The nation's largest wheat-growing province of Henan is expected to be hit by more rain in the coming days, state forecasters said on Thursday, complicating efforts to harvest grain already damaged by wetter-than-normal weather in late May.

Known as the "granary of China", the south of Henan had been struck by higher-than-normal rainfall in the last week of May days ahead of the harvest of wheat planted last winter. The rains have caused some grain to sprout or be hit by blight.

China's winter wheat accounts for the majority of the country's annual wheat output.

Medium to heavy rain can be expected in the south of Henan on Saturday, national weather forecasters said on Thursday.

Most of the country's other key wheat-growing provinces, including the central and southern parts of Henan, Anhui, Hebei, Jiangsu, and southern Shaanxi, have also been drenched by recent rain, said Ma Wenfeng, a senior analyst with Beijing Orient Agribusiness Consultancy.

"The area affected is relatively large ... This basically covers the wheat-producing areas in central and southern China," said Ma.

The world's top grower of the grain was expecting a bumper crop this year of 137 million tonnes.

Around 30 million tonnes will be impacted by the precipitation, estimated Ma, with between 10 million and 20 million tonnes likely to have sprouted, meaning it is unfit for human consumption.

The agricultural ministry earlier this week urged local authorities to dispatch emergency teams to drain water from fields, speed up access by harvesters and mobilise drying machinery to save as much of the crop as possible.

On Wednesday, the agriculture bureau of Anhui province urged farmers, "Don't wait ... speed up the harvest!", as it sought to protect the crop from further damage.

China, including provinces like Henan, is no stranger to floods and droughts. But abnormal weather patterns still risk devastating key crops and squeezing supplies.

The recent wet weather was the worst to occur during the wheat ripening period in a decade, the Henan provincial agricultural department told state media, affecting all 17 cities in the province. (Reuters)