Locoweed Poisoning on the CSU Ranch

In 1994, Gary Greathouse faced his biggest challenge ever as manager of the Colorado State University (CSU) Maxwell Ranch. That spring brought plenty of rain to the cattle operation on the Colorado and Wyoming border, but by early summer the moisture stopped. The grass slowly turned brown, giving dormant locoweed seeds the opportunity to quickly leaf out and multiply – making a tasty, but dangerous situation for the cattle.

Greathouse knew that a few locoweeds lurked around the 12,000 acre pasture, but he wasn’t concerned about the cattle being poisoned. In his 15 years of managing the ranch, he had never had a problem with the weeds.

Unaware of the situation in the pasture, Greathouse stayed busy with his duties as a CSU animal science professor. He only made it out to the ranch once or twice a week. In July, while making a routine run to the pasture, he found a devastating site. Though the Angus/Hereford cows were in great condition, their calves were depressed and thin. Greathouse immediately suspected their vaccination program failed and the calves suffered from bovine viral diarrhea (BVD). He quickly sent a trailer load of calves and a cow to CSU vet teaching hospital to be tested.

The test results shocked Greathouse – the calves had locoweed poisoning. Over the summer, the changes in the animals were so subtle, Greathouse didn’t realize what was happening until it was too late.  

“Ranchers and horse owners should be concerned about locoweed,” Greathouse said. “Locoweed poisoning is one of those things that can really sneak up on you. The symptoms occur slowly over time and are often difficult to tie to the weed. “

Though cattle, sheep, horses, and wildlife are susceptible to locoweed poisoning, animals rarely eat the weed unless feed is scarce. However, the plant has a relatively high nutrient value – similar to that of alfalfa – so once animals develop a taste for the plant, they will graze it even when other forage is available. If one animals starts to eat locoweed, she’ll teach the rest of the herd how. That’s just what happened on the CSU ranch that summer.

Locoweed is the most common and costly cause of livestock poisoning in the Western United States. The weeds can grow in all parts of Colorado, Utah, Kansas, Nebraska and Wyoming. They are one of the first spring forages to appear – usually beginning in mid April – and they grow throughout the summer. All parts of the weeds are always poisonous, even during the winter months when they are mature and dry.

According to Brad Gillmore, livestock agent for the Weld County Cooperative Extension Service, locoweed poisoning is more common during a drought because there is less competition from other plants. “Locoweeds look attractive no matter what,” he said. “The bad thing about locoweed is that it’s like candy to an animal. Once an animal tries it, it will seek it out for the rest of its life.”

Swainsonine, the toxin found in all parts of the plant, affects the central nervous system of an animal resulting in the “locoed” or crazy behavior often seen in poisoned animals. The poison is secreted in a highly concentrated form in the milk of infected lactating livestock, so suckling animals often show symptoms first. In addition, Swainsonine readily crosses the placenta, often resulting in fetal death, abortion, and weak newborns.

“Calves that have been exposed to locoweed through milk are severely stunted in growth – they’re almost like miniatures,” Gillmore said.

The CSU ranch not only experienced poor growth rates in their locoed calves, but also severe reproductive problems in the cows. “Even after we moved the cattle from the locoweed pastures and were intensively managing the situation, we were still experiencing a 25% abortion rate,” Greathouse said. “The wildlife was devastated by a 100% abortion rate.”

  Depending on the duration of locoweed consumption, a poisoned animal can be permanently damaged. If an animal’s exposure to locoweed is short, the animal often recovers with few noticeable side effects, if any. However, the neurological signs of poisoning may unpredictably recur. Horses often show more severe neurological effects than cattle and sheep. The unpredictable behavior of locoed horses makes them dangerous to work around or ride. Animals rarely die from locoweed poisoning, but often perish from a secondary illness caused by their depressed immune system.  There is no effective treatment for a locoed animal.

Usually chemical eradication of the weeds isn’t economical or feasible, so it’s important to take management steps to prevent poisoning.

1. Don’t overgraze pastures or put too many animals on a limited acreage.

2.      Don’t turn livestock onto spring ranges until desirable forages have made sufficient growth to support grazing.

3.      Restrict access to locoweed during critical periods when the plant is more palatable than other forages in the area. 

4.      Remove animals that begin eating locoweed to prevent them from influencing others to start eating the plant. Once an animal is a locoweed eater, she will always be one. 

The CSU ranch battled locoweed until 2002 when a severe drought killed most of the locoweed plants. However, Greathouse knows it is just a matter of time before the weeds become a big problem again. “There are thousands of locoweed seeds per square yard on the ranch,” he said. “Those seeds can last in the ground for over 50 years, so it’s inevitable that they’ll be back. But we’ll be ready.”

 Though locoweed isn’t a problem for most ranchers and horse owners, it is an important weed to be aware of.  “Undoubtedly, there are ranchers out there who have trouble with locoweed in their pastures and they don’t even know it,” Greathouse said.