(Adds Vilsack, analyst comments, details about farmer demographics)
WASHINGTON, Feb 20 (Reuters) - The proportion of U.S. farms with annual sales and government payments of $1 million or more doubled in the five years through 2012, the U.S. Department of Agriculture said on Thursday, even as the total number of farms fell.
The figures were part of the USDA's "Census of Agriculture," which surveys all U.S. farms and ranches and the people who operate them every five years. Results of the Census form the backbone of U.S. agricultural policy.
The Census also showed average farm size rose to 434 acres from 418 acres five years earlier.
Overall, the number of farms in the United States fell 4.3 percent, to about 2.1 million, continuing a decades-long decline. Most of the decline came in medium-sized farms, the USDA said. The number of large and very small farms did not change significantly.
"I am concerned about the middle. It appears as if you are a very large operator, you are going to be okay," U.S. Agriculture Secretary Tom Vilsack said at the agency's annual Agricultural Outlook meeting.
Overall, agricultural sales per farm averaged $187,000 in 2012, up more than $52,000 or about 39 percent over 2007.
That added up to "a strong rural America that has remained stable during difficult economic times," Vilsack said. During the period covered by the Census, the overall U.S. economy suffered its worst downturn since the 1930s.
U.S. farms sold nearly $395 billion in agricultural products in 2012, up 33 percent from five years earlier. In that year, when many major producing areas experienced a severe drought, the market values of crops and livestock were at record highs.
Vilsack and others sounded the alarm about the aging farm population, the smaller number of farmers who are just starting out, and what might lie ahead.
Almost 258,000 principal operators of farms were 75 or older, according to the Census, up about 6 percent from the previous survey.
"As folks age out of farming, it leaves fewer and fewer people potentially in these rural communities. We have to rebuild the middle," Vilsack said.
The number of farmers in each of USDA's older age groups (those 55 and older) increased and the average age of farmers rose to 58.3 years from 57.1.
"We have an aging farm population and they are carrying a debt load that is small, historically. But we have also reached that point where this aging population is starting to push over to the younger farmer and it looks like his debt load is going to be sizable," said Don Roose, president of U.S. Commodities in West Des Moines, Iowa.
Roose said young farmers will have to cope with interest rates that are likely to rise, farmland values forecast by USDA to slip and flat or lower grain prices for the next few years.
The vast majority of principal farm operators in the United States remained white (92 percent) and male (86 percent) in the latest Census. The number of minority-operated farmers rose, though, with the number of Hispanic principal operators rising 21 percent.
(Additional reporting by Karl Plume in Chicago and Julie Ingwersen in Washington; Editing by Jan Paschal and Meredith Mazzilli)