Don’t Panic: Managing for Toxicity Risk in Panicum Grass Species

The exceptional rainfall this growing season brought forth an abundance of forage on range and pasture across the state. In some cases, this moisture prompted a flush of growth from annual plant species not commonly seen. 

A recent contact to the Johnson County Extension Office reported non-predator losses of mature ewes while grazing upland pasture and the presence of an unknown grass. A discussion with the local weed and pest district identified an abundance of Panicum dichotomiflorum, also known as fall panicum, on the piece of land where sheep were grazing. 

While correlation does not equal causation in most cases, this time the pieces fell together. Fall panicum can be toxic to livestock.

Panicum grasses 

Fall panicum is a native annual warm-season grass related to the annual witch grass Panicum capillare and the perennial warm-season switchgrass Panicum virgatum, all in the panicum genus. 

Panicum grasses are known to contain elevated levels of steroidal saponins, which are chemical compounds causing photosensitization, gallbladder obstruction, liver failure and death of livestock in severe cases.

These grasses are sometimes called panic grasses in reference to their open, airy panicle seed head structure. Of note, P. capillare and P. dichotomiflorum sometimes share the common name “witchgrass,” which can create confusion. 

These may be correctly distinguished by the hairy stems of P. capillare and the hairless stems of P. dichotomiflorum. For the purpose of this article, witchgrass will refer to P. capillare.

Panicum toxicity

Panicum toxicity causes many signs in livestock. 

Photosensitization includes skin redness and sores, especially on the muzzle, around the eyes and other areas not covered by hair or wool. Shade-seeking behaviors can become pronounced. 

Animals may also appear lethargic and stools may become loose. Swelling of the lips and then of the entire head may occur. Yellowing of the eyes and jaundice can set in as liver failure progresses.

Incidents of fatal liver failure in horses, goats and sheep have been associated with the consumption of Panicum grasses.

Ruminant livestock are typically able to tolerate the toxic components of these grasses under normal conditions because they are metabolized and neutralized by microbes in the rumen. However, the precise mechanism of toxicity is unknown, and sometimes animals can graze Panicum grass species with no complications. 

There is evidence fungal mycotoxins can compound the effects of the saponins in a way the rumen cannot overcome. Fungal growth requires humidity, which many parts of Wyoming have experienced this growing season due to greater than normal precipitation. 

As with many things, the dose makes the poison. 

A few stray plants in a hayfield or grazing allotment will not mean immediate death for any animal which dares to nibble a bite. 

Likewise, pastures in which Panicum grasses are the predominant species should be grazed with caution. Feeding Panicum-free hay prior to turnout on pasture containing a high proportion of Panicum grasses can help reduce appetite and intake, while also making use of the available forage.

Pasture and hay containing fall panicum, witchgrass and switchgrass is never recommended for horses because they do not have the biological ability to neutralize the toxins.

Switchgrass is not common in Wyoming, except for at the eastern margins of the state. Witchgrass is a widely distributed weedy species frequently seen in disturbed areas. Fall panicum is not common in Wyoming, which makes the above report a standout case study.

Panicum management

The simplest and best management is to remove livestock from an area with a large population of Panicum grasses when possible, especially when conditions have been wet and likelihood of fungal growth is high. 

Affected livestock typically recover rapidly following this change in diet. Early interventions lead to better outcomes. 

Since fall panicum and witchgrass are both annuals grasses, a targeted pre-emergent herbicide application can provide management while minimizing harm to desirable perennial forage grasses. 

Herbicide applications should be considered only when these species are overtaking a pasture. Consult a local Wyoming Weed and Pest District office for product recommendations and application rates.

The above average moisture this growing season caused some unusual plants to emerge. It is a good reminder to keep an eye out for unrecognized plants and be aware of what is growing in the local environment. 

When Panicum grass species are present in forage, graze only with ruminant livestock. If signs of toxicity occur, immediately remove them from the site and provide a Panicum-free recovery diet.

Micah Most is the University of Wyoming Extension agriculture and natural resources educator serving Johnson County. He can be reached at mmost@uwyo.edu or 307-684-7522.