America's corn and soybeans are showing signs of suffering from being left late in the field by a rain-delayed harvest, with the quality of both crops falling to its lowest level this season.

The proportion of US corn rated "good" or "excellent" slid by two percentage points to 67% last week, the lowest since this season's ratings began in May, Washington data showed.

For soybeans, the proportion in the top two bands was 63%, down two points on the week and the lowest since the crop was first assessed in June.

The data, revealed as the US Department of Agriculture revealed a further week of modest progress in the slowest harvest since records began in 1985, follow growing concerns that the slow harvest will take a toll on the quality of what have appeared near-ideal crops.

'Big concern'

US Commodities, the Iowa-based broker, has already warned that "crop quality will be a big concern this winter".

Many crops are coming in light on so-called "test weights" – the actual weight of the crop per bushel of volume, and which can vary to between 45-60 pounds per bushel.

Low test-weights signal a weaker protein composition in the grain, and higher concentration of starch, and are typical of crops that have not matured entirely.

Higher moisture levels in damper crops are also a concern, limiting how much crop can be pushed through limited drier capacity.

Slow progress

US farmers managed to harvest only a further 5% of their corn crops in the week to Sunday, leaving 75% in the field. Typically, just 29% of corn is still left to be harvested by now.

In some states, the slow progress in corn has represented in part an emphasis on soybeans, viewed as a more vulnerable crop, during breaks in the cloud.

The proportion of soybeans harvested rose by seven points to 51%.

Even so, that is well behind the average of 87% completed by now.

Illinois - typically America's second-biggest soybean state after Iowa, and which has normally all-but completed its soybean harvest by now - still has 65% of its crop to bring in.