CHICAGO, March 28 (Reuters) - The Chicago Board of Trade has won regulatory clearance to shorten the trading cycle for U.S. grain markets after a move to extend activity hurt liquidity. The Board of Trade, owned by CME Group Inc, said it will cut futures and options trading for crops like wheat, corn and soybeans to 17.5 hours per session from 21 hours starting on April 8. The Commodity Futures Trading Commision said it did not object to the plan following a 10-day review. Under the new schedule, electronic trading will run from 7:00 p.m. CT to 7:45 a.m. CT (1245 GMT) Sunday to Friday. Trading will pause for 45 minutes before resuming on the screen and in Chicago's historic open-outcry pits until 1:15 p.m CDT. The change comes less than a year after the Board of Trade increased the cycle to 21 hours a session from 17 hours in response to a challenge from arch-rival IntercontinentalExchange Inc. ICE in May began electronic trading on a 22-hour basis. Grain traders said the Board of Trade's increase spread out volume and reduced liquidity, and the exchange last week confirmed that "quantitative evidence" supported the complaints.