More info for the terms: competition, fire suppression, presence, shrub
Increases in broom snakeweed may be due to livestock grazing, drought, weather, or fire suppression [44]. Broom snakeweed quickly invades overgrazed rangeland. Cattle sometimes leave broom snakeweed almost untouched while grazing grasses to within 0.5 inch (1.27 cm) of the soil surface [14]. An abundance of this shrub is considered by some authorities as an indicator of range deterioration [62]. Many researchers believe that broom snakeweed may be reduced and species such as blue grama increased by protection from grazing.
Grass production increased from 976 lbs/acre to 2,024 lbs/acre during the first year after complete removal of broom snakeweed in Texas [68]. However, increases in broom snakeweed may be due to climatic fluctuations rather than overgrazing [15,49]. West and Tueller [104] maintain that climatic factors are more important than grazing in determining the extent of broom snakeweed populations.
Broom snakeweed populations fluctuate in response to annual moisture patterns, with rapid increases commonly occurring after drought periods [25]. This plant apparently exhibits only a slight response to late-season irrigation [12].
The root system of broom snakeweed occurs at approximately the same soil depth as do the roots of many perennial grasses [60]. Competition for water and nutrients is suspected. Broom snakeweed competition can apparently cause decreases in big bluestem (Andropogon gerardii) and blue grama. However, bottlebrush squirreltail (Elymus elymoides) and black grama appear to be unaffected by the presence of broom snakeweed [49].
Broom snakeweed can be difficult to kill with herbicides, and opinions vary on the optimum time to spray. Clary and others [17] noted erratic results from herbicides such as tebuthiuron in the Intermountain region. Stands were often reduced by applications of pelleted tebuthiuron, but reestablishment sometimes compensated for the loss of the original plants. Detailed information on the chemical control of broom snakeweed is available [17,19,35,66,91]. In cases where application of herbicide is effective, broom snakeweed populations are controlled for up to 5 years, although grazing practices and fall and winter precipitation received after application can also influence results. In some cases, fire can be used in combination with herbicides to extend the longevity of chemical treatment to beyond 5-7 years [92].
Mechanical control is generally ineffective in controlling broom snakeweed [29]. Hoeing the plants just below the soil surface can be effective but is difficult or impractical in stony ground [94]. In Arizona, chaining resulted in increases in broom snakeweed and harrowing in central Arizona reduced populations by only 5 to 10% [34].
Broom snakeweed is susceptible to severe damage by the round-headed borer (Crossidius pulchellus) and mealybugs (Chorizococcus spp.). These insects were believed to be the major factor causing a broom snakeweed die-off in Texas and New Mexico during a particularly dry summer. Eighty-two percent of the mortality was attributed to insects, while the other 18% was thought to be drought-induced [100].
Biological control agents may have potential for control of broom snakeweed [21,36].