Toxic Plants on the Range

BY MEGAN CLAYTON, Professor & Extension Range Specialist, Texas A&M AgriLife Extension Service

Owning livestock is not for the faint of heart. A living investment, prone to mishaps like pneumonia, calving trouble, getting stuck in the mud, and internal parasites, is enough to make a manager worried. Add in the art and science behind determining what grazing system is best to protect the long-term health of the soils and grazing forages, and then watch all these plans slowly change as drought sets in. Livestock producers are no doubt a tough breed!

Toxic plants are one concern that often surfaces when managing pastures for livestock. Livestock may be more inclined to eat toxic plants during drought because less of their preferred foods are available. Additionally, slowing plant growth causes some plants to become more toxic than usual. The National Weather Service is predicting South Texas will likely develop drought conditions through May. While this is a ‘best guess’, intermittent droughts are a part of our climate, and understanding what potential toxicities to look out for can make management easier, no matter the weather.

Less than four percent of plants in Texas are considered toxic to livestock, with only 80 out of approximately 5,000 plant species thought to cause economic loss to livestock producers (Rangeland Risk Management for Texas: Toxic Plants by Charles Hart). Keep in mind that some stages of plants can be more toxic than others. For example, broadleaf milkweeds are more toxic when young and silverleaf nightshades are more toxic when mature. Certain plant parts, like coyotillo seeds, are more toxic and even some environmental extremes like drought or the first frost can cause toxicities to build up in plants, such as in Johnsongrass. In general, livestock learn to avoid toxic plants, usually from their mother. Like an unsupervised child in the store candy aisle, what would drive an animal to consume toxic plants in large enough quantities to produce harm? Let’s review the main situations where poisonous plants may cause issues:

• Most livestock losses from toxic plants can be attributed to overgrazing or lack of adequate forage resources available. Consequently, drought often limits the amount of forage available and increases bare ground, promoting toxic plant growth. Consider running low to moderate stocking rates to prevent overuse of the better grazing grasses and follow proper range management practices to better range condition.

• When penning livestock, make sure good forage is available and

toxic plants are not present in high amounts. This can also apply to moving livestock to a new pasture. Always scout for potential weed issues and ensure the forage grasses are adequate to support the herd. Evaluate the need for a supplemental feeding program.

• Toxicities can occur when naïve livestock are moved to a new location with unfamiliar plants. Be sure new livestock are turned into a pasture on a full belly, so they do not aggressively consume large quantities of the wrong plants. Avoid leaving livestock in pastures with dense infestations of toxic plants, especially if they are new to the area.

• Herbicide applications can affect the palatability of toxic plants, where some livestock might graze plants usually avoided. Additionally, as plant growth is slowed after herbicide applications, toxins could build up to dangerous levels. Err on the side of caution and remove livestock from a pasture for a few weeks if you spray dense populations of toxic weeds with herbicides. In general, reducing poisonous plant numbers through chemical control, grazing if appropriate, mechanical control, and prescribed burning helps limit toxic plant issues longterm.

For more detailed information about toxic plants and their identification, go to RangePlants.tamu. edu or order the Toxic Plants of Texas book by Hart et al.