> Anybody know if any man-produced chemicals were in agricultural use back then?

As people settled into established societies they began looking for ways to
protect their crops. Sulfur was used as an insecticide long before 500 BC.
Toxic formulations of lead, arsenic and mercury were applied to crops in 1400s. 

In the 1600s nicotine compounds were extracted from tobacco leaves and used
as insecticides. By the mid 1800s, the heads of chrysanthemum flowers were
used to obtain pyrethrum and rotenone was extracted from the derris plant.

In the 1930s the crop yields in the United States were comparable to those
of India, England, and Argentina. Since the 1950s the use of
petroleum-derived pesticides, fertilizers and a host of governmental
policies have vaulted the U.S. into the biggest farming economy in the
world. Today, fewer farmers feed more people than ever before in the history
of food production. 

from:
Commercial Agriculture: Facts and Figures
by J. Robert Hatherill, Ph.D, 
Environmental Studies Program, 
University of California at Santa Barbara

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