NMSU Researchers Study Low-Level Locoweed Consumption
Date: Jan. 27, 1997
Editor: D'Lyn Ford (505) 646-6528, dlford@nmsu.edu
LAS CRUCES -- When 2 to 5 percent of the cattle in a herd have gone "loco" from grazing heavily on locoweed, ranchers have cause to wonder what's happening to those that just nibble a little of the poisonous plant.
Animal scientists with New Mexico State University's Agricultural Experiment Station are researching how grazing locoweed at low levels affects ruminant animals like cattle and sheep.
"Locoweed is a common, wild plant that contains a compound called swainsonine, a toxin that causes locoism in animals," said James Strickland, a nutritional toxicologist.
"Locoism affects animal growth, the reproductive system and, probably the most detrimental, the nervous system with the classic symptoms of tremors and nervous activity."
Loco animals may run in circles, stagger, drool, or even charge at ranchers.
Strickland studies the effects of poisonous plants on animals' tissue and organs. Graduate student Bret Taylor is working with Strickland on the locoweed project.
Locoweed is a problem throughout the West and some parts of Mexico, with producers losing millions of dollars each year. Ranchers are the most concerned in the early spring and late fall, when locoweed may be the only green forage cattle can find on the range, Strickland explained. Controlling the weed is troublesome, since the hardy seeds can lay dormant for years before springing to life.
While there is much information about the most severe or acute cases of locoism, which occur when locoweed makes up 10 to 15 percent of the diet, not much is known about low-level consumption, Strickland said. Ranchers haven't reported livestock exhibiting lesser symptoms of the disease.
What the scientists suspect is that if cattle and sheep are grazing with others that have gone loco, they are probably eating locoweed, too. "Ruminant animals are susceptible to peer pressure, and they are likely to have similar grazing habits as others in the herd or flock," Taylor said.
Strickland and Taylor are concentrating their research on how low levels of swainsonine from locoweed affects the immune system of ruminants. The immune system is generally one of the first systems in the body to be affected by toxins.
A weakened immune system could leave animals susceptible to bacterial and viral infections, Strickland said. Calves that have grazed on locoweed and are then stressed by moving to the feedlot may be especially vulnerable, he added.
In an initial laboratory study, Strickland and Taylor found that low levels of swainsonine didn't negatively affect cattle lymphocytes, some of the immune system's infection-fighting white blood cells. This could be good news for ranchers.
The researchers are hoping their findings hold when they feed low levels of locoweed to 16 sheep in an upcoming, follow-up study.
"The laboratory research only tested part of the immune system," Strickland said. "By studying the sheep, we'll get to look at how the whole immune system reacts to swainsonine."
Taylor said he will feed the sheep a low-energy diet, primarily made of alfalfa and grass to mimic range conditions. Varying amounts of locoweed will be included in the mix. Taylor will take blood samples from the sheep once a week for a month to study the immune response to the toxin.
"With different levels of locoweed, we'll look for a difference between major locoweed consumers and nibblers," Taylor said. "This study may also tell us if there is any cumulative affect of sheep eating small amounts of locoweed over time."
After they figure out how swainsonine affects the sheep, the researchers may return to the laboratory to find out if it can actually boost the immune system of animals. In fact, in some initial studies with humans done elsewhere, swainsonine has been used to fight cancer.