Heavy rain across northern China this month delayed the corn harvest, submerged fields in water and raised concerns about the quality of the crop in the world's second-largest producer.

China is expected to harvest one of its largest corn crops in years this season after tight supply last year pushed prices to record levels.

But rare heavy rainfall last week hit swathes of northern China just as the harvest was due to start, hampering gathering of the crop and drying of the grain.

"Generally speaking it will still be a bumper harvest, but the rain affects the harvest pace and the quality. With too much rain, farmers can't dry freshly harvested corn and it will cause high toxin levels," said Rosa Wang, analyst at Shanghai JC Intelligence.

Adding to the worries, a severe coal shortage has forced China to curb power supplies to industry recently, and may hinder the scope for large scale industrial crop drying in coming weeks.

Continuous rain has submerged crops across Shandong, Hebei and Shanxi provinces, as well as farther south in Henan and in Liaoning in the northeast.

In Qihe county in Shandong, the corn harvest would normally be completed by now, but has not started because of severe waterlogging in the fields, said Paul Niven, a China-based dairy consultant.

"The roots are rotting in the ground, and the stalks are dry and unusable for silage," he said in a social media post.

Videos posted on social media showed farmers in Shanxi province pumping water out of flooded fields.

In Hebei, farmers have had to buy special harvesters with excavator tracks to harvest their corn, said farmer Liu Ligang.

"It's been raining for the last two weeks. We'll finish our harvest in a couple of days, but the quality won't be as good as last year," he said.

A few million tons of corn could be of poorer quality as a result of the rain, said Meng Jinhui, senior analyst with Shengda Futures.

Chinese farmers sharply increased corn planting this year to cash in on record prices, with analysts expecting output up at least 6% on last year.

That had raised expectations of a drop in imports. Analysts said they are still assessing whether the rain would lead to significant crop losses, changing the outlook for imports.

The harvest in China's top two corn producing provinces, Jilin and Heilongjiang in the far northeast, does not get into full swing until next month.

Meanwhile, rains in central Argentina have helped farmers there plant the 2021-22 corn crop, with 23.2% of expected area sown so far, the Buenos Aires Grains Exchange reported.

The key corn-growing provinces of San Luis and Cordoba are enjoying good ground moisture, the exchange said in its weekly report. Argentina is the world's second-biggest corn exporter, after the United States.

And in the U.S., soybean and corn production will be bigger than previously expected, the government said on Tuesday.

The U.S. corn harvest was seen at 15.019 billion bushels and the soybean harvest was seen at 4.448 billion bushels, the U.S. Agriculture Department said in its monthly World Agricultural Supply and Demand Estimates report. USDA forecast corn yields of 176.5 bushels per acre and soybean yields of 51.5 bushels per acre.

Analysts had been expecting the report to show a corn crop of 14.973 billion bushels, based on an average yield of 176 bushels per acre. The average estimate for soybean harvest was 4.415 billion bushels, based on an average yield of 51.1 bushels per acre.

Hugh Bronstein in Buenos Aires and Tom Polansek in Chicago contributed to this report.