"Perhaps you heard the story going around today. A genetically modified grass started pumping out cyanide gas, killing a herd of cattle. CBS News had the scoop, as seen at WTVR.com in Richmond: 'Genetically modified grass linked to cattle deaths.' It’s basically a story custom-built for rapid spread around the internet. And it is basically completely wrong. The grass at issue, Tifton 85, was not genetically modified at all, but rather is a hybrid."
"Confusion between hybridized crops (which is a process that is basically as old as the idea of 'crops') and GMOs is not uncommon.
Nor did the plants suddenly start pumping out cyanide. Examiner.com was one of the first sites with a refutation of the story ... ."
Philip Bump reports for Grist June 25, 2012. 
"GMO Food: Hybrid Poison Grass That Kills Texas Cattle Not Genetically Modified" (Examiner: Linda
"The GM Grass Scare That Wasn’t" (Knight Science Journalism Tracker) 
"Grass Tied To Texas Cattle Deaths Hybrid, Not GM" (CBS News 6/25/2012) 
[Ed. Note: Just for the record, EJ Today Headlines aggregated the original CBS story of 6/23; we have since taken it down as a standalone story, because it was incorrect. The original link is reposted above, in the context of the correction.]
"Mystery Ahoof As U.S. Cattle Dying After Eating Grass" (Reuters)