Locoweed's Effect on Horses and Livestock

Posted by Mark J. Platten, Teller County Extension Director

White locoweed (Oxytropis sericea)

This post's focus will be on the various locoweed (Oxytropis and Astragalus spp.) species that can be found in Colorado.  I noticed they really sprang forth around the second week of June in Teller County last year but may be emerging already in the lower elevations.

Locoweed gets its name from the Spanish word loco (crazy) and is the most widespread poisonous plant problem in the Western United States.  As opposed to other noxious weeds we've discussed in previous posts, locoweeds are native species and not covered by the Colorado Noxious Weed Act.

There are three primary species that can be found in Colorado: 1) Purple (Oxytropis lambertii), 2) White (Oxytropis sericea), and 3) Woolly (Astragalus mollissimus).  The flowers can be purple, white, or a variety of shades in between due to cross pollination.  Purple locoweed tends to flower after white locoweed is finishing blooming.  

Horses, cattle, sheep, goats, and wildlife (elk, deer, and antelope) are poisoned by eating any part of the plant, even when dry. Signs of poisoning appear after 2 to 3 weeks of continuous grazing on the plant. Locoweed has four principal effects on affected animals: 1) neurological damage; 2) emaciation; 3) reproductive dysfunction and abortion; and 4) congestive heart failure when grazed at high elevations.

Signs and Lesions of Poisoning

·         Depression

·         Dull dry hair coat

·         Eyes dull and staring

·         Irregular gait or some loss of muscular control

·         Weakness

·         Some animals show extreme nervousness

·         Loss of sense of direction

·         Withdrawal from other animals

·         Some animals develop inability to eat or drink

·         Abortions are common

·         Skeletal malformations may occur

·         Animal may become violent if stressed

·         Reduced libido in males and altered estrous behavior in females

How to Reduce Losses

Purple locoweed (Oxytropis lambertii)

Many minerals and feed additives have been investigated to prevent locoweed poisoning but none have been proven to be effective.  Most locoweed species are endemic, growing only in certain habitats or on specific soils.  Fences could be constructed on soil or vegetation boundaries to provide seasonal control.  Restricting access to locoweed during critical periods when the plant is more palatable than associated forages (spring and early summer).  Maintain conservative stocking rates to avoid forcing animals to consume locoweed when desirable forage becomes limited.  Locoweeds are palatable and of similar nutrient value to alfalfa which helps explain why animals eat them even when normal forages are present. Through social facilitation, animals learn to eat locoweed from each other. 

Pea-like Flowers

Remove animals that begin eating locoweed to prevent intoxication and to keep them from influencing others to start eating locoweed.  Recovery depends on the duration and severity of the lesions.  Although some of the toxic effects may resolve after animals are removed from infested areas, there is no effective treatment for locoweed poisoning, and once affected, they are more susceptible to future poisoning.  Locoed horses should be considered permanently affected since neurologic signs may unpredictably recur, making them of little value as saddle or draft animals.

Control Options

Seeds may remain viable in the soil for 50 years or more, so management requires a long-term plan.  Most varieties of woolly locoweed are naturally controlled by the four-lined locoweed weevil (Cleonidius trivittatus).  Although weevils can be reared in the lab or collected from the field, neither is practical for control due to labor costs, so you will need to rely on their natural presence for biological control. 

Locoweed may also be chemically controlled by spraying actively growing or budding plants with clopyralid, picloram, or metsulfuron by following label directions.  If plants are scattered, treatment of individual plants or patches may not be practical.  Follow precautions when handling herbicides.

Information for this post was gathered from the CSU “Guide to Poisonous Plants” website:  http://southcampus.colostate.edu/poisonous_plants/index.cfm