The condition rating of the soybean crop in North Dakota tumbled by nine points to 60%, in terms of the proportion rated in "good" or "excellent" health, while that in Wisconsin fell by four points to 75%.
"Hard frost brought an early halt to the growing season" in northern Wisconsin, the US Department of Agriculture said, noting temperatures in the 20s Fahrenheit in many areas.
"While spared a killing freeze, crops in southern Wisconsin were still contending with moisture shortages," the department added.
However, more significant to the national picture was the impact on crops in Minnesota – one of the four US states to produce more than 1bn bushels of corn and the third-ranked soybean-growing state.
While the Minnesota Corn Growers Association estimated that the freeze had little impact on yields, the percentage of the state’s corn crop seen good or excellent was cut by seven points to 56%, with the figure for soybeans slashed by 10 points to 51%.
"Crop ratings in Minnesota showed pronounced falls," Paul Deane at Australia & New Zealand Bank said.
The USDA said that "overnight temperatures on September 15 fell into the low-30s to mid-20s [Fahrenheit] setting record lows in several Minnesota communities.
"Freezing temperatures ended the growing season in portions of the state."
Worse than 2007
The rating of Minnesota’s soybean crop has now fallen below that of 2007, when the state’s soybean production tumbled by more than 20% year on year.
The national crop, too, is now rated below that in 2007, when US output fell 16% on a yield of 41.7 bushels an acre.
The USDA has the current crop, which will have been sown largely with more advanced seed, at 41.8 bushels per acre.
The US corn crop, downgraded by two points to 51%, is in significantly worse condition than anything seen in the previous five years.
In Canada too, the frost had significant impact on crops, thanks to temperatures as low as minus 8 degrees Celsius in Saskatchewan.
The "widespread frost" had, in Canada’s Prairies region, brought the growing season "to an end for all but western Alberta", the Canadian Wheat Board said.