Cheatgrass, aka "downy brome." An acre of the stuff in ideal conditions is able to produce over 500 pounds of seed per acre per year.

The seed is viable for over 10 years, dried and laying on the soil top. It chokes out other forages and grasses by out-producing by seed. Every day above freezing is a growing day for cheatgrass.

The best thing to do is spray it while it is still green, which is around this time of year, then look in the autumn for when it germinates again in fall moisture and spray the newly emerged cheatgrass then.

Some species of birds love the stuff here in the west - for example, Chukar (which are from the mountains of South Asia) feed extensively on newly-emerged cheatgrass in the west. 

Your seed probably came down the Mighty Missouri from Montana.