Tall larkspur (wild delphinium) is a cattle killer. The poisonous plant claims average death losses of 4-5% annually in some allotments in Utah, Colorado, Wyoming, Idaho and Montana. Some ranchers experience death losses of more than 15%, says James Pfister of the USDA Poisonous Plant Research Laboratory (PPRL) in Logan, UT.

As a result, the presence of tall larkspur on many ranges forces stockmen to avoid some pastures early in the season. That reduced value of forage is the greatest economic loss, far exceeding animal deaths and management headaches, Pfister says.

The best "treatment," however, is prevention - not using pastures with larkspur during seasons and situations when the plants are most deadly. Or, going in ahead of the cattle and pulling up or breaking off the plants (if located in a few specific areas where they can be eliminated).

Graze At The Proper Season The poison content of larkspur is high during early growth, then drops off rapidly after maturity, except in the seed pods, Pfister says. Poisoning of cattle is less likely after flowering is past.

Cattle may not eat larkspur during preflower stage (when it's most toxic) unless other forage is sparse. In a typical year, cattle begin eating larkspur during flower stage, says Pfister. Consumption usually peaks during late flower or pod stage. Toxicity is declining at that point but consumption is increasing.

He says, "This results in a toxic window when most cattle deaths can be predicted. Depending on weather conditions, this toxic window may be 4-5 weeks long in a typical season."

Researchers say tall larkspur contains up to 20 different alkaloids that vary in toxicity from very poisonous to almost harmless. Alkaloid levels can vary from one patch to another, and from year to year in the same patch. These differences may be part of the reason for variations in annual death losses. It takes more pounds of larkspur consumption in some years to be fatal.

Nutritionally, larkspur is similar to alfalfa hay (60% digestibility and 12% crude protein). Small amounts have no detrimental effects on digestion.

For instance, if plants contain only 0.2% of the most toxic alkaloid, a 1,000-lb. cow must eat more than 14 lbs. of larkspur to kill her, according to Utah studies. By contrast, a cow only needs to eat 3 lbs. of tall larkspur to be fatally poisoned, if plants contain 1% of the most toxic alkaloid. For fatal poisoning, cattle must eat these amounts within a couple of hours.

Patterns Of Consumption Weather patterns and stage of maturity can affect consumption and extent of death loss. For example, cattle eat less larkspur during drought if plants are dry. But if larkspur grows in a wet area and stays green longer than surrounding grasses, larkspur consumption may increase.

Cattle losses during wet years tend to be higher, probably due to changes in the chemistry of the plants, says Mike Ralphs, range scientist with the PPRL.

In late summer, larkspur may make up as much as 25-30% of a cow's diet, especially if she spends a lot of time in the moist bottomlands where there are large patches. At this stage of maturity, this amount of larkspur may not be fatal, especially if cattle are eating other plants along with it and spreading their larkspur consumption throughout the entire day.

But during or just after a storm, larkspur consumption often increases; cattle eat more in a short time, causing more death loss. This may be because cattle "hole up" in the bottoms during the storm instead of spreading out to graze. Also, wet larkspur may be more palatable than when it is dry.

In experimental studies in Utah, Idaho and Colorado in 1988, and 1991 to 1996, cattle were grazed early, while plants were immature or in the bud stage. In none of those years did the cattle eat more than a few bites of larkspur before the plants flowered.

The researchers felt it might be safe to graze some of these ranges very early in the summer, then remove cattle after larkspur starts to flower, returning again in late summer. This type of use could minimize (though never eliminate) risk of poisoning, while allowing greater use of forage.

In some situations, sheep can be grazed ahead of cattle since tall larkspur is not toxic to sheep. If sheep eat or trample the larkspur before cattle are put into a pasture, cattle losses may be reduced.

Mineral supplementation has been tried by some ranchers to reduce consumption of larkspur. But after four years of study, researchers found that no level of mineral supplement changed the amount of larkspur eaten.

PPRL researchers say the best management is for livestock producers to take note of larkspur defoliation.

"We suggest ranchers directly observe what cattle are actually eating one or two days each week when cattle begin to graze troublesome pastures. When cattle start eating larkspur, move them," Pfister says.

Use Of Herbicides Another avenue of control is herbicides, but tall larkspur can be hard to kill. The entire tap root and underground buds must be killed, or it will regrow the next year.

Total eradication is nearly impossible, but reducing larkspur density can significantly lower the amount eaten and reduce death losses, according to PPRL studies.

In a study completed in 1993, researchers looked at the effectiveness of various herbicides. The value of cattle saved by controlling dense patches far outweighed costs of control, even when using the most expensive herbicides and labor-intensive spot applications.

Utah State University Extension economist Darwin Neilsen says, "Even though herbicides are much higher priced than they were a few years ago, returns from control are economically feasible. Sometimes, the cost of control more than paid for itself during the first year."

The goal in larkspur control, says PPRL range scientist Ralphs, isn't eradication, but reduction in density of patches where losses are highest, making sure that a cow can't eat larkspur fast enough to kill her.

One effective herbicide is Tordon (Picloram), which can be used throughout the growing season. Another is Escort (Metsulfuron), which works well during the early stages of growth, but is less effective as larkspur matures, according to Ralphs. Roundup (Glyphosate) can be selectively applied by hand spraying or with a wipe-on applicator to kill larkspur in the bud stage. It, however, is not as effective after the plants have flowered.

Ralphs recommends keeping cattle off treated areas until plants are completely dead and dry.

"Larkspur's toxicity and palatability may actually increase after the plants are sprayed. To be safe, a rancher should keep cattle off the area for the remainder of the grazing season," he says.

If done properly, herbicide application can give long-lasting control. On some areas that were treated in the early '70s, Ralphs says that larkspur still isn't a problem.

Tall larkspur contains numerous alkaloids which can be very toxic. The toxic alkaloids interfere with the central nervous system and paralyze muscles.

Symptoms develop within a few hours after plants are eaten. Muscles become fatigued - beginning with mouth, ears and legs and progressing to the diaphragm, making breathing difficult. This causes muscle tremors and collapse. Typically, it occurs so fast that the first symptom is a dead animal.

The gut becomes paralyzed and the animal bloats. Inability to burp up and swallow the cud can result in bloat or spillage of rumen contents into the windpipe and lungs. Exertion can make the effects worse (due to muscle fatigue), so poisoned cows that are still alive should not be moved.

Many, but not all, affected cattle die unless the amount eaten has been small, according to researchers.