KEYE TV CBS | June 23, 2012

A mysterious mass death of a herd of cattle has prompted a federal investigation in Central Texas.

Preliminary test results are blaming the deaths on the grass the cows were eating when they got sick.

The cows dropped dead several weeks ago on a ranch in Elgin, just east of Austin.

Jerry Abel opens the gate on his 80-acre ranch in Elgin, walking on a field of grass he's been using for cattle grazing and hay for 15 years.

"This is it, a lot of leaf, it's good, grass, tested high for protein - it should have been perfect," said Abel.

The grass is a genetically modified form of Bermuda known as Tifton 85 which has been growing here for 15 years, feeding Abel's 18 head of Corriente cattle.  Corriente are used for team roping because of their small size and horns.

"When we opened that gate to that fresh grass, they were all very anxious to get to that," said Abel.

Three weeks ago, the cattle had just been turned out to enjoy the fresh grass, when something went terribly wrong.

"When our trainer first heard the bellowing, he thought our pregnant heifer may be having a calf or something," said Abel.  "But when he got down here, virtually all of the steers and heifers were on the ground.  Some were already dead, and the others were already in convulsions."

Within hours, 15 of the 18 cattle were dead.

"That was very traumatic to see, because there was nothing you could do, obviously, they were dying," said Abel.

Dr. Gary Warner, an Elgin veterinarian who specializes in cattle, conducted the 15 necropsy.  Preliminary tests revealed the Tifton 85 grass, which has been here for years, had suddenly started producing cyanide gas, poisoning the cattle.

"Coming off the drought that we had the last two years, we're concerned it was a combination of events that led us to this," said Warner.  "The problem is, we don't know, and there needs to be some caution exercised until we know more about the situation."

Until scientists can determine why this tried and true grass suddenly began producing cyanide, Abel is keep his livestock far away.

"The grasshoppers are enjoying it now," said Abel.

What is even more worrisome - other farmers have tested their Tifton 85 grass, and several in Bastrop County have found their fields are also toxic with cyanide, although no other cattle have died.

Scientists at the U.S. Department of Agriculture are dissecting the grass to determine if there might have been some strange, unexpected mutation.