Noxious plants are a part of life in the Mountain West and commonly occur in our high-elevation pastures. These plants can cause a variety of health problems including birth defects and even death. These plants can cause serious problems and represent a significant financial loss to our operations.
Plants that generate some concern here in the West include lupines, larkspur, thistles and many others. Properly identifying plants is crucial to understanding their life cycle, including periods of toxicity to livestock, and when it is safer to put cows onto the pasture. Having a good field guide to identify plants on the range is a good idea. I have found success using phone apps that use the camera to identify the plant. This method is not always correct, and does require cell service, which is why I recommend having a hard-copy book as well.
Properly identifying plants and plant characteristics can help inform management decisions and control methods. Once you have a basic understanding of the plant itself, research its effect on cattle. Some plants like lupines will cause birth defects if consumed in early gestation and are especially toxic to livestock species such as sheep. Lupine affects cattle through alkaloid poisoning, which is believed to slow movement of the fetus by blocking brain signals in the fetus.
Larkspur is another common poisonous plant on rangelands, which also poisons through alkaloids. Larkspur can cause death in a dosage as low as 0.5% of bodyweight, which in a 1,200-pound cow equates to 6 pounds of larkspur consumed. Larkspur is most poisonous from late April through June. Once the seeds have dropped, cattle are less susceptible to poisoning by larkspur.
Thistles as well as many other plants can contribute to poisoning through high nitrate levels. Nitrate poisoning can occur in many forage types, and it is impossible to know the nitrate load of the plants without testing. Nitrates accumulate in many plants; however, some plants are known as nitrate accumulators, such as thistles and kochia weed.
Pastures with poisonous plants can and should be grazed; however, additional management is needed. Do not turn hungry animals into a pasture with a heavy load of poisonous plants. Do not allow late-term gestation cows or sick animals onto pastures with poisonous plants. I encourage you to research more about poisonous plants in your area. If you have questions, reach out to your relevant extension personnel.
A great resource on poisonous plants can be found on the University of Idaho website.
