One of our On Pasture readers in east Texas wrote us to get some ideas for controlling bitterweed on her place. She raises natural sheep and hay, so would prefer not to use herbicides. But she now has one four-acre pasture that is about 80% bitterweed and pulling and burning it is very time consuming. Are there alternatives?
Bitterweed, also known as western bitterweed or bitter rubberweed (Hymenoxys odorata) is native to the southwestern and south-central United States, northern Mexico and other dry areas. A single plant can produce more than 50 flowers and 5,000 seeds in a growing season. It thrives in cases where there is little competition. So when other plants in a pasture are closely grazed or overgrazed, bitterweed populations expand.
Grazing is NOT an Option
The problem Hymenoxis odorata and 3 related species (H. richardsonii, H. lemmonii, and H. subintegra) are poisonous to livestock, as sheep ranchers began to discover in the early 1920s. Overgrazing in the late 1800s and early 1900s seriously weakened the choice grasses and forbs so bitterweed flourished. Drought exacerbated the problem and sheep, with very little else to eat, began grazing bitterweed. In his 1949 paper on “The Control of Bitterweed on Texas Ranges” Omer Sperry said that sheep losses of 28% were recorded in 1928, and that losses of 10 to 25% were frequent into the late 1940s.
The lethal dose (LD) of green plants for sheep is 1.3% of their body weight, though during drought the lethal dose can be as little as .5%. Goats are less susceptible and can eat twice that amount before succumbing. Cattle and horses can also be affected. Omer Sperry, in his 1949 paper described the symptoms: “The usual symptoms of illness are loss of appetite, weakness, depression , indications of abdominal pain, and occasionally a dark green discharge from the mouth and nose. A laggard gait and an arched back indicating abdominal pain are the early symptoms of chronic field poisoning. Ranchmen find the first symptoms usually become obvious 7 to 12 days after sheep are placed in bitterweed infested pastures.” As the illness progresses, animals will have difficulty breathing and they will tremble. Head pressing and seizures indicate that death will occur in 24 to 48 hours.
The effects of the toxins in this plant are cumulative so animals can become poisoned by eating a lot of the plant rapidly, or by eating smaller amounts over a long period of time. As little as .1% of bodyweight eaten daily can produce chronic symptoms in 44 days. Burrows and Tyrl say, “Depending upon the rapidity of plant consumption, three forms of the disease are observed. Relatively uncommon, the acute form causes severe clinical signs and death in 24-48 hours. In the more common subacute form, signs are less severe and death occurs in 4-15 days. In the third, more chronic form, there are few distinct clinical signs, but eventual death due to starvation and dehydration is seen when small amounts of Hymenoxys are consumed over a prolonged period of time.” (1)
Grass is the Best Option
“Twenty years’ experience with bitterweed led [Joe] Vander Stucken to conclude that grass was the only effective means of control once the plant had become widely established.”
Edward Keng, “Conservation Ranching in the Edwards Plateau Region of Texas,” 1956 Journal of Range Management
Since the discovery that bitterweed was poisonous to livestock in the 1920s, scientists and ranchers have been looking for ways to control it. The best answer was discovered early on by ranchers like Joe Vander Stucken: Manage pastures in a way that allows perennial vegetation to thrive and crowd out the bitterweed.
Ranchers figured this out after they fenced off pastures with the worst bitterweed infestations to protect their livestock. Omer Sperry observed two pastures, one of 400 acres and one of 10 sections (6,400 acres) that were rested for a period of 4 years. He writes, “The reduction of bitterweed the first and second years was not noticeable but there was a marked improvement in the vigor of the grasses. During the third growing season the bitterweed showed a sharp decrease and, except for a few flooded and disturbed spots, all bitterweed had been crowded out by the perennial growth the fourth year.” Another rancher took a different approach on his 24-section ranch. He split it into 8 pastures, reduced stocking by half (lower during low rainfall years) and implemented a rotational-deferment grazing system. His ranch was cleared of bitterweed in less than 10 years.
Joe Vander Stucken used a variety of treatments on his 27-section ranch. He noticed that bitterweed was abundant in the southeast corners of large pastures where prevailing winds caused sheep to concentrate. He solved the problem by simply moving the fence so that area was the northern part of a pasture. Then he cut and piled live oakbrush on the bitterweed infested areas to prevent sheep from entering the areas. This thin cover of branches on solid bitterweed spots would exclude enough grazing to allow grass to recover and crowd the bitterweed out. He also found that by deferring grazing on small pastures during the summer, the grass increased rapidly and choked out the bitterweed. He changed his management to maintain bitterweed free pastures.
Sperry describes erratic success with herbicides and depended on proper timing and how well drained the soils were. (2,4-D was the chemical available when he worked and wrote.) Based on this he considered herbicides useful in the early stages of a management program. But in the end he concluded that “The only permanent method of controlling bitterweed is to maintain a good cover of perennial vegetation.” His prescription for bitterweed control:
“The management program must include rest periods for the bitterweed infested areas to allow the grasses and other desirable herbs to regain vigor. The initial renewal of root growth is retarded if the grass tops are continually removed. When above ground growth is allowed to remain throughout the growing season, the roots are able to expand resulting in increased top growth and improved vigor. Range grasses which are continuously and closely cropped can not produce sustained amounts of forage year after year. If grass is to control the habitat, controlled grazing must be followed so as to maintain a good top-root balance. When grasses and the more desirable forage are in control of the habitat, there is no apparent
Progress in controlling bitterweed is being made in the area of most severe infestation and ranchmen are talking more in terms of grass and pounds of production and less in terms of the number of head of sheep the ranges can carry. Through the efforts of Soil Conservation Districts, County Agricultural Agents, and Experiment Station workers, more ranchmen are adopting good range management practices but until all operators combine action, bitterweed will not be controlled on Texas ranges.”
As usual, there is no silver bullet and we just have to improve our management. It may take a little time to recover, but it’s still the best answer.
1. From “Toxic Plants of North America” George Burrows and Ronald Tyrl, 1st Edition, 2001, pg. 176 -177
Thanks to the University of Arizona for making the Journal of Range Management archive available!