Ok, so the grass is Johnson grass.
Looks like I CAN have horses and/or livestock, during the fall, winter, and spring. But during the summer...

[b]Negative aspects of johnsongrass[/b]
Johnsongrass is on the noxious weed list in several U.S. states (including Oklahoma) and has even made the list of the 10 most noxious weeds in the world. Johnsongrass can accumulate nitrates during the summer if exposed to several dry, cloudy days in a row. It can also produce prussic acid (hydrogen cyanide) after stressful conditions such as drought, freezing weather or exposure to a herbicide that kills grasses. If your johnsongrass is subjected to any of these conditions, keep cattle away for about a week to allow the prussic acid to dissipate.

http://www.noble.org/ag/pasture/johnsongrass/