Dyers woad. Purple loosestrife. Rush skeletonweed. Black henbane. Hoary cress. Sounds like the recipe for the witch’s brew in Macbeth. But these weeds are actually non-native plant species that threaten Colorado’s plant diversity and ecosystem. Horse owners need to be especially concerned about weed growth in their pastures, because many noxious weeds lead to illness and even death if horses eat them.The Colorado Department of Agriculture cites three main reasons that horses are harmed by weeds: the increasing number of poisonous weeds now found in the state; a lack of knowledge on how to identify these weeds; and poor pasture management practices.The number of poisonous and noxious weeds in Colorado is increasing rapidly, spread when soil is disturbed by various activities, such as construction, off-road motorized vehicles, improper livestock grazing or natural disturbances like fire. Seeds are carried in the wind and by people and animals.These invasive weeds are aggressive and quickly dominate plant communities. Native grasses and plants are crowded out, and the weeds threaten biodiversity and ecosystem stability. They also pose a real danger to livestock. Poisonous plants can kill quickly, and continued ingestion of noxious weeds can lead to acute illnesses in horses and cattle.The first step in protecting your horses from noxious weeds is to learn how to recognize them. A wealth of information is available from a variety of governmental and private sources (see below). In our area, the most common poisonous weeds are senecio (actually a genus of weed that includes more than 1,000 species), Russian knapweed, yellow starthistle, houndstongue, locoweed and two species of sages. “We had a horse pass away last year in Ellicott from Riddell’s groundsel [a species of senecio],” said Mark Johnston, forestry and noxious weed manager at the El Paso County Environmental Services Department. “We were looking for locoweed, but the autopsy came back with the same toxic agents as Riddell’s groundsel, and that’s what we determined it was.”A certain weed may be present in a pasture, but the horses will usually not eat it unless they have nothing else to eat. That’s why good pasture management is ultimately the most important preventative measure you can take. “Most livestock prefer the grasses over the broadleafs [weeds] and won’t eat things that are bad for them,” said Johnston. “That’s good cultural weed management practice: provide a good environment for species you want and a bad environment for the species you don’t want.”Once you have identified the noxious weeds that are present in your pasture, how do you get rid of them? Johnston identified five pasture management principles to control weeds.
- Cultural: To prevent weeds from invading your pasture in the first place, do everything you can to create a positive environment for desired vegetation. Proper grazing management (including installing cross fencing and allowing parts of your pasture to rest), irrigation and seeding vigorously can all encourage the growth of grasses and discourage weeds.
- Preventive measures: “Go after those one or two weeds before they become an infestation,” said Johnston. Be sure that any soil or gravel you bring on your property is free of weeds.
- Biological controls: Various natural enemies of weeds such as insects or disease organisms can be added to your pasture. The county environmental services department can provide various insects free of charge to add to your property if appropriate. Another natural control that is growing in popularity is introducing goats to your pasture. Goats tend to prefer broadleaf plants to grasses. “Just like any other livestock, it’s important that you move them at the appropriate time and leave the grasses high,” said Johnston.
- Mechanical: Tilling, mowing, mulching, burning and flooding can disrupt weed growth.
- Chemical: Various herbicides can be used to control weeds.
