Chicago wheat was poised on Friday to post its biggest weekly decline in more than three months, with lack of demand for pricey U.S. shipments and the improving condition of the winter crop dragging on the market.

Corn, though, has risen more than 2 percent this week, and soybeans have lost ground after five consecutive weeks of gains, on expectations that U.S. farmers will switch acres to beans from corn in the upcoming planting season.

The Chicago Board of Trade most-active wheat contract has shed 3.8 percent this week, the most since late December.

Soybeans were headed for a 1.5 percent weekly drop, the first such fall in six weeks. Corn has gained 2.2 percent, the biggest gain for a week in four months.

Forecasts for rains across U.S. Plains pressured wheat.

"Weather forecasters have some rain relief for the dry parts of the U.S. hard red winter wheat crop," said Tobin Gorey, director of agricultural strategy at Commonwealth Bank of Australia. "The rain is modest in the near term. A larger rain event sits about a week away and that will relieve the worry for a time," Gorey said.

Still, U.S. wheat exporters continue to face difficulties in selling wheat amid competition from cheaper suppliers in Europe and the Black Sea region.

The U.S. Department of Agriculture export sales report showed net cancellations of old-crop U.S. wheat totalling 58,100 tonnes in the week to March 31, the weakest results in the current marketing year.

Analysts had expected old-crop net sales of 150,000 to 350,000 tonnes.

The USDA has rated 59 percent of U.S. winter wheat as being in good-to-excellent condition, better than expected.

The agency also reported export sales of U.S. corn in the week to March 31 at 945,200 tonnes for 2015/16 and 175,100 tonnes for 2016/17, in line with trade expectations.

Commodity funds bought an estimated net 10,000 CBOT corn contracts on Thursday, trade sources said.

The funds also were net sellers of 6,000 soybean contracts and 3,500 wheat contracts, traders said.

China is aiming to reduce corn planting by about 8.2 million acres by 2020 and boost soybean output, in order to meet domestic food demand, the ministry of agriculture said on Thursday.