U.S. corn futures ended mixed, with the old crop spot July contract rallying in volatile trade on tight supplies and supportive demand.

The market fluctuated wildly ahead of Thursday's government reports on acreage and quarterly stocks, opening sharply higher amid renewed optimism about export demand, analysts said.

The rebound this week was first fueled by a U.S. Department of Agriculture report after Monday's close that showed deteriorating crop conditions. Further momentum has come from traders buying contracts to exit short positions, or bets the market would fall, ahead of a key agency report due out Thursday morning.

The advances were also fueled by crop worries and broad-based commodity strength.

Futures prices pulled back 16% in mid-June from all-time highs as global economic concerns grew with Greece struggling to pass austerity measures needed to win support for a plan to address its sovereign-debt crisis. Greece's parliament passed the measure Wednesday, likely refocusing grain traders on the precarious balance between supply and demand for corn. Support from a lower U.S. dollar, higher crude oil futures and firm cash prices in the face of precariously tight corn inventories buoyed the nearby July contract.

However, deferred contracts that represent developing crops for harvest in the fall relinquished its early gains, succumbing to selling as traders booked profits in an effort to reduce risk before Thursday's crop reports.

Nevertheless, traders remain optimistic about future price potential. The uncertainty of crops in a year where large production is needed to replenish depleted supplies and reports of export demand emerging on recent price declines help underpin prices, analysts said.

Otherwise, traders are taking a cautious approach awaiting Thursday's acreage and stock report for near-term guidance.

CBOT July corn ends up 2.2% at $6.98/bushel, and December dropped 0.4% to $6.50 1/2.

U.S. wheat futures rose for a second straight day as worries about the spring wheat crop in North Dakota continued to fuel the market. Millions of unplanted acres in North Dakota, a key wheat state, along with more rains that have threatened the planted crop, could lead to significant shortfalls, analysts said. MGEX spring wheat led the way higher, with the September contract jumping 16 1/2 cents, or 2%, to $8.49 1/2 a bushel, while September KCBT wheat ended up 1 cent to $7.64. September CBOT wheat ends up 2 1/2 cents to $6.741 1/4.

U.S. soybean futures finished higher after a choppy session as traders evened positions ahead of Thursday's key government updates on acreage and inventories. The market was supported by a combination of strength from external financial markets and supply uncertainties. CBOT November soy ended up 4 cents at $13.23/bushel.

Other Markets

CBOT December soyoil ended up 0.42 cents/pound; December soymeal finish down $0.60 at $338.10/short ton.

U.S. rice futures end lower as a modest rebound started Tuesday fails to hold. Prices are down sharply since June 10 thanks to widespread commodity selling and ample world rice supplies. CBOT November rice end down 5 1/2 cents to $13.67 per hundredweight.

Oat futures climbed, continuing to recovering from prior declines as traders brace for Thursday's USDA reports. The September contract advanced 2.2%, to $3.52 a bushel. Ethanol futures edged higher, with strong gains in crude oil futures helping offset pressure from lower weekly U.S. production. Ethanol for December delivery rose 0.1%, to $2.363 per gallon.

--Ian Berry contributed to this article