That's the question asked in this recently published paper by authors at the USDA Economic Research service appearing in the journal Applied Economics Perspectives and Policy. They find:
Our empirical results show that two types of structural change have occurred since WWII, and both have influenced the U.S. agricultural productivity. First, we identified a break in trend in 1974. Prior to 1974, productivity grew at an average annual rate of 1.71%, but this rate slowed to 1.56% per year after 1974. The break in trend coincided with the 1973 oil embargo, which resulted in a rapid and unexpected rise in energy prices.
A different type of structural change occurred in 1985 when we observed an upward intercept shift for the level of productivity. This may have been due to a number of factors including a U.S. economic recovery, and a liberalization of farm policy since the mid-1980s. The 1.56% annual trend rate of growth persisted after the 1985 breakpoint.
Farmers are getting more productive, just not as fast as they once were.