Sheep vs. Weeds: Biological Control Agents to Combat Noxious Weeds

Final Report for FW05-004

Project Type: Farmer/Rancher
Funds awarded in 2005: $4,570.00
Projected End Date: 12/31/2007
Region: Western
State: Oregon
Principal Investigator:
Cameron Gillespie
Gillespie Grazing Co.
Sabrina Gillespie
Gillespie Grazing Co.
Expand All

Project Information



Umatilla County has a cropland weed-control problem due, in part, to a shift in agricultural practices and the conversion from traditional tillage systems to no-till or reduced tillage systems. This has significantly altered the weed spectrum, which is becoming increasingly difficult to control. We conducted a large-scale demonstration of integrated pest management (IMP) using sheep as biological control agents on 330 acres north of Pendleton, Oregon. The project began in October 2005 and concluded in May 2006. We had 778 sheep (ewes and lambs) graze the 330 direct seed fallow wheat stubble field for 40 days. Because of grazing, the broadleaf weeds were controlled at 95%, compared to the control plots (non-grazed), while grassy weeds were controlled by approximately 10% compared to the control plots. Soil moisture and soil infiltration rates were not significantly impacted by the presence of sheep and grazing pressure.


1) Spark an interest in agricultural producers to use IPM programs on no-till or reduced tillage fields as opposed to costly chemical applications to control weeds.
2) Provide science-based information regarding soil moisture, soil compaction and the number of weeds present in the grazed and the non-grazed areas of the field.
3) Illustrate the weed control benefit of IPM programs as opposed to costly chemicals when shifting tillage practices from traditional to no-till systems.


On the 330 acres, we established a grid of six 1-meter-diameter plots, consisting of three control plots and three experimental plots. We conducted several tests to establish whether weed control, soil moisture and soil infiltration rates were impacted by our project, including pre-treatment and post-treatment tests for each of the measures.

Regarding weed control, sheep were fenced out of the control or non-grazed plots with hog panels shaped into a 1-meter-diameter circle and secured with hog rings and rebar stakes. We measured weeds numerically in each plot using a 3-square-foot area, within an agronomic hoop. No weeds were present in October 2005 when we conducted the pre-treatment test, but weeds were present in May 2006 during the post-treatment test. We found no noxious weeds on the field, though summer weeds such as kochia, Russian thistle, prickly lettuce, Canada thistle, Scotch thistle and yellow starthistle may be present in the control plots later in the summer as we found a larger proportion of weeds in the non-grazed areas.

To test moisture, we collected soil samples using a soil probe. We collected samples at a 0-6 inch depth, 6-12 inch depth, and at 1-foot intervals following the first foot to a depth of 6 feet. We weighed the samples, dried them, and weighed them again to determine the soil moisture content; we subtracted the net weight from the gross sample weight. We conducted soil moisture tests on both the experimental and control plots in October 2005 and May 2006 and found that soil moisture directly affects potential crop production and, ultimately, crop yields in semi-arid eastern Oregon.

We conducted soil infiltration rates using a ring infiltration test. We drove 1-foot-diameter cylinders 6 inches into the soil. A calibration mechanism, a float and a pressurized tubing system were used in each ring to supply a method of adding and measuring water to the ring. We flooded the rings for one hour then recorded measurements from the pressurized tube system on three 5-minute intervals and four 15-minute intervals. We recorded the measurements in centimeters. We conducted soil infiltration rates on both the control and experimental plots in October 2005 and May 2006.

After analyzing the data and test results, we conclude that grazing significantly reduced the broadleaf weed population by 95%, while controlling grassy weeds by approximately 10% compared to the control plots. Grazing pressure did not significantly impact soil moisture and soil infiltration rates. The observed fluctuation in the soil moisture and soil infiltration rates may be due in part to a dry summer and fall in October 2005 and an exceptionally wet winter and spring in May 2006.


Potential benefits or impacts on agriculture revolve around the reduction of herbicides on direct seed fallow fields. Our project illustrated a win-win solution for agricultural producers interested in pursuing low-cost direct seed fallow and livestock producers who are seeking alternative forage types.

The use of IPM positively affected overall farm production levels. Grazing direct seed fallow allowed for the breakdown of crop residue, control of weeds and reduced costly chemical inputs. Symbiotic relationships such as this benefits two entities in rural communities, creating a self-sustaining environmental and economical relationship within society.


Hill Ranchers have worked to adopt the use of sheep to graze summer weeds and noxious weeds on a large portion of their property. They implemented grazing on approximately 4,500 acres by the fall of 2006. This represents a nearly 90% adoption rate over a two-year period, originating from the demonstration area provided by this project. Hill Ranches anticipate the full adoption of sheep grazing on direct seed fallow in the near future.


We assessed reactions from agricultural producers following the Demonstration Day Event, which was held in conjunction with the 2006 Umatilla County Weed and Crop Tour. Of the surveys handed out, 32% were completed. Nearly 80% of survey participants had heard of the large-scale demonstration prior to the tour; 60% farm under direct seed systems and would consider using an IPM system such as grazing to control weeds.

Overall, the reaction from agricultural crop producers attending the tour was favorable, with several tour participants interested in pursuing grazing systems on their direct seed fallow. Others were concerned about additional costs. The division between the two groups appears to be based on tillage practices. Those individuals who have adopted direct seed or reduced tillage systems were more interested in the possibility of grazing their fallow fields, which was the target audience of this project.


For others examining similar issues and conducting large-scale demonstration projects, we recommend focusing on the following three points: 1) economics, 2) additional tests and 3) piospheres.

After observing Demonstration Day participants and reviewing survey comments, the economic conditions of projects like ours must be pursued to have widespread adoption. Definite dollar-to-dollar comparison with traditional fallow, direct seed or chemical fallow and gazing livestock must be produced.

The conclusion reached by the soil-infiltration-rate test was inconclusive. There were no significant differences between grazed and non-grazed locations; however, the information revealed in the test results may have resulted from too few data points. If the demonstration project were repeated in eastern Oregon or elsewhere, we recommend a greater number of plots be established over a larger area of land.

Gradients of animal impact known as piospheres tend to develop around artificial watering points. One could examine the impacts of watering point provision on weed control and soil compaction with a focus on the trails the sheep create and use to access the watering points.


1) Morrow County OSU Extension Service. “Umatilla County Weed and Crop Tour.” Ag News. May 2006: 1-2.
2) Corp, Mary K. “All Aboard . . . 2006 Umatilla County Weed & Crop Tour.” Cereal Newsletter, OSU Extension Service, April 2006: 3-4.
3) Corp, Mary K. “Umatilla County Weed and Crop Tour.” OSU/Umatilla County Extension Service. May 2006: 1-4.
4) Corp, Mary K. “All Aboard . . . 2006 Umatilla County Weed & Crop Tour, May 25th.” Pendleton Business, May 2006, vol. 12, no. 5.
5) Clarke, Jaime L. “Sheep vs. Weed.” e-Notes, Umatilla County Soil and Water Conservation District. June 2006: 3.
6) Clarke, Jaime L. “Sheep vs. Weeds: Biological Weed Control Agents Combat Noxious Weeds,” 2006 Umatilla County Weed and Crop Tour handout, produced by the Umatilla County Soil and Water Conservation District, May 2006.
7) 2006 Umatilla County Weed and Crop Tour, May 25th, 2006.


Click linked name(s) to expand/collapse or show everyone's info
  • Jaime Clarke


Participation Summary

Research Outcomes

No research outcomes
Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.