2022 weeds in review

Nov 14, 2022

2022 weeds in review

Nov 14, 2022

This year, I received a variety of questions from livestock owners in the region. All of the weeds I mention here were of concern to owners managing smaller (less than 20 ac) properties in Fresno and Madera Counties, impacting multiple kinds of livestock – horses, sheep, and cattle. The most common weeds infesting larger properties may be different, and this list is not comprehensive of the weeds found in this part of California.

Weed common name

Scientific name

Livestock of concern*

Key weed concern


Asclepias spp.


Potential toxicity


Malva spp.


Potential toxicity


Brassica spp., Hirschfeldia incana, others


Potential toxicity, lack of forage

Thistles (blessed milkthistle and Italian thistle)

Silybum marianum, Carduus pycnocephalus

Sheep, Horses

Physical deterrents, / injury, lack of forage

Palmer amaranth

Amaranthus palmeri


Potential toxicity

Sacred thornapple

Datura wrightii


Potential toxicity

* NOTE: 'Livestock of concern' here only reflects the type of livestock owned by people who asked me about the weed. Each of these weeds have varying levels of impact to other livestock species.

The dominant concern this year was the potential toxicity of most species. The only species where toxicity wasn't the primary concern were the thistles, including blessed milkthistle (pictured below in the rosette stage) and Italian thistle. These plants were more of an issue due to their prickly stature and out-competing more desirable forage species.

milkthistle closeup

In drought years like this year we often see increased issues with toxic plants. Many toxic plants are forbs with long taproots able to reach deep water and to thrive through the driest of summers. Indeed, some of the species in the table above are native plants, well adapted to our dry-summer climate (many milkweeds, for instance, and sacred thornapple, pictured below).

datura wrightii closeup

Other toxicity issues during drought can include nitrate accumulation in forage grasses which are drought-stressed. In addition, pasture and rangeland forages tend to be less productive when rainfall is limited, so there is less good forage available. All of these factors result in an increased likelihood that animals might eat toxic plants. Hay can be especially challenging. Some weeds, like fiddleneck (a native wildflower), are more of a concern in hay, due to the inability of livestock to avoid the plants and their seeds once they are dried and baled. In a pasture scenario, animals can often avoid living harmful plants if there is sufficient good forage for them to eat.

For any livestock owner concerned about toxic plants, I strongly recommend the publication Livestock-Poisoning Plants of California, which can be downloaded for free (click the title to download). It discusses not only the most common toxic plants in different regions of California, but also describes the signs of poisoning from different kinds of plant toxins, and has images of many toxic plants.

To manage these species, often the solution is to graze moderately to support a healthy forage base that will reduce the likelihood of your animals even encountering a toxic plant! However, sometimes toxic plants can become the dominant species in a field, especially on smaller properties where the owner may not have enough different pastures to reduce pressure on certain areas. When a toxic plant becomes a real problem, then I suggest referring to the excellent book, Weed Control in Natural Areas in the Western United States (currently $37). If you don't want to purchase the entire book, then you can find its weed reports for many species as free PDF downloads here.

By Rebecca Ozeran
Author - Livestock and Natural Resources Advisor