"Extreme" heat has sent the rising tide of Australian wheat production forecasts into reverse, stoking doubts that the grain export heavyweight will manage its best crop for four years.

Commonwealth Bank of Australia reduced its forecast for the crop by 700,000 tonnes to 21.6m tonnes, citing weather damage.

ProFarmer Australia, the consultancy, cut its estimate by 1m tonnes to 20.9m tonnes, and warned over crop quality too.

At that level, the crop would fall more than 400,000 tonnes behind last year's harvest, which was the best since Australian production plunged three years ago thanks to a drought blamed on the disruptive El Nino weather pattern.

Quality and quantity 

The revisions, which follow months of rising hopes for Australia's production, come amid a heatwave which has brought record temperatures to some growing regions, with officials this week issuing "code red" warnings over the risk of bushfires in parts of the country.

For New South Wales and Western Australia too – the biggest wheat producing states – "lack of rainfall during the grain filling period for late maturing grain is expected to impact on yields".

The group also noted that the hot weather had caused some crops to ripen "prematurely", heralding "some reduction in quality in the worst affected areas".

Barley trimmed 

ProFarmer also cut its estimate for Australia's rapeseed production by 40,000 tonnes to 1.67m tonnes, and its forecast for barley production by 440,000 tonnes to 6.99m tonnes.

Nonetheless, that would leave barley farmers on course to beat last year's harvest by nearly 200,000 tonnes, despite lower plantings.

Separately, AWB, the grain handler, raised its estimate for returns to farmers selling through its barley pools by Aus$6-11 a tonne, taking the forecast for best grade feed barley to Aus$175 a tonne, and malting barley to up to Aus$223 a tonne.

The revisions reflected "improved barley values and a softening of the Australian dollar", Mitch Morison, AWB's general manager commodities, said