U.S. soybeans were on track on Friday for a ninth consecutive weeks of gains, the longest bull run in 43 years, as Chinese demand helped to tighten supplies and concerns began to emerge about U.S. production prospects.

Wheat prices eased on expectations of a bumper U.S. winter crop as the market awaited the release of monthly data from the U.S. Department of Agriculture later in the day. Corn was little changed.

Most-active July soybeans on the Chicago Board of Trade was up 0.4 percent at $11.80-1/2 per bushel at 1100 GMT and remained on track for a ninth consecutive weekly gain.

The last time soybeans rose for nine weeks in a row was in 1973.

Warm weather this week and the potential for a La Nina in the U.S. Midwest this summer have made investors nervous about stress to soybean plants.

"La Nina is the big story," said one Sydney-based trader. "The supply-and-demand situation in beans is really tight and it has caught everyone off guard. China is buying large volumes."

The rally was initially fueled by diminished crop potential in Argentina following heavy rains although dealers said the situation in the world's third biggest producer was stabilizing.

"A drier weather pattern in Argentina has allowed the country's soybean harvest to progress, with fieldwork now about 87 percent complete," analyst Tobin Gorey of Commonwealth Bank of Australia said.

Corn prices were underpinned by the current strong pace of U.S. exports as the market awaited USDA data.

"Analysts are expecting (USDA to report) higher U.S. exports given that weekly sales continue to power along," Gorey said.

Export sales of U.S. corn to be shipped ahead of the fall harvest have topped last year's pace for the first time this season, government data on Thursday showed, fueled by competitive prices and a lack of available grain from drought-hit Brazil.

CBOT July corn was unchanged at $4.26-1/2 a bushel with the market on track for a fifth consecutive weekly gain.

Wheat prices were lower with an improving crop outlook in the U.S. adding to concerns about excess global supplies.

Analysts polled by Reuters expect USDA on Friday to boost its forecast for U.S. winter wheat production due to good yield results in early-harvested fields in Kansas and Texas.

July CBOT wheat fell 0.5 percent to $5.07-3/4 a bushel while September wheat in Paris was off 0.6 percent at 170.00 euros a tonne.