Wheat farmers' hopes of bumper US crops to make up for weaker market prices are coming under pressure from poor weather.

The US winter wheat crop, hit first by drought and last week by frost, is continuing deteriorate in states which grow the hard red variety, the US Department of Agriculture said in its weekly Crop Progress report.

Oklahoma's winter wheat has suffered the biggest decline, with half now rated poor or very poor compared with 37% in last week's report. The Texas crop remains the worst affected, with 67% rated unsatisfactory.

Meanwhile, growers in the spring wheat states are being held back by floods. In South Dakota, where more than a quarter of the crop is typically planted by mid-April, just 2% has been drilled. North Dakota farmers have yet to plant any spring wheat.

Traders said that it was early days yet to get too concerned over US wheat, and forecasts of a 21% rise to 68.0m in US production, with time remaining for farmers to make up for the slow start to the planting season.

Nonetheless, the lack of progress in South Dakota was "a little bit worrisome", Joe Victor at commodity broker Allendale said.