China's grain output will increase substantially this year despite persistent rainfall that has caused "unprecedented difficulties" for the autumn grain harvest in Shanxi and Shaanxi provinces, officials said on Wednesday.

Autumn grains constitute around three-quarters of China's grain production, which also includes summer grains and early season rice.

The year's summer grain output was 145.8 million metric tons, up by 2.97 million tons year-on-year, said Zeng Yande, who oversees development planning at the Ministry of Agriculture and Rural Affairs.

The harvest of early season rice in July and August increased by 725,000 tons, reaching 28 million tons, he said at a news conference in Beijing. Zeng said almost 80 percent of the autumn grain harvest has been completed, and an increase in yield seems certain.

"The year's grain output will set another record and stay above 650 million tons for seven straight years," he said.

Pan Wenbo, director of the ministry's planting management department, said the rain that hit the provinces over the National Day holiday caused the Weihe River, a tributary of the Yellow River, to overflow, damaging 367,000 hectares of grains. It also caused flooding in another 200,000 hectares of farmland.

"In normal years, the corn harvest is almost finished at this time of year," he said. "However, this year's harvest is only about 80 percent done."

Pan said some farmers were forced to equip harvesters with caterpillar tracks to prevent them from getting stuck in moist fields and some had to do the job manually, which pushed up the cost of harvesting by between 750 and 1,500 yuan ($117 to $234) a hectare in some places.

But he said the loss of grains in parts of provinces hit by extreme weather, including Shanxi, Shaanxi and Henan, had been more than compensated for by increases in other grain-growing areas, such as in northeastern provinces.

Pan said about 6.67 million hectares of crops were affected, compared with an average of 24 million hectares in the past 10 years.

"This year's agricultural disaster is the least severe in the past 10 years," he said.

The precipitation has also delayed up to 60 percent of winter wheat planting in Shanxi, but officials said the risk was manageable and expressed confidence in a bumper harvest next summer.

Citing experts, Pan said winter wheat planting can wait till Nov 10, which gives farmers a lot of leeway.

Data provided by the ministry showed that the average price of rice, wheat and corn in the first nine months of the year was 2.58 yuan a kilogram, up 6.2 percent from the same period last year.

Tang Ke, director of the ministry's market and informatization department, said the increase was mainly led by corn prices, adding that prices had since dropped due to increased supply.