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

2006 Annual 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.

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


In Umatilla County, OR, a shift in agricultural practices, from traditional tillage to no-till or reduced-tillage systems, has altered the weed spectrum, making weed control more difficult. A large-scale demonstration of integrated pest management (IPM) using sheep as biological control agents was conducted on 330 acres north of Pendleton, OR. The demonstration presents benefits for biological weed control using fewer herbicide applications in direct-seed fallow.

Seven hundred and seventy-eight sheep (ewes and lambs) grazed the 330-acre direct-seed fallow wheat stubble field for 40 days. As a result of grazing, the broadleaf weeds were controlled at 95% compared with control plots (ungrazed areas), while grassy weeds were controlled by about 10% compared with control plots. Soil moisture and infiltration rates were not significantly impacted by the presence of sheep and grazing pressure.

• Spark an interest among agricultural producers to utilize integrated pest management programs on no-till or reduced-tillage fields as opposed to costly chemical applications to control weeds.
• Provide science-based information regarding soil moisture, soil compaction (illustrated by infiltration rate) and the number of weeds present in the grazed and non-grazed areas of the field
• Illustrate the weed control benefit of IPM programs as opposed to costly chemicals when shifting tillage practices from traditional to no-till systems.

The research design consisted of a grid of six plots, each representing a 1-meter-diameter area. Placement of control plots (ungrazed) and experimental plots were selected at random with a flip of the coin. Of the six plots, three were control and three were experimental. Tests were conducted to establish whether 1) weed control, 2) soil moisture and 3) soil infiltration rates were impacted by the large-scale demonstration project. Pre- and post-treatment tests were conducted for each of the three measures.

For the weed control tests, sheep were fenced out the control plots with hog panels shaped into 1-meter-diameter circles and secured with hog rings and rebar stakes. Weeds were measured numerically in each plot.

For soil moisture, soil samples were collected, using a soil probe, at depths of 0-6 inches, 6-12 inches and at 1-foot intervals below that to 6 feet. Samples were weighed, dried and weighed again with the net weight subtracted from the gross weight to determine soil moisture content.

Soil infiltration rates were tested using a ring infiltration test with 1-foot-diameter cylinders driven 6 inches into the soil.

After analyzing the data and test results, it was concluded that, compared with the control plots, grazing significantly reduced the broadleaf population, 95%, while grassy weeds were controlled by 10%. Soil moisture and soil infiltration rates were not significantly impacted by the sheep and grazing pressure.

The observed fluctuation in the soil moisture and infiltration rates may have been caused by a dry summer and fall, reflected in the October 2005 data, and an exceptionally wet winter and spring, observed in the May 2006 data. The May 2006 data may have been skewed by the soil retaining a maximum level of moisture during the winter and spring, thus saturating the soil and reducing the overall ability of the soil to absorb, decreasing the soil infiltration rate.

The “Sheep vs. Weeds” project demonstrated a win-win solution for crop producers seeking low-cost direct-seed fallow and for livestock producers seeking alternative forages. Grazing direct-seed fallow broke down crop residue, controlled weeds and reduced chemical inputs. The ensuing symbiotic relationship benefiting the two entities in rural communities helps to create a self-sustaining environmental and economic relationship within society: soil can be preserved through direct seeding; water quality can be preserved and improved because of less erosion and reduced herbicide and pesticide inputs; and the productive capacities of both entities can be enhanced.

Hill Ranches, which provided the original demonstration ground for the Western SARE grant, has worked to adopt the use of sheep to graze summer weeds and noxious weeds, and had implemented the practice on 4,500 acres by fall 2006, a nearly 90% adoption rate over two years. Hill Ranches, an innovative farming operation that led the way in 2000 in conversion to no-till from conventional tillage, anticipates full IPM adoption of grazing sheep on direct-seed fallow in the near term. Gillespie Grazing Co. is also using the practice of grazing sheep to manage weeds.

In a survey of participants in a Demonstration Day Event, held in conjunction with the 2006 Umatilla County Weed and Crop Tour, 80% of the respondents had heard of the “Sheep vs. Weeds” demonstration before the tour, and 60% farm under direct-seed systems and said they would consider using an integrated pest management system like grazing to control weeds. Overall, the participants had a favorable reaction to the demonstration – several said they were interested in pursuing grazing, while others expressed concern over potential additional costs.

Those considering examining similar issues or conducting large-scale demonstrations may want to consider 1) economics, 2) additional tests and 3) grazing distribution, or piospheres.

Economics: Based on observation and the survey comments, the economic perspective of such large-scale demonstrations must be pursued to achieve widespread adoption. Dollar-to-dollar comparisons must be made with traditional fallow, direct seed or chemical fallow and grazing with sheep or cattle. If such a demonstration were to be replicated, investigators must keep in mind that grazing occurs sporadically as weeds appear, which reduces the number of days on a field and increases the number of fields needed for forage. The length of time on a particular field depends on the previous crop and crop yields. These variables fluctuate in eastern Oregon, and other areas, with rainfall gradients, soil types and farming history.

Additional tests: The conclusions reached in the soil infiltration rate tests were inconclusive, with no significant differences between grazed and ungrazed locations. The data revealed may have been because of too few data points. If this demonstration were to be repeated, it is recommended that a greater number of plots be established over a larger land area.

Piospheres: Gradients of animal impacts, known as biospheres, tend to develop around artificial watering points. These watering points and the trails that lead to them should be examined in the future.

The project and its results were communicated through several publications and as part of the 2006 Umatilla County Weed and Crop Tour, ranging from Oregon State Extension advertisements, local newspapers and the Umatilla County Soil and Water Conservation District newsletter.


Jaime Clarke
Watershed Technical Specialist
Umatilla County SWCD
Pendleton, OR
Office Phone: 5412768170