Many Yellowstone Deer, elk, antelope, and bison have exhibited varying malformations from under bites to malformed genitalia since the 1990s. Some of these populations have experienced declines in their long term populations as well.
As many as half of the elk and most of the bison from Yellowstone migrate out of the park to their winter ranges. To the South the longest elk migration occurs with elk leaving the park for the National Elk Refuge and surrounding areas. Herbicides are used on the elk refuge to control weeds.
To the North, elk and bison winter in the Gardiner Montana area.
In a 2005 habitat restoration plan for the Gardiner area, it was proposed that several herbicides be used in large scale habitat restoration efforts. These herbicides included Roundup(Glyphosate), Buctril(Bromoxynil), Transline(Clopyralid). These efforts began in earnest in 2008. Yellowstone National Park itself uses herbicides for the control of weeds across the park.