Bob Larson, DVM, PhD
Reproductive physiologist and Epidemiologist
Beef Cattle Institute
Kansas State University
RLarson@vet.k-state.edu
Cattlemen throughout history have known that some plants can be toxic to their animals. Some poisonous plants are only present in a few small areas of the U.S. while others are found over a wide geographic area. Some of these plants are poisonous during all phases of growth, while others are only poisonous (or more dangerous) during certain stages of growth or at certain times of the year.
Plants that cause damage to the heart or lungs will generally cause a very rapid death if a large enough dose is eaten – but if the amount consumed is less than a lethal dose, cattle may appear weak and depressed. Plants that are toxic to the liver or kidney seldom cause a rapid death, but instead a slow decline in health and body condition. Consuming plants that cause damage to the liver can result in the buildup of toxins that would normally be removed by a healthy liver. These toxins can cause the skin to become very sensitive to sunburn and can cause other signs of liver failure such as weight loss and poor performance. Some plants are toxic to the kidney – particularly plants such as oak trees that accumulate tannins. Cattle with kidney damage may show signs of reduced appetite, rapid weight loss, and increased water intake.
Some plants are toxic to the nervous system such as some types of ryegrass, locoweeds, and water hemlock. Cattle that eat these plants may suffer from rapid death or longer-term weight loss and nervous system signs depending on the specific plant and the amount consumed. Signs of nervous system problems include staggering, apparent blindness, exaggerated movements, and hyper-excitability.
Abortion or the development of birth defects can occur when pregnant cattle consume certain toxic plants. Abortions caused by toxic plants are often difficult to diagnose, and the birth defects caused by some toxic plants are the same or similar to defects caused by genetic problems or viral infection. Loco weeds, tobacco, lupine, and poison hemlock are known to cause birth defects in calves in certain situations when pregnant cows consume these plants.
In order to diagnose suspected plant poisonings you should work with your veterinarian and a veterinary diagnostic laboratory. Your veterinarian can help you to develop an accurate history and time-line for the problem. To identify the cause of losses due to poisonous plants, the pasture or pen should be inspected, both alive and dead animals should be examined, and diagnostic samples of plants, blood, and tissues should be properly collected and then evaluated by trained diagnostic laboratory personnel. The presence of toxic plants in pastures or hays is not proof that cattle have been harmed by the plants because many time cattle will refuse to eat them. However, finding evidence of consumption of potentially toxic plants by animals, either by observing evidence of grazing of suspected plants or by finding parts of these plants in the digestive tract of a dead animal is highly suggestive of poisonings.
Because removing all toxic plants from a range or pasture is not likely to be practical or successful, good grazing management using stocking density, fencing, water development and salt/supplement placement to maintain good pasture health will
minimize the incentive for cattle to graze toxic plants. If it appears that a toxic plant problem is occurring, cattle should be removed from the suspected pasture immediately and a veterinarian should be contacted. It is important to work with veterinarians, Extension agents, natural resource conservation service specialists, and range specialists who can all help develop a plan to keep pastures healthy and minimize the risk of toxic plant poisonings.
