yellow foxtail.  when other grasses get thin or overgrazed, it starts and grows mid june into october here.  
mow the heads off, the heads irritate the gums of animals, especially horses.  

new heads will show up in a few days, the shorter you mow it , the shorter the new heads will appear.

glyphosate or clethodium kills it easy (as well as any other grass nearby), also any type of triflurilan crabgrass preventer in the spring will prevent annuals, but not harm perrineals.