Canada's wheat harvest faces a double whammy of hits to both quantity and quality thanks to the impact of cold and dry weather, the world's biggest seller of the grain has warned.

The Canadian Wheat Board has cut by 600,000 tonnes to 20.2m tonnes its forecast for wheat production in western states this year, leaving the country's grain heartlands on course for an output drop of more than 20%.

The revision reflected continued adverse weather in much of the region, which typically produces about 90% of Canada's wheat, with well over half of it getting - at best - 75% of normal rainfall during June and July.

Big swathes had suffered temperatures 3% below normal last month on top, said the CWB, which has estimated regional plantings only marginally lower.

'Drought and cold weather'

The poor growing conditions threatened to lower the quantity of the crop deemed fit for higher-value uses, such as milling, too.

"It is now quite certain that drought and cold weather will take a toll," Larry Hill, the CWB chairman, said.

"We will need near-perfect weather for the rest of the growing season and into the fall to preserve the quality of this year's crop."

Nonetheless, a higher proportion of the harvest looks likely to be downgraded to feed, Bruce Burnett, director of the CWB's weather and crop surveillance department, said.

While 8-10% of the spring wheat crop is downgraded in a typical year, that figure could rise by five percentage points this season.