When did the big gains in agricultural productivity come?

I had the general and indeed correct impression that U.S. agricultural productivity has been rising for a long time.  It’s less commonly known how big was the productivity acceleration in agriculture in the 1940s. Between 1880 and 1940, agricultural productivity in this country rose at the rather modest rate of about one percent a year.  After World War II, the trend rate of growth jumps to 2.8 percent annually.  Multi-factor productivity in agriculture skyrockets in the mid-1930s, not earlier.

The information is from Bruce Gardner’s American Agriculture in the Twentieth Century: How it Flourished and What it Cost, pp.20-21, p.44.