* Ethanol output falls 36,000 bpd to 821,000 bpd * Stocks fall 761,000 to 19.53 mln bbls July 11 (Reuters) - U.S. ethanol output plunged 4 percent to its lowest level in nearly two years last week as soaring corn prices pushed many biofuel refineries into the red. Ethanol production totaled 821,000 barrels per day, down 36,000 bpd, in the week ended July 6, according to the Energy Information Administration. It was the lowest production since the week that ended July 23, 2010, and the fourth straight weekly decline. Stocks of the fuel fell 761,000 barrels to 19.53 million barrels. At least three ethanol plants -- two in Nebraska and one in Indiana -- are idling due to rising corn prices and negative profit margins. Chicago Board of Trade corn futures this week rose to a 13-month high amid dwindling old-crop supplies and as hot and dry conditions reduced yield potential of the developing crops. In a monthly supply and demand report released early Wednesday, the U.S. Agriculture Department cut its estimate of the corn used in U.S. ethanol production by 100 million bushels.