"I was on the Pampas and had a mare that I was fond of go to grass"
by cm
The Pampas
Creative Commons Attribution Share AlikeThe Pampas - Credit: ko:사용자:Jjw

The Pampas are grass plains that spread across parts of Brazil, Uruguay and Argentina.  This vast and fertile region provides valuable grazing for cattle and a variety of wildlife.

"Going to grass" usually means retiring or resting, but Quincey seems to be suggesting something rather more precipitous.

 

Grazing on the Pampas
Creative Commons Attribution Share AlikeGrazing on the Pampas - Credit: Jesús Gorriti