Cattle Grazing as a Biological Control for Broom Snakeweed: Vegetation Response

Authors: Ralphs, Michael; BANKS, JEFFERY - USU

Submitted to: Rangeland Ecology and Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/8/2008
Publication Date: 1/1/2009
Citation: Ralphs, M.H., Banks, J. 2009. Cattle Grazing as a Biological Control for Broom Snakeweed: Vegetation Response. Rangeland Ecology and Management, 62:38-43.

Interpretive Summary: Broom snakeweed is one of the most wide spread range weeds in western North America. It increases and dominates plant communities following overgrazing and fire. Although overgrazing is a principal cause of its increase, targeted grazing may be an effective biological control. A 4 year grazing study was conducted to determine if cattle could be induced to graze snakeweed as a biological control, and this paper reports the vegetation response. Grazing trials were conducted in spring before culm elongation of grasses when snakeweed was rapidly growing, and in late summer when grasses were dormant and snakeweed was in flower. Cattle were restricted to narrow grazing lanes to limit herbaceous forage and force them to graze snakeweed, and they were moved to new lanes each day. Density of mature snakeweed plants declined to a greater degree under grazing, than in ungrazed areas. Intensive grazing did not reduce crested wheatgrass cover under spring grazing, and it actually increased in the summer grazing trials. Snakeweed was controlled without adversely affecting associated species in a crested wheatgrass plant community.

Technical Abstract: Broom snakeweed (Gutierrezia sarothrae) increases and dominates rangelands following disturbances such as overgrazing, fire and drought. However, if cattle can be forced to graze snakeweed, they may be used as a biological tool to control it. Cattle grazed snakeweed in May and August in 2004-2007. Narrow grazing lanes were fenced to restrict availability of herbaceous forage to force cattle to graze snakeweed. They utilized 50 – 85% of snakeweed biomass. Mature snakeweed plant density declined due to prolonged drought, but the decline was greater in grazed lanes. At the end of the study, density of mature plants in grazed lanes was 0.31 plants/m2, compared with 0.79 plants/m2 in ungrazed pastures. Spring precipitation in 2005 was 65% above average, and a new crop of seedlings established following the spring grazing trial. Seedling establishment was greater in the spring grazed lanes in which the soil had been recently disturbed, compared with the ungrazed transect and summer grazed lanes. The cattle were not able to utilize the large volume of new snakeweed plants in the spring grazed pasture. They did reduce the number of seedlings and juvenile plants in the summer grazed pasture. Intense grazing pressure and heavy utilization did not adversely affect crested wheatgrass cover, and it was actually higher in the summer grazed lanes than the ungrazed control transects. In moderate stands of snakeweed, cattle can be forced to graze snakeweed and reduce its density without adversely affecting the associated crested wheatgrass stand.