How much has milkweed growth diminished?

Milkweed growth has decreased substantially in the Midwest over the past 10-20 years, especially in agricultural areas treated with herbicides. Some studies estimate the decline in milkweeds to be over 58% since 1999 (Pleasants and Oberhauser, 2013). As of 2014, only 1.34 billion milkweed stems remained from the estimated 2.2 billion milkweeds present in 1999 (Pleasants, 2016).