Two head of cattle grazing in field. Thankfully without sunburn.

Managing Sunburn in Cattle

Livestock | May 2025

Written by:

Jennifer Ryan has been writing about veterinary topics for nearly 20 years. She has a master’s degree in journalism, a bachelor’s degree in agricultural communications and decades of experience caring for both livestock and pets.

Watch out for toxic plants in the pasture that can photosensitize cattle.

The combination of specific plants and ultraviolet light can interact to cause a serious skin condition in cattle. Photosensitization, or sunburn, can occur when grazing cattle ingest certain plants that create a phototoxic reaction, receive certain pharmaceuticals, or have compromised liver function resulting in severe skin burns and sloughing.

Recognizing and treating cattle quickly can help save animals from infection and culling, advised Scott Fritz, DVM, Dipl. ABVT, clinical assistant professor of toxicology at Kansas State University.

Identify and manage plants

The best way to prevent photosensitization is through range management. Key plants to identify and remove from pastures include:

  • Bishop’s weed (Ammi majus) furocoumarin
  • St. John’s Wort (Hypericin)
  • Buckwheat (Fagopyrin)

When these plants are ingested, specific compounds circulate in the animals’ bloodstream. These compounds interact with ultraviolet light in the skin, causing inflammation. Affected skin cells can die, which causes sloughing. This is most visible on animals with light patches, such as the white areas of Holstein breeds, faces of black baldies or teats. These areas will begin to burn and slough off.

 

Scott Fritz, DVM, Dipl. ABVT headshot
Scott Fritz, DVM, Dipl. ABVT

 

“St. John’s Wort is the poster child for photosensitization in cattle,” Fritz said. “Cattle will typically have to eat more than 1% of their bodyweight. This can be a problem if producers turn out cattle before there is adequate grass or overgraze.”

St. John’s Wort typically grows in the eastern or western regions of the United States, but there can be hot spots of growth in any given year depending on weather conditions.

Photosensitivity can occur after accumulation of chlorophyll metabolites due to reduced liver function. Many things can disrupt liver function and producers should seek a veterinarian to evaluate liver function in affected animals to aid in the decision-making process, Fritz advised.

 

Veterinarian and owner survey field with dairy cattle in it.

 

Treatment of cattle

Once photosensitized cattle are identified, Fritz recommended producers immediately stop grazing and remove animals from direct sunlight if possible.

“You’re probably not going to have a bunch of them affected at once,” he noted. “First, you’ll probably see animals itching and rubbing on everything. Then, their skin begins to slough off. It can be up to a dinner plate size of skin, which will take a long time to recover.”

Fritz recommended working with a veterinarian to choose appropriate antibiotic therapy and pain control options, if necessary.

Next, seek help to identify the cause and determine the next steps for management.

“Some plants can actually become more palatable once they are dead or dried, so they can be a bigger issue when included in hay or after herbicide use,” Fritz said. “The sloughing can sometimes be confused for a laceration. Be sure to get a veterinarian involved in making the diagnosis.”

Key points

  • Signs of photosensitization include rubbing or scratching of lightly pigmented or exposed areas of skin. In addition, producers may see lesions on exposed areas.1
  • Affected skin will be red, oozing fluid and swollen. In severe cases, large areas may slough off.1

1 Texas A&M Veterinary Medical Diagnostic Laboratory. Photosensitization: Causes and Testing Options. Aug. 21, 2024. Accessed April 11, 2025. Available at: tvmdl.tamu.edu/education-library/photosensitization-causes-and-testing-options.

 

Photo credits: istockphoto.com/maunzel, istockphoto.com/tomazl

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