CANBERRA, March 13 (Reuters) - Chicago soybean futures fell on Wednesday from one-month highs in the previous session as a production forecast downgrade by Brazil's crop agency tests speculative investors who have bet heavily on further price declines.

Corn futures edged higher and wheat futures also rose after falling on Monday to their lowest since August 2020 amid lower Russian export prices and cancelled U.S. sales.

The most-active soybean contract on the Chicago Board of Trade (CBOT) was down 0.7% at $11.88-1/4 a bushel by 0552 GMT, having hit $11.97 on Tuesday, its highest since Feb. 13.

Prices remained down around 8.5% this year and last month hit their lowest since 2020.

"It is too early to say whether this is a readjustment or a 'dead cat bounce'", said Andrew Whitelaw, an analyst at Australian consultants Episode 3.

He said speculators had grown their net short position in soybeans and the recent rise could be due to some of them being forced to cover those positions, with downward pressure on prices remaining.

Commodity funds were net buyers of CBOT soybeans on Tuesday, traders said.

Crop agency Conab on Tuesday lowered its projections for Brazil's soybean production by 2.6 million metric tons to 146.858 million metric tons and for its corn production by nearly a million tons to 112.753 million tons due to adverse weather conditions.

However, Brazilian soy supply remains plentiful after a record harvest last year and some Brazilian soybean farmers are hoarding new crops in the hope that prices improve, a survey found.

Also pushing down prices are expectations of a bumper Argentinian crop. Intense rains in Argentina are beginning to leave the soil too wet in key agricultural areas, which could make it harder to harvest.

CBOT corn was up 0.1% at $4.42-1/4 a bushel after climbing to $4.45 on Tuesday, its strongest since Feb. 6. Prices are still down around 6% this year and fell to their lowest since 2020 last month.

Wheat rose 0.5% to $5.50-1/4 a bushel but was not far from Monday's low of $5.24.

Falling Russian export prices and hefty global supplies continue to pressure wheat futures and reduce the competitiveness of U.S. wheat on the global market.

U.S. exporters cancelled sales of more than 504,000 tons of wheat destined for China in the past week.

Ukraine's 2024 combined grain and oilseed harvest could shrink to 76.1 million metric tons from 82.6 million tons last year and 107 million tons before Russia invaded, Ukrainian grain traders union UGA said.

The UGA said the 2024 harvest could include 26.3 million tons of corn, 20 million tons of wheat and 13.7 million tons of sunflower seeds.

(Reporting by Peter Hobson; Editing by Sherry Jacob-Phillips and Sohini Goswami)