SIERRA VISTA - When local residents learned that nearly 40 head of cattle on a ranch in Chino Valley near Prescott may have died after eating pigweed, the news sparked concern among livestock owners.
Whetstone resident and horse owner Annette Gerhardt first learned about pigweed from her neighbors who believe they lost two goats to the plant around this time last summer.
After the conversation with her neighbors, she then saw a news story reporting the cattle in Chino Valley that died after eating pigweed. While the story reported 40 cattle had died, a state agriculture official said the number had climbed to nearly 50.
Because the plant can be seen growing all over her property, Gerhardt was immediately concerned about her horses.
Phil Blair, the assistant state veterinarian with the Arizona Department of Agriculture, said pigweed does not present a problem for horses. However, for goats, cattle and other ruminants - or animals with a multi-compartmented stomach - the weed can cause serious problems.
"Unlike ruminants, the horse has a simple stomach and digestive system, much like a human's," Blair said. "Because of the digestive tract in a horse, pigweed doesn't bother them."
Under certain environmental stressors, pigweed can become high in nitrate, which is why some cattle and other ruminants have a problem digesting it.
When cattle eat higher than normal amounts of nitrate, it accumulates in an area of the stomach called the rumen. As cattle digest food, nitrate becomes nitrite, which then becomes ammonia, Blair said.
In addition, nitrite converts hemoglobin to methemoglobin in the bloodstream, making it unable to transport oxygen. The cattle then die from nitrate poisoning because of a lack of oxygen to their cells and organs.
"There are times when cattle can eat pigweed, and do just fine with it," Blair said. "Sometimes cattle have problems because they get a double dose of nitrate. Since nitrogen is a basic fertilizer, pigweed picks up the nitrate in the ground. And it can be in the water the animals drink, as well."
Gerhardt's neighbors, Barbara and Matt Ford, lost two goats last year after the animals were staked out in an area where pigweed was prevalent.
Matt Ford, who has worked on cattle ranches most of his life, was out of town at the time the goats died. Upon his return, he saw where the goats had been staked out, and immediately attributed their deaths to pigweed. The cause of death, however, was never confirmed by a veterinarian.
After losing two horses last October to blister beetle poisoning, Gerhardt was not taking chances when she learned there might be potential problems with pigweed. Gerhardt's two horses became sick and then died after eating alfalfa hay infested with blister beetles.
With the memory of the blister beetle poisoning and the loss of her horses still fresh in her mind, Gerhardt said she wasn't taking chances with pigweed.
"I have pigweed all over my property," said Gerhardt, who raises gaited horses. "We've been hacking away at it, but it grows faster than we can get it knocked down. That's why Matt and Barbara and I started talking about it to begin with, because they were at my place ... and Matt warned me about it."
Years of drought followed by recent rains have caused the weed to grow rapidly, which state agriculture officials say is the right formula for higher than usual levels of nitrate.
While common throughout the Southwest, cattle do not typically graze on pigweed. But if grasses are scarce, and it's the only green plant around, they will go for it.
Dr. J. Phelps, a veterinarian with a practice in Sonoita, said the same is true for horses.
"Even though we have a lot of pigweed this year, horses really aren't affected by it. You can only get a starving horse to eat it, and even then, it doesn't seem to affect them much. It's more of a concern for sheep, goats and cattle."