Most horse ranchers are probably aware of poisonous plants that can affect horses (please refer to a brief list at the end of the manuscript). Yellow Starthistle (Centaurea solstitialis) is one poisonous plant that can cause horses a slow and painful death. Symptoms and eventually death generally appear in horses less than two years of age. The early symptoms include nervous syndrome - “chewing disease” (Equine nigropallidal encephalomalacia). The afflicted horses are unable to chew or drink. There is no treatment for this disease. Signs are irreversible, and if the animal is not destroyed, it will eventually die of dehydration, starvation, or aspiration pneumonia.

In experimental feedings, the symptoms appear after the horse ingests 50% to 200% of its body weight in Yellow Starthistle over one to three months. This can be as little as 600 pounds during a one to three month period. Horses consume about 2% of their body weight per day; therefore, a 500 pound weanling eating 10 pounds per day can easily consume 250 pounds, (50% of their body weight) in one to three months. The principle poison is unknown at this point, however it does cause large lesions in the brain. Poisonings are observed year round in California. Generally, the disease affects young horses; it is speculated that young animals begin curiously nibbling or “picking” at anything “green,” eventually becoming “addicted” to the plant. In dry, summer pastures, Yellow Starthistle can be the only green plant available.

Yellow Starthistle was first recorded in California in 1869. Yellow Starthistle seed was thought to contaminate alfalfa seed imported from Eurasia. Once touted as a haven for honey-producing bees, it is now considered a noxious weed in California. It is now common on roadsides, rangeland, hay fields, pastures, and waste areas. Yellow Starthistle is an annual plant, meaning it completes its life cycle in one year. The previous summer’s seed germinates with the first fall rains. Yellow Starthistle begins sending down its taproot in the fall before other plants start to grow. This deep taproot (4 to 6 feet deep) robs moisture and nutrients from the surrounding grasses, enabling healthy Yellow Starthistle growth into California’s dry summer months. A stand or group of yellow star thistle can produce an estimated 50 to 200 million seeds per acre. A single plant can produce up to 29,000 seeds in a season. Two million seeds per acre are needed to repopulate next year. Consequently, only 70 Yellow Starthistle plants are needed to produce the 2 million seeds required to repopulate. Yellow Starthistle infests over 12 to 15 million acres in California. This is the equivalent of about 25,000 square miles. Additional infestations are spreading to Arizona, Nevada and Oregon, Washington, Idaho, and Montana.

Control methods for Yellow Starthistle vary, depending on the degree of infestation and the source of potential seeds. The key to prevention begins with hay free of Yellow Starthistle. It also is necessary to prevent invasion of Yellow Starthistle from roadside ditches and borders. Once Yellow Starthistle is present, there is a spectrum of control methods to consider. Repeated tillage and mowing can be used late in the season, if it is done when less than 5% of the plant population is flowering. However, these procedures are labor intensive, expensive and require cumbersome equipment. These methods fail if it rains after tilling or mowing as the moisture causes new plants germination from the soil seed bank.

Insects used for bio-control have produced mixed results. The insects are released to “feed” on the flowers or seed head of Yellow Starthistle to reduce future reproduction. Due to the previously mentioned seed production, an astronomical number of bio-controls would be needed to eradicate Yellow Starthistle. Bio-controls have not been a reliable or effective option for eradication because of the quantity required and their relatively fragile environmental nature.

Control through competition using perennial grasses, such as Harding-grass and annual clovers (legumes) can decrease Yellow Starthistle populations. However, planting competitors usually requires additional farming practices. Alternatively, grazing, by ”ruminants” such as goats, sheep or cattle can be effective, as they are not susceptible to the poison in Yellow Starthistle if it occurs before thistles acquire barbs. Burning is another alternate control method. However, it has safety concerns and requires repeated treatment for maximum effectiveness. The final and probably most effective control method is a newly registered herbicide. The California Department of Pesticide Registration registered Transline[1] herbicide in the fall of 1997. The active ingredient in Transline is clopyralid. Transline is an effective post emergent herbicide. This herbicide will kill the mature Yellow Starthistle through contact and control seedlings through soil residual. Transline is a “selective” herbicide because it controls primarily broadleaf weeds. It is safe for grasses In addition, there are no grazing restrictions. Therefore, all livestock including pregnant or lactating mares may graze freely in treated pastures.

In University of California trials, Transline was 100% effective in controlling Yellow Starthistle for the season when applied at the highest labeled rates. To be most effective, Transline should be applied during the rainy season (December through March) and at a rate of 4-oz Transline per acre. Early treatment encourages grass growth by eliminating Yellow Starthistle competition.

To achieve maximum cost effectiveness combined and the highest quality results, a surfactant (a wetting agent or “sticker”) may be added to the chemical mixture of water and Transline. Surfactants help Transline adhere to Yellow Starthistle for maximum effectiveness, especially late in the season.

Transline is a non-restricted herbicide found in many farm or ranch retail locations. Transline herbicide has a “caution” label; the lowest human hazard signal words, and is not a “Restricted Use” pesticide. Operator identification numbers are required for all herbicide applications to agricultural ground including rangeland. Non- crop Transline applications may not require operator identification number. For more information concerning application practices contact your county agricultural commissioners office.

To achieve long- term control, Transline should be applied consecutively for a minimum of three years to reduce the seed bank in the soil. After three seasons, the natural seed bank should be depleted, and other methods of controls may be used. Other follow-up methods may include bio-control and/or spot treatment with Transline or other non-selective chemicals. These procedures should provide a long-term and sustainable control program to help prevent unnecessary Yellow Starthistle poisoning.

Gary Veserat, PAS, and Valerie Veserat, MBA, Veserat Consulting - provide comprehensive consultation services for improved ranch management and livestock production. For more information, contact Gary or Valerie at (530) 668-4884 (telephone & fax), (916) 798-7825 (mobile), or Email - gveserat@cattlemen.net.