DESERT MULE DEER RESEARCH
Effects of Spike 20P on Mule Deer Habitat
Spike 20P (or tebuthiron) is a herbicide that is growing in popularity in the Trans-Pecos. Resource specialists prescribe Spike 20P for controlling invasive brush species and increasing grass production. Despite the increased interest in Spike 20P, few data exist on its impact on mule deer habitat.
To evaluate the impacts of the herbicide Spike 20P on mule deer habitat, we conducted a study on a private ranch 20 miles east of Van Horn, Texas. Spike 20P was applied at a rate of 0.75 lb./acre by a fixed wing aircraft during winter. Riparian areas with higher brush diversity were left untreated as well as steeper slopes, which are more prone to erosion. Each year following application, light rains allowed the pelleted herbicide to dissolve into the soil.
Forb production (%cover) was greatly reduced by Spike 20P even 5 years after initial application compared to control areas (C-rip, C-upland) where herbicides were not applied.
Spike treated areas were effective in controlling creosotebush and tarbush resulting in 94-100% mortality. Likewise, canopy cover of brush was reduced from approximately 17% on control sites to 2% on treated sited. The reduction of brush species allowed grasses to flourish from 388 pounds/acre (control sites) to 635 pounds/acre (5-year post treatment).
However, Spike treated areas showed noticeable reduction in forbs between control sites. Forb diversity was 60% less than forb diversity on control sites. Forbs had not recovered to pretreatment levels even 5 years later.
Riparian areas (which were not treated with Spike) had the highest value to wildlife with respect to forb and brush diversity and abundance. Browse assessment also supported the importance of riparian habitats compared to other habitats evaluated for desert mule deer.
Our study demonstrates the effectiveness of controlling two invasive brush species using Spike 20P resulting in a substantial increase in grass production. However, the effectiveness of Spike 20P may come at a price to important wildlife foods including annual and perennial forbs and important browse species (e.g., range ratany, sumac species). We noted reduced forbs on our treated sites which still had not recovered after 5 years to pretreatment levels.
If resource managers are interested in applying Spike 20P in the Trans-Pecos, they need to consider the habitat needs of desert mule deer and other wildlife. We recommend (1) using mosaic patterns rather than uniform application, (2) avoiding riparian and other diverse habitats, (3) deferring livestock grazing for a minimum of 2 growing seasons post-treatment, and (4) contacting your local natural resource specialist for specific application rates that are best suited for your goals.