* Optimism over U.S.-China talks tempered by lack of detail
* Smaller-than-expected Brazil soy crop cuts weigh on prices
* CBOT wheat weak, Russian sale to Egypt cools export hopes (New throughout, updates prices, market activity and comments to U.S. market open; new byline, changes dateline, previously PARIS/SINGAPORE)
CHICAGO, Jan 10 (Reuters) - U.S. soybean futures dropped nearly 2 percent on Thursday, the steepest slide since Nov. 26, reversing a week of gains due to a lack new purchases by China since trade talks with the United States.
Forecasts for a bumper Brazilian soybean crop added pressure despite recent hot, dry weather in parts of the top exporting country that has tempered forecasts for a record harvest.
Corn and wheat futures followed soybeans lower as optimism that China would be soon buying "a substantial amount" of U.S. goods, including both grains, sputtered.
"Technical selling is weighing on the market and we've been building a pretty large premium anticipating that these meetings in China would give us some sort of resolution and more buying of products. That does not seem to be the case at the moment," said Brian Hoops, president U.S.-based brokerage Midwest Market Solutions.
China's commerce ministry said on Thursday this week's trade talks were extensive and detailed, while the U.S. Trade Representative's office said on Wednesday that China had pledged to purchase "a substantial amount" of U.S. agricultural, energy and manufactured goods and services.
However, grain markets were still seeking evidence of more Chinese demand, after several purchases of U.S. soybeans by China in the past month as part of a 90-day truce agreed by U.S. President Donald Trump and Chinese counterpart Xi Jinping in their countries' tariff dispute.
In Brazil, statistics agency Conab cut its estimate of the 2018/19 soybean harvest that is underway to 118.8 million tonnes from 120.1 million in December, near the high end of trade expectations.
Others, including consultancies AgRural and Agroconsult have also lowered their crop forecasts.
Poor South American weather, including excessive rains in Argentina, have also trimmed some corn production forecasts.
Chicago Board of Trade (CBOT) March soybeans fell 17-3/4 cents to $9.06-1/4 per bushel by 11:57 a.m. CST (1757 GMT).
March corn fell 5-1/4 cents to $3.76-3/4 a bushel, breaching chart support at its 20-, 50- and 100-day moving averages.
CBOT March wheat dropped 6 cents to $5.14 a bushel, the first decline in six sessions.
U.S. wheat export sentiment has cooled following a clean sweep for Russian wheat in an import tender held by Egypt on Wednesday. The 415,000-tonne deal countered recent expectations that Russian wheat was becoming uncompetitive overseas after a brisk first half of its export season. (Additional reporting by Gus Trompiz in Paris and Naveen Thukral in Singapore; Editing by Kirsten Donovan and David Gregorio)