LOVELOCK - I recently had a producer bring in a Yellow Starthistle plant that they discovered in one of their hay fields. Yellow Starthistle is another noxious weed that grows in Nevada. Although, we have not seen a lot of this weed in the local area, a 2001 study indicated that 11 of Nevada's 17 counties have reported Yellow Starthistle populations.
Yellow Starthistle (YST) was first discovered in Sacramento in 1869. This invasive weed now occupies approximately 22 million acres in California, and several million acres in the Pacific Northwest. It is a major threat to pastures, rangelands and roadsides. Dense stands of this weed drastically reduce forage production by quickly displacing native plants and shrubs and forming a monoculture. This in turn affects wildlife populations, grazing capacity, recreation and many other economic aspects of a community.
The key to keeping this invasive weed out of Nevada is prevention. Prevention requires teaching property owners, managers, land users and the general public how to identify YST and eradicate early infestations. The least costly alternative for land managers and communities is to prevent the establishment of an infestation. When prevention does not work, or is not implemented in time, several expensive control methods may be necessary. Biological, physical and chemical controls, when used in combination, have the greatest chance of eradicating or controlling Yellow Starthistle.
The total cost of an invasive weed is more than the mere cost of eradicating the infestation. Ecosystems are damaged by the weed and those damages cause economic losses to recreation, agriculture, conservation and preservation. If not controlled, YST will overwhelm Nevada landscapes as cheatgrass and perennial pepperweed have done.
The best and most economical way to manage weeds is to stop them from advancing into new areas. Unfortunately, many land managers, recreationists and the general public are unfamiliar with invasive weeds or what an infestation looks like until it is large and noticeable, which is often too late.
Land managers can prevent the introduction of invaders in these ways:
1. Checking and cleaning the undercarriage of vehicles before leaving an infested site or entering uninfested property.
2. Feeding only weed-free forage.
3. Grazing animals before weeds set seed and feeding livestock grazed on infested range weed-free forage before moving them to uninfested land.
Biological controls of YST include the use of grazing animals, plant diseases, insects and competition from other plant species.
The most promising of the insects used to control YST are the false peacock fly and the hairy weevil. However, the success of biological controls is directly related to the amount of site disturbance. Control methods such as grazing, mowing, tilling or burning cannot be used for several years in conjunction with insect control and if the infestation can be eliminated by other means, then it does not make sense to release bio-agents.
Some livestock grazing has been successful in controlling YST, however, timing, duration and intensity are critical. Grazing reduces weed biomass and seed production. Cattle, sheep and goats will eat the weed before the spines have appeared and after it has bolted. Cattle and sheep will not graze Yellow Starthistle after the spines appear. Goats eat the spiny flowers and will graze it all season. Consequently, goats are best for controlling relatively small infestations.
Horses should be kept away from YST infestations. Grazing YST causes horses to develop Equinenigropallidal encephalomalacia or "chewing disease." This neurological disease causes horses to stop eating but continue to chew and eventually die. Horses may develop chewing disease within weeks after grazing infested forage, although some may graze infested forage for years before getting the disease.
Herbicides are valuable in managing invasive weeds. Dense stands of YST can be significantly decreased if chemicals are applied at the proper rate and time. They are an excellent tool as part of an integrated weed management program.
To be effective, pre-emergent herbicides must remain in the soil throughout the season because YST seeds germinate whenever moisture and temperature conditions are favorable. Post-emergent herbicides (Clopyralid, 2,4-D, Triclopyr) work best on seedlings. Adequate control with a single application rarely occurs and follow-up spot treatments are usually necessary. Delaying control will allow more weeds to be treated; however, the plants will be larger and require more herbicide.
Mowing, tilling, hand-pulling and controlled burning are physical control methods used to reduce YST infestations. Mowing is effective if it occurs when two to five percent of the seed heads are flowering. This prevents the production of viable seed and also ensures that the lowest branches of the weed are above mower height. If mowed too early, the architecture of the plant changes from upright to horizontal and more seeds will be produced per unit area. In field studies, control was directly related to height of cutting, i.e., the closer to the ground, the better the control.
Each control method will differ in effectiveness in different years during a long-term invasive weed control program. The key is to optimize the success of any control method by using it when it is most effective. Herbicides are most effective if used in the first year of a long-term control program. They are also important in follow-up spot treatments. Grazing, re-vegetation and release of biological controls can be used in most multi-year, integrated programs, but not in the same year.
If you need help identifying Yellow Starthistle or other noxious weeds, feel free to contact me at the Pershing County Cooperative Extension office, (775) 273-2923 or e-mail, fosters@unce.unr.edu.
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