Cocklebur can be a pasture peril

If you’ve ever had to disentangle spiny cockleburs from a dog’s fur, you know just how problematic these prickly little pests can be. But inconvenience isn’t the worst of the problems. The cocklebur plant itself is poisonous to livestock.

A coarse, herbaceous annual weed, cockleburs start growing in the spring, mainly along fence­rows, ditches and low-lying areas of pastures. The cocklebur is found throughout the world, but here in the United States, Xanthium stru­marium is the species involved in livestock toxicity. Cocklebur poison­ings are also common in Australia and South Africa.

Toxicity to animals can occur at different times of the year, but newly emerged plants are the big­gest concern. The toxic principle, carboxyatractyloside, is present in the seeds and young seedlings, especially during the two-leaf stage of growth. The toxin concentration drops rapidly when the first true leaves appear.

Early spring is a problem time in pastures, but animals can be poisoned later in the year when mature seeds drop and start to grow. Hogs are particularly sus­ceptible to cocklebur poisoning, but the seedlings will also poison chickens, sheep, cattle and horses. Usually animals don’t eat the seeds for obvious reasons, but problems can also occur when cattle are fed whole cottonseed or hay contaminated with cocklebur. Seedlings are toxic even when dead and dry.

Signs of toxicity in livestock include de­pression, reluctance to move, a hunched pos­ture, nausea, vomiting, weakness, prostration and labored breathing; they will also show an abnormal posture with the back extremely arched due to muscle spasms. You may see leg paddling and convul­sions when lying down. Coma and death are possible. Other indi­cations of cocklebur toxicity are hypoglyce­mia, increased vascular permeability and gastro­intestinal tract irritation.

The minimum lethal dose of cocklebur seeds is about 0.3% of body weight; for rough figuring, that is 10-20% of cattle feed intake. Thirty years ago, the Journal of Veterinary Diagnostic Investigation reported an incidence of cockle­bur toxicosis in cattle. The cattle were fed hay contaminated with mature cocklebur plants. Clinical signs ranged from death to hy­perexcitability, blindness, tense musculature, spastic gaits, lying down and convulsions. Research­ers reported 100-200 ppm of carboxyatractyloside in the rumen contents.

Good pasture management can greatly reduce the risk of cocklebur poisoning. Clipping pastures before seeds mature can help eliminate the spread of this weed. Many herbicides are labeled for cocklebur control, so a solid weed-control program is also effective. Rotation­al grazing will allow your pasture grass to thrive and discourage the growth of weeds and toxic plants. In addition, ensuring your animals have a quality feed and source of hay free of cocklebur contamina­tion will help you avoid potential problems.