"Cover crops were in use on less than 2 percent of total cropland (for all crops) during 2010-11 (6.8 million
acres), with adoption rates higher in some regions (e.g., the Southern Seaboard and the Mississippi Portal).
Although the benefits of cover crops and no-till/strip-till are enhanced when these practices are used on the
same fields, the low cover crop adoption rate suggests that these benefits are realized on few acres." (From the report summary.)
Cover crops have come into style though since then. They are a lot more common. Not nearly as common as they should be, but they still a lot more than 10 years ago.
https://www.ers.usda.gov/publications/pub-details/?pubid=440...