Corn monoculture.

I'm glad I avoid fast food:

Americans spend >100 billion dollars on restaurant fast food each
year; fast food meals comprise a disproportionate amount of both
meat and calories within the U.S. diet. We used carbon and
nitrogen stable isotopes to infer the source of feed to meat
animals, the source of fat within fries, and the extent of fertilization
and confinement inherent to production. We sampled food
from McDonald’s, Burger King, and Wendy’s chains, purchasing
>480 servings of hamburgers, chicken sandwiches and fries within
geographically distributed U.S. cities: Los Angeles, San Francisco,
Denver, Detroit, Boston, and Baltimore. From the entire sample set
of beef and chicken, only 12 servings of beef had delta 13C < -21‰; for
these animals only was a food source other than corn possible. We
observed remarkably invariant values of delta 15N in both beef and
chicken, reflecting uniform confinement and exposure to heavily
fertilized feed for all animals. The delta 13C value of fries differed
significantly among restaurants indicating that the chains used
different protocols for deep-frying: Wendy’s clearly used only corn
oil, whereas McDonald’s and Burger King favored other vegetable
oils; this differed from ingredient reports. Our results highlighted
the overwhelming importance of corn agriculture within virtually
every aspect of fast food manufacture.

Hope Jahren and Rebecca A. Kraft, Carbon and nitrogen stable isotopes in fast food: Signatures of corn and confinement, PNAS 2008, November 10, 2008, doi: 10.1073/pnas.0809870105has more.

Effect Measure provides an interesting analysis of this work.

Every day I drive past on the Burger Kings the authors sampled from in Baltimore. Always wondered who ate there. Now I know. ;)