Locoweed: Identification and Prevention

The Plant That Makes Cattle Go Crazy

Locoweed is arguably the most economically damaging toxic plant affecting cattle in the western United States. The term "locoweed" covers plants in two genera — Astragalus (over 350 species in North America) and Oxytropis (about 20 species) — that contain the toxin swainsonine. This alkaloid causes irreversible neurological damage, earning affected animals the description "loco" (Spanish for "crazy").

In Texas, locoweed costs ranchers millions of dollars a year through deaths, reduced performance, abortions, and permanent neurological damage in surviving animals. If you're in an affected region, you need to know how to identify it, understand its toxicity patterns, and have a management strategy.

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Understanding Locoweed Toxicity

The Toxin: Swainsonine

Swainsonine is an indolizidine alkaloid that inhibits essential enzymes (α-mannosidase and mannosidase II), disrupting cellular function throughout the body.

• Causes accumulation of abnormal compounds in cells
• Damages neurons, leading to permanent brain lesions
• Affects developing fetuses
• Impacts immune system function
• There is no antidote
• Damage is permanent once clinical signs appear
• Some poisoning effects appear weeks after consumption stops
• Animals become addicted to locoweed

The Addiction Problem

Unlike most toxic plants that cattle avoid, locoweed is palatable and addictive. Once cattle begin eating locoweed:

• They actively seek it out
• They may ignore good forage to eat locoweed
• Addiction can spread through social learning
• Removing cattle from pasture is the only way to break the habit
• "Loco'd" cattle teach naive cattle to eat it

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Species Identification

Common Texas Locoweed Species

#### Woolly Locoweed (Astragalus mollissimus)

• 6-18 inches tall
• Covered with silky, silvery hairs
• Compound leaves with 21-31 leaflets
• Purple to lavender flowers in dense clusters
• Inflated, egg-shaped pods covered in woolly hairs
• Prairies and open rangeland
• Road rights-of-way
• Overgrazed pastures

#### Wooton's Locoweed (Astragalus wootonii)

• 4-12 inches tall
• Silvery-green foliage
• Pink to purple flowers
• Bladdery pods
• Rocky slopes
• 3,500-6,000 feet elevation

#### Spotted Locoweed (Astragalus lentiginosus)

• Typically low-growing
• White to purple flowers
• Mottled or spotted pods (hence the name)
• May have reddish stems
• Sandy washes
• Open desert scrub

#### Lambert's Crazyweed (Oxytropis lambertii)

• 6-16 inches tall
• Leaves arise directly from root crown
• Purple, pink, or white flowers
• Flowers have pointed "keel" petal
• Pods divided internally
• Dry hills
• Roadsides

Visual Identification Chart

Recognizing Locoism

Progression of Symptoms

• Slight depression
• May appear dull or listless
• Weight loss despite adequate forage
• Rough coat
• Difficulty judging distance
• Exaggerated response to stimuli
• Reluctance to move through gates or narrow openings
• Visual impairment
• Difficulty drinking (head movements uncoordinated)
• Inability to eat or drink normally
• Falls when startled
• Extreme weight loss
• Abortion in pregnant cows
• Social isolation
• May stand in one place for extended periods
• Unable to find water
• Severe emaciation
• Heart failure (especially with exertion)
• Death

Fetal Effects

Pregnant cows consuming locoweed may produce calves with:

• Limb deformities (contracted tendons)
• Cleft palate
• Cardiac abnormalities
• Nervous system defects
• Hydrocephalus (water on the brain)

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High-Risk Periods

Early Spring (February - April)

• Cattle are hungry after winter
• Locoweed may be only green forage available
• This is when most addictions begin

Drought Conditions

• Locoweed may remain green longer than grasses
• Cattle forced to eat whatever is available
• Stressed cattle are less selective

Following Fire

• Reduced competition from grasses
• May actually increase locoweed density

Winter

• Less palatable but still consumed
• May be only "green" option under snow

Prevention Strategies

Grazing Management

• Hungry cattle in spring are most likely to start eating locoweed
• Supplemental feeding before turnout reduces consumption
• When grass is abundant, cattle rarely eat locoweed
• Reduce stocking if forage is limited
• Remove any animal showing signs immediately
• Cull confirmed addicted animals—they will re-addict
• Addicted horses can teach cattle to eat locoweed
• Separate species on infested range

Pasture Scouting

• Map locoweed locations
• Estimate density and coverage
• Plan grazing rotation to minimize exposure
• Look for locoweed in manure
• Watch for early signs of locoism
• Note which areas cattle are using

Supplemental Feeding

When locoweed risk is high:

• Provide high-quality hay before turnout
• Mineral supplementation may reduce consumption
• Protein blocks in high-risk areas can reduce grazing pressure

Control Methods

Herbicide Control

|-----------|------|--------|-------| | Picloram (Tordon) | 0.5-1 lb/acre | Early growth | Most effective; long residual | | Tebuthiuron (Spike) | 0.5-1 lb/acre | Any time | Soil-applied; slow acting | | Metsulfuron (Escort) | 1-2 oz/acre | Early growth | Add surfactant | | Clopyralid (Transline) | 4-10.5 oz/acre | Early flower | Selective for legumes |

• Treat before flowering for best results
• May require repeat treatments
• Protect desirable forages if possible
• Follow label directions exactly

Mechanical Control

• Does not kill established plants
• May need multiple cuttings per season
• Must remove entire root
• Labor-intensive but avoids herbicide
• Best when soil is moist
• Effective for scattered plants
• Can disturb soil, allowing reseeding

Biological Control

• Not yet widely available
• Research ongoing in some states

Reseeding After Control

After successful locoweed reduction:

• Reseed with competitive grasses
• Established grass competes with locoweed seedlings
• Maintain good grass stand to prevent reinfestation

Managing Affected Animals

Early-Stage Cases

If caught very early (before obvious neurological signs):

• Remove from locoweed pasture immediately
• Provide good-quality feed
• Reduce stress
• Some animals may show partial recovery
• Never breed affected animals

Moderate Cases

Animals showing clear neurological signs:

• Permanent damage has occurred
• Remove from pasture
• Provide supportive care
• Evaluate for culling
• Do not breed
• Monitor carefully—stress can cause death

Severely Affected Animals

• Humane euthanasia may be indicated
• Animals will not recover
• Risk of suffering from starvation, injury
• Consult veterinarian

Breeding Decisions

• Animals known to have consumed significant locoweed
• Bulls that may have consumed locoweed (affects sperm)
• Genetic selection for avoidance is not proven

Economic Considerations

Costs of Locoweed Poisoning

Control Economics

• Value of land for grazing
• Cost of alternative forage
• Long-term management plan
• Broadcast treatment of heavily infested range may not be economical
• Prevention through management is usually cheapest

Case Example: Recognizing the Problem Early

• Seemed to have difficulty judging distance
• Weight loss despite eating
• Manure contained purple flowers
• Identified suspect animal and separated
• Walked pasture and found patches of woolly locoweed
• Veterinarian confirmed suspicion
• Remaining herd showed no signs
• Early recognition prevented herd-wide problem
• Pasture managed to avoid early turnout in future

Bottom Line

• Locoweed is addictive - Once cattle start eating it, they seek it out
• Damage is cumulative and permanent - There is no cure; prevention is everything
• Early spring is highest risk - Keep cattle off infested pastures until grass is growing
• Overgrazing causes poisoning - Maintain adequate forage at all times
• Remove affected animals immediately - They teach others to eat locoweed
• Scout before grazing - Know where locoweed grows on your ranch
• Herbicide control works - Treat when plants are actively growing
• Never breed affected animals - Damage includes reproductive effects

Related Resources

• Complete Guide to Texas Toxic Plants
• Spring Poisoning Risks
• Pasture Walk Inspection Guide
• Poisoning Symptom Checklist

References

• Ralphs, M.H. & Panter, K.E. (2018). "Locoweed Toxicity and Research in the Western United States." Rangelands.
• James, L.F. et al. (2005). "Locoweed (Astragalus and Oxytropis) Poisoning in Livestock." Veterinary Clinics of North America.
• Texas A&M AgriLife Extension. (2024). "Locoweed Poisoning in Texas."
• Knight, A.P. (2001). A Guide to Plant Poisoning of Animals in North America. Teton NewMedia.
• Pfister, J.A. & Cook, D. (2011). "Maternal and Fetal Effects of Locoweed Ingestion." Journal of Animal Science.
• Stegelmeier, B.L. (2015). "Equine Locoweed Poisoning: Current Status." AAEP Proceedings.

"The loco cattle never come back"