Broom snakeweed Gutierrezia sarothrae is a small (growing to about 50 cm), dense bushy shrub, native and widespread over much of western USA. It can be invasive, especially in over-grazed prairies and grassland ranges, and is toxic to livestock in large quantities. This study evaluated vegetation response to various herbicidecontrol trials of broom snakeweed on blue gramma Bouteloua gracilis range near Laramie, Wyoming, northwest USA.
The study area had been heavily sheep grazed and in 1966 supported dense stands of snakeweed with occasional plains pricklypear Opuntia polyacantha. Blue grama comprised most of the perennial grass cover. Herbicides in water were applied (25 gal/acre) on 26 June 1966, on unreplicated, 43 x 200 ft (13.11 x 60.96 m) plots by truck-mounted sprayer:
1). Untreated (control);
2). Silvex 2 lb/acre;
3). Silvex 4 lb/acre;
4). Picloram 0.5 lb/acre;
5). Picloram 1 lb/acre;
6). Picloram at 0.25 lb/acre + 2,4-D at l lb/acre;
7). Picloram at 0.5 lb/acre + 2,4-D at 2 lb/acre.
Vegetation response was recorded by visual evaluations each year (1967-71). In 1971, vegetation was surveyed along 10 randomly located 30 ft (9.14 m) transects in each plot (line intercept method). To assess yield, herbage was clipped by species in 15 randomly located circular (76 cm diameter) plots in each main plot; samples were dried and weighed.
All four picloram treatments effectively controlled snakeweed (95-100% control for 3 years after application; 80-95% in year 5). Picloram also killed pricklypear.
Silvex failed totally in killing broomwood and was partly effective against Opuntia (at 2 lb/acre 25% control year 1 and 20% year 3; at 4 lb/acre 80% control year 1 and 65% year 3).
Blue grama (and some other grasses) was initially ‘injured’ by 0.5 and 1 lb/acre picloram rates but recovered after two years. Needle-and-thread Hesperostipa comata was damaged at the 1 lb/acre rate.
Subsequent recovery in combination with broomweed control resulted in range improvement 5 years after treatment. Blue gama cover increased relative to the control (control 3.8%; silvex treatments 1.5.- 2.2%; picloram treatments 7-14%) and dry yield (1b/acre) was significantly higher in picloram treated plots (control 105 lb/acre; silvex treatments 140-150 lb/acre; picloram treatments 350-680 lb/acre; note: approximate values read of figures in original paper).