|Spoilt crop [Photo:BBC]|
Crops including corn and soybeans have been hit by the dry conditions, and several states have seen wildfires.
Mr Vilsack announced in a press conference on Wednesday that the US Department of Agriculture had added 39 counties in eight states as "natural disaster areas", making farmers in those counties eligible for low-cost emergency loans.
Almost 1,300 counties across 29 states have received the designation this year.
Temperatures are expected to hit around 100F (37C) across much of the affected area, including Missouri, Iowa, Illinois, southern Minnesota, Nebraska and South Dakota.
Some rain is expected in parts of Ohio, Kentucky and eastern Indiana later in the week.
On Monday, the National Climatic Data Center reported that this year's drought is the most widespread since 1956.
In a press conference, Mr Vilsack said that while crop prices would likely rise, consumers would not see the effect immediately. He cautioned about potential price gouging in the coming months.
The intensity of the drought was not as great as one in 1988, he said, but much larger areas of the country are now affected.