Milk prices for farmers have plunged to their lowest point since October 2009 as cheese stockpiles swell.Dairy farmers drowning in cheap milk begged agricultural officials on Friday to buy up tens of thousands of tons of cheese to help bail them out. Jim Mulhern, chief executive of the National Milk Producers Federation, asked U.S. Agriculture Secretary Tom Vilsack to buy $150 million worth of cheese to protect struggling dairy farmers and provide 90 million pounds of food to needy Americans. “Dairy producers here in the United States need assistance,” Mr. Mulhern wrote to Mr. Vilsack. A spokesman for the Agriculture Department said the regulatory body “shares the concerns for our nation’s dairy farmers, who like many in the farm community are facing tight margins,” and that it would review the letter.