Agriculture and animal health officials said Ike's winds and storm surge tore down fences throughout Chambers, Jefferson and Orange counties, allowing the livestock to flee from enclosed pastures -- and away from water sources.
Dead cows lay stiff on the sides of roads in Orange County.
Without feed and water for three days, livestock become stressed and many are drinking salt water left behind by the storm. The water will not quench their thirsts.
Contributions of hay, water troughs, feedstocks, such as range cubes, or cash are needed to help keep livestock alive, agriculture officials said.
Cowboy Loren Doucet of the Edwards Ranch in Southeast Texas rode a chestnut quarter horse named Sister on Monday to herd cattle from a highway into a raised pasture.
"They were stuck in a field that was full of water, salt water," he said. "We're just getting them in where they can drink fresh water."
It is not yet known how many animals were killed by the storm.
"We can't put out numbers until we know for sure," Texas Animal Health Commission spokeswoman Carla Everett said.
The estimate on wandering livestock is preliminary and could rise, officials said. Agriculture officials were surveying the area by helicopter late Monday and could give a more accurate assessment of losses Tuesday, Everett said.
In Galveston, about 20 Longhorns roamed empty neighborhoods with one herd sunning themselves in front of an empty, storm-battered hotel named Escapes!.
Elsewhere, two calves wandered amid the ghost town that Galveston has become, spooked when a reporter approached.
"They'll have a hard time making it," Kathleen Phillips, spokeswoman from Texas AgriLive Extension Service, said of the several that have been born since the storm hit.
The west end of Galveston Island, while dotted with vacation bungalows and expensive homes, also has undeveloped grazing lands where herds escaped after ranchers fled. Water from Ike's surge stormed over the shore and across a main beachfront road, and the longhorns apparently had to swim or wade through floodwaters to find high ground.
Those wishing to make contributions can call the Texas Department of Agriculture's hay hotline at 1-877-429-1998 or another hotline set up by the state's 4H association at 1-979-845-1213.