The declaration will make affected ranchers and farmers eligible for low-interest loans and speed processing of disaster claims.
"Agriculture remains a bright spot in our nation's economy," said Vilsack. "We need to be cognizant of the fact that drought and weather conditions have severely impacted on farmers around the country."
The declaration covers counties in California, Oregon, Nevada, Utah, Arizona, New Mexico, Colorado, Nebraska, Wyoming, Kansas, Oklahoma, Texas, Louisiana, Arkansas, Missouri, Kentucky, Tennessee, Illinois, Indiana, Alabama, Mississippi, Georgia, South Carolina, Florida, Delaware and Hawaii. It does not include Iowa, the country's biggest corn producer.
The first six months of this year were the warmest on record, the National Oceanic and Atmospheric Administration said. Twenty-eight states east of the Rockies set temperature records.
Those record-breaking temperatures deepened drought conditions across much of the American west, triggering an early and violent season of wildfires in Utah, Idaho, Colorado, Arizona and New Mexico.
The heat also destroyed expectations of a bumper corn crop. American farmers planted more than 96m acres of corn this year, the most in 75 years.
The early spring got the crop off to a good start but , after June's extreme heat, only 40% of the crop was in good condition, according to USDA figures.
From the mid-west to the mid-Atlantic, meanwhile, there were triple digit temperatures, breaking hundreds of heat records. On Thursday, St Louis confirmed 18 deaths due to extreme heat conditions.
"The recent heat and dryness is catching up with us on a national scale," Michael Hayes, director of the national drought mitigation center said in a statement.
Intellpuke: You can read this article by Guardian U.S. Environment Correspondent Suzanne Goldenberg in context here: www.guardian.co.uk/environment/2012/jul/13/drought-declaration-natural-disaster-states