I've wondered about this for a while. Looks like there's no real differences in nutritional content between present day broccoli and what was grown 30-40 years ago, except for the one variety that was never really eaten much anyway. However, varieties that produced larger heads did have less nutrients per ounce, which makes sense to me -- bigger broccoli heads means more fiber and more water. It'd be neat to see similar studies for other vegetables.
Scientists at the U.S. Department of Agriculture took 13 different genetic lines of broccoli that were released by seed companies over the past 46 years, grew them side-by-side, and measured levels of many micronutrients, including calcium, magnesium, iron, and zinc. They also grew an older variety of the vegetable, called Waltham 29, that was created by plant breeders in Massachusetts in 1950.
They found no clear trend of rising or falling nutrients among the commercial broccoli lines. Older hybrids did not, on average, have higher or lower concentrations than more recent ones. But the researchers did find bigger is worse; the genetic lines that produced larger heads of broccoli also had lower concentrations of nutrients — that is, there was less nutrition per ounce of vegetable.
And then there's the geezer broccoli: Waltham 29. It does have significantly higher levels of many nutrients. But there's a catch. Waltham 29 is a very different version of broccoli. The flower is looser, more bush-like, with bigger flower buds. It's less predictable; many plants don't produce any flowers at all. It doesn't keep as long, and it's harder to ship.
As a result, broccoli like Waltham 29 never was very widely consumed. Mark Farnham, a co-author of the new study, says that back in the day when plant breeders created Waltham 29, Americans ate less than one pound per person per year, on average. In the 1960s and 1970s, breeders came up with hybrid lines that could be more easily stored and shiped. "It's really after these dramatic changes that people started eating it," Farnham says. Today, the average American eats seven or eight pounds per year.
Link:
http://www.npr.org/blogs/thesalt/2011/10/20/141557743/new-varieties-havent-taken-the-nutrition-out-of-broccoli]