The Statistics

Here are a few statistics provided by the USDA’s Economic Research Service:

U.S. beef exports (commercial carcass weight and value):
    2002: 2.447 billion pounds, $2.629 billion
    2003: 2.518 billion pounds, $3.186 billion
    2004: 460 million pounds,   $631 million
    2005: 697 million pounds,   $1.031 billion
    2006: 1.132 billion pounds, $1.617 billion
    2007: 1.434 billion pounds, $2.187 billion 
    2008: 1.996 billion pounds, $3.014 billion 
    2009: 1.935 billion pounds, $2.909 billion
    2010: 2.300 billion pounds, $3.839 billion
    2011: 2.788 billion pounds, $5.042 billion

The sudden appearance of Mad Cow disease may have affected the dramatic price drop from 2003 to 2004.

U.S. beef production (commercial carcass weight):
    2002: 27.09 billion pounds 
    2003: 26.24 billion pounds  
    2004: 24.55 billion pounds
    2005: 24.68 billion pounds
    2006: 26.15 billion pounds
    2007: 26.42 billion pounds 
    2008: 26.56 billion pounds 
    2009: 26.07 billion pounds
    2010: 26.41 billion pounds 
    2011: 26.29 billion pounds

After the appearance of Mad Cow disease in 2003, production decreases.

U.S. beef exports as percent of production: 
    2002: 9.0 percent
    2003: 9.6 percent
    2004: 1.9 percent
    2005: 2.8 percent
    2006: 4.4 percent
    2007: 5.4 percent
    2008: 7.1 percent 
    2009: 7.4 percent 
    2010: 8.7 percent 
    2011: 10.6 percent

Finally, we can see the impact Mad Cow disease had on exports, from 2003 to 2004, dropping nearly 8%. 

Link: http://www.ers.usda.gov/news/BSECoverage.htm
"U.S. Beef and Cattle Industry: Background Statistics and Information." Background Statistics: U.S. Beef and Cattle Industry. Web. 09 May 2012. <http://www.ers.usda.gov/news/BSECoverage.htm>