Pasture full of tarweed
Thank you for reaching out to Extension. Yes, Tarweed (also known as Fiddleneck) is on the list of plants that are toxic to horses. It is in a group of plants that contain pyrrolizidine alkaloids which are the most common cause of liver damage, but also can cause kidney damage, heart failure, cancer and photosensitization. Animals typically will not readily eat plants with pyrrolizidine alkaloids, unless no other forage is available. https://extension.oregonstate.edu/sites/default/files/documents/1/pppyrrolizidinealkaloidpoisoning.p.... Check out that link to confirm that is what you have. With all toxic plants, the effect on the animal (if any) depends on the life stage of the plant and the amount consumed. The following information is taken from a longer article linked here:https://extension.oregonstate.edu/pests-diseases/weeds/poisonous-plants-commonly-found-pastures
Pastures often contain weeds that are potentially dangerous to livestock. The toxic compounds in plants are usually a defense mechanism against predation and have a distinct, unpleasant odor or a bitter taste and are not preferentially grazed. Consumption of unpalatable plants will increase under some circumstances, primarily if other forage is not available. Understanding the dangers and various management strategies to control toxic plants will reduce the risk to your livestock.
Grazing management is a critical component to maintaining pastures free of poisonous weeds. Avoiding overgrazing will help maintain an abundance of desirable forage plants that are able to compete with weeds and reduce the risk of livestock being forced to eat poisonous plants because no other forage options are available. Grazing pressure should be reduced during dry periods as drought can increase consumption of poisonous plants if there is a decrease of other forage.https://www.jswcd.org/files/885b82f95/Poisonous+Weed+Fact+Sheet.pdf
Managing the Tarweed: At this time of the year, the best management strategy would be keeping the tarweed from setting and distributing more seed. If you can mow or pull the plants and remove them from the field that would be good. You will want to mow the plants when stalks are elongating, but before flowering (if possible).
For additional reading and overall management planning I highly recommend you check out "Managing Small-acreage Horse Farms", which will give guidelines to minimize stress to your pastures. Also if applicable, the Pasture and Hayland Renovation Guide provides information about soil preparation and timing for successful establishment.
Feel free to contact me directly with any follow up questions you may have.
I just wanted to send a follow up to see if you were able to confirm what plant you had. Since sending this response I have learned that the identification of Tarweed can be tricky because it's a very common named used to describe several plant species. All tend to have yellow flowers, but commonly, the tarweed people complain about this time of year in our region is not Amsinckia intermedia, a fiddleneck in the Boraginaceae family, but a Madia species (of which there are many!) in the Asteraceae family. More information can be found here:
Some Madia species are actually edible. Regardless, due to the fact that the plants are sticky and have a pungent aroma, livestock species do not eat them in any quantity.
If you want to actually know if that plant is a real concern or not, and are not confident in your identification - you can bring in a sample to your local Extension office for a positive identification. Alternatively we could try to identify from photos, especially those of the flowers with the rest of the plant.