The late US harvest, combined with "excessive wetness", threatens soybeans with mould damage, Washington crop experts have warned, as markets continues to gauge the scale of vomitoxin contamination in corn.

US soybean crops have deteriorated over the last month, particularly in the states of Arkansas and Mississippi, parts of which had four times average rainfall last month, the US Department of Agriculture said.

"The longer those crops are left in the fields, the greater the potential for losses of yields and quality," the department said.

"When pods on mature soybeans cannot be harvested due to excessive moisture, they are subject to shattering, sprouting and mould damage."

Farmers should prepare for "substantial price discounts" on damaged crops, or those with a high moisture content.

'Reason to be concerned'

The comments came as traders continued to assess the threat to feed rations of corn and distiller's grains posed by vomitoxin, a fungal residue which causes loss of condition, and potentially death, to livestock.

"Farmers have a reason to be concerned with grain quality this harvest season because of wet weather," Linda Mason, a pest management specialist at Purdue University, in Indiana, one of the states at the centre of the vomitoxin scare.

"The moisture content in corn is 18-22%. Ideally we would like to see it at 12-14%."

The price of soybeans and soymeal, viewed as likely to benefit if farmers seek alternative livestock feeds, rose further on Friday, with Chicago's January contract adding 1.4% to cross back above $10 a bushel.

High contamination 

Opinions differed as to the scale of the vomitoxin problem, with researchers at the University of Missouri reporting hearing of contamination of up to 40 parts per million – eight times the level needed to sicken pigs.

US Commodities, the Iowa broker, reported that a "large pork producer" had rejected some distiller's grains.

However, Don Roose, the US Commodities president, told Agrimoney.com: "I do not think we have a full-blown problem", noting that vomitoxin contamination appeared to be limited to some crops in Ohio and Indiana.

Illinois-based Allendale said its poll of grain companies and buyers in seven states had revealed only one finding of "significant positives" for contamination.

"Two [respondents] said they had some early corn test positive but were not even docking so it was not an issue," the broker added.