Here is a pretty good article on the subject.  Note that the index used for calculating the model were based on the years from 1982 to 1998.  Agman has been challenged to use the numbers in this index to show what happened from 2001 to 2003.  Real movers of demand include advertising, food safety issues and demographics.

http://www.agmanager.info/livestock/marketing/bulletins_2/todays_market/Focusonbeefdemand.pdf