Wheat plantings in Canada's agricultural heartland are to fall to their lowest for nearly 40 years thanks to the "extraordinary" rains which may force growers to abandon an area bigger than Denmark.

The data from the Canadian Wheat Board, the grain marketing giant, sent prices of US spring wheat up nearly 8%.

Wheat farmers in Canada's Prairies, who are responsible for more than 90% of the country's production, will sow 19.2m acres this year, the smallest area since 1971, the board said.

While some fall in plantings, particularly of durum, had been expected because of lower grain prices, the 18% cut is three time the size that analysts had pencilled in early in the spring.

The extent of the decline reflected "the extraordinary rains" which had "halted progress" on plantings, Bruce Burnett, the CWB director of weather and market analysis, said.

"Significant amounts of farmland remain unseedable at this late date."

'Unprecedented severity'

Indeed, 8.25m-12.5m acres of cropland will be abandoned, the largest area to go unsown since Canada was subsidising set-aside programmes some four decades ago.

Across the Prairies, 22% of crops remain to be seeded. Plantings are normally complete by now.

Saskatchewan, Canada's biggest agricultural province, had suffered worst, with 36% of fields unplanted and "prospects for additional seeding dim". Only 5% of land has been sown in some areas of the state, according to provincial data.

Production to tumble

The lower plantings were likely to feed through into a 22% slump to 18.9m tonnes in the region's wheat production, assuming the yield of 37.5 bushels per acre suggested by CWB models.

Durum will suffer the deepest cut, down 40% at 3.2m tonnes.

Even assuming a decent yield in the rest of the country, the world's second biggest wheat exporter, the figures imply a national crop significantly lower than the 24.5m tonnes forecast by the US Department of Agriculture among benchmark crop estimates released on Thursday.

Indeed, the CWB said that the USDA data did "not account for the seeding difficulties in the Canadian Prairies".

Wheat price soars

The data helped revive US wheat prices after a weak start, notably those in Minneapolis, which trades spring wheat rather than the soft red winter wheat traded in Chicago.

Minneapolis's July contract jumped 7.7% to $5.28 a bushel at one stage before retreating to close up 2.3% at $5.04 ¾ a bushel.

Chicago's July lot ended 1.7% higher at $4.40 ¾ a bushel.