, corn and oilseed contracts. Aiming to leverage its benchmark Canadian futures contracts to grab bus