(New throughout, updates prices, market activity and comments through close, adds details on China waivers)
By Mark Weinraub
CHICAGO, Sept 24 (Reuters) - MGEX spring wheat futures rallied for the fifth day in a row on Tuesday as farmers harvesting in the northern U.S. Plains found that wet conditions had lowered the quality of the crop there.
"They are talking about protein levels really taking a hit with the wet weather," said Mark Gold, managing partner with Top Third Ag Marketing in Chicago.
The front-month MGEX contract, which has gained 7.6%during the winning streak, hit its highest since June 28.
Winter wheat futures were weaker, pressured by good soil moisture levels in key growing areas as well as light demand on the export market.
Corn futures were slightly higher on technical buying but better-than-expected crop conditions kept the gains in check. Soybeans rose on renewed optimism about Chinese demand for U.S. supplies of the oilseed.
U.S. President Donald Trump sharply criticized China's trade practices on Tuesday in an address at the U.N. General Assembly. Trump also held out hope that the two economic superpowers could reach a deal to end their trade dispute.
China has given new waivers to several importers to buy U.S. soybeans exempt from retaliatory tariffs, in a goodwill gesture ahead of high-level trade talks next month, two sources familiar with the matter said. The waivers, offered in two batches, total around 5 million to 6 million tonnes, according to one of the sources.
Chicago Board of Trade November soybean futures ended up 1-3/4 cents at $8.94-1/4 a bushel. Soybeans retreated from their session highs after hitting resistance near Monday's peak.
CBOT December corn was up 1-1/2 cents at $3.74-3/4 a bushel, recovering from early weakness after finding support at its 10-day moving average.
The Commodity Weather Group said a frost was possible late next week in the northern Plains and northwest Midwest, posing a "limited threat" to 10% to 15% of corn and soybean crops.
MGEX spring wheat for December delivery was 7-1/4 cents higher at $5.44-1/2 a bushel. Chicago Board of Trade December soft red winter wheat was down 1-1/4 cents at $4.82-1/4 a bushel and K.C. December hard red winter wheat was 1-1/2 cents lower at $4.05 a bushel.
The U.S. Agriculture Department said on Monday afternoon that 22% of U.S. winter wheat had been planted, up sharply from 8% last week. Some 87% of U.S. spring wheat was harvested, topping market expectations.
Additional reporting by Naveen Thukral in Singapore and Michael Hogan in Hamburg Editing by Edmund Blair, Alexander Smith and David Gregorio