It appears that cows are smarter than many humans, John.
In 1998, Howard Vlieger harvested both natural corn and a
genetically modified Bt variety on his farm in Maurice, Iowa.
Curious about how his cows would react to the pesticide-
producing Bt corn, he filled one side of his sixteen-foot trough
with the Bt and dumped natural corn on the other side. Normally,
his cows would eat as much corn as was available, never leaving
leftovers. But when he let twenty-five of them into the pen, they
all congregated on the side of the trough with the natural corn.
When it was gone, they nibbled a bit on the Bt, but quickly
changed their minds and walked away.