Spotted knapweed has invaded much of Montana. This is, perhaps, the top noxious weed in Montana.

   Some of those habitats have restrictions on the tools used for control. An experiment from the University of Idaho was designed to examine the effects of grazing spotted knapweed with goats. The study explored the utilization of spotted knapweed by goats, the effects on plant cover, plant counts, and seed head production. Three years of study determined that goats would consume spotted knapweed, reduce plant cover, plant density, and seed head production. An extensive education outreach program has assisted with adaptation and recognition of goat grazing as a viable tool for noxious weed control. 

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   Spotted knapweed (Centaurea biebersteinii) is a perennial, deeply tap rooted plant from Eurasia. Spotted knapweed plants can live up to 9 years and produce seeds each year (Sheley, Jacobs, & Carpinelli, 1999). Seeds are viable in the soil for more than 7 years (Davis, 1993). Spotted knapweed contains a compound that inhibits growth of other plants (Bais, 2003). Its reduced biomass through winter leaves much of the soil exposed and susceptible to erosion in the spring (Lacey, 1989).
 

Grazing has been utilized to manipulate the composition of forage on open rangelands. Grazing has been incorporated into the tool set for weed management and can reduce both biomass and seed production of targeted weeds. For example, sheep have been utilized to control many noxious weeds and goats have been utilized to control leafy spurge. (Olson, 1994; Lacey, 1992) In May of 2001 an experiment was established to determine if goats could be utilized to control spotted knapweed.

 

Methods of this study

 

Questions to be answered by the demonstration included:

1. At what stage of plant growth would grazing reduce spotted knapweed cover?

2. At what stage of plant maturity would grazing reduce the number of plants per square meter?

3. At what stage of plant growth would grazing decrease seed head production?

Four grazing treatments for spotted knapweed were compared in the experiment.

   Treatments included grazing spotted knapweed at the following times: (1) non-grazed control (2) rosette to bolt stage (3) bud to bloom stage (4) grazed twice - rosette to bolt stage and seed set to fall rosette stage. Treatments were replicated three times in plots that measured 405 m2 each. Thirty-six permanent measurement plots of 1.35 m² in size were established. Treatments within each plot were repeated each year. Individual plants were counted and percent canopy cover was measured in May of 2002, May of 2003 and May of 2004 after grazing treatments were completed the summer before. Seed heads were counted in September of 2001, 2002, and 2003. Each cell received the same treatment each year of the three year study. Five mature goats were utilized in each cell. Grazing time ranged from two to five days and goats were removed from a cell when consumption of spotted knapweed ceased and they started to consume other vegetation.

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Results

Goats readily consumed spotted knapweed, and plant maturity affected the grazing pattern of the goats. At the rosette to bolt stage, goats consume the plant from the top down, leaving the bottom 2 inches of the plant. At the bud to bloom stage, goats grazed the buds and blooms first, then stripped the leaves from the stems. When grazing at seed set to fall rosette stage, they grazed the rosette and green parts of the plant along with a few seed heads.

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Timing of grazing was significant in each year, and density and cover changed each year. Plant density was highest in 2003, with greater survival than in 2002 and 2004. There was no timing by year interaction, so all treatments changed in the same way each year. Over all years, density was lower for the bloom time grazing (0.607 plants m-2) than for control (0.847 plants m-2) and rosette timing (0.976 plants m-2); bloom timing was not different from grazing at rosette and bloom (0.788 plants m-2); and grazing at rosette and bloom did not differ from any other treatment. With respect to cover, bloom (19%) and rosette and bloom (19%) grazing reduced cover when compared to control (26%) and rosette timing (25%) 

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   Grazing at the bloom period was better than grazing at the rosette timing for reducing plant density. In fact, grazing at the rosette stage resulted in more spotted knapweed plants in 2004 than were found in the plots not grazed. No treatment consistently reduced spotted knapweed plant density when compared with not grazing.

   Spotted knapweed cover did decline for the bloom timing in both 2003 and 2004. Grazing at the rosette timing was not beneficial even when followed with a second grazing period. Plant competition is associated with plant cover, so reduction in plant cover should eventually result in increased grass yield. If grazing at the rosette timing is necessary, it should be followed with grazing at bloom in order to avoid increasing plant density.

Grazing at all stages of plant growth reduced seed head production. Grazing at the bud to bloom stage and grazing twice consistently reduced the number of seed heads.

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Application of Results

The knowledge and information gained from the goat grazing experiment encouraged land management agencies and private land owners to utilize goat grazing for controlling spotted knapweed in areas where there is sensitivity to weed control using other tools. Salmon City has utilized goats for 3 years on 17 acres at the city's water supply and settling ponds. The area had a 75% spotted knapweed infestation. Based on demonstration results, grazing was focused on the bud to bloom stage. In 2002, a herd of 250 does and kids grazed for 10 days. In 2003, the grazing days was reduced to 7 days, and, in 2004, grazing required only 4 days. The reduction in days indicates a reduction in plant biomass with repeated grazings.

 

   Goats can be used as a tool to control spotted knapweed. When grazing is applied at the bud to bloom stage over a series of 3 years, a reduction of seed head production and plant biomass can be achieved. When spotted knapweed is reduced, desirable plants have an opportunity to increase.

 

ABOUT THE GOAT GRAZING STUDY

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Principal Investigator: Bonnie Jensen

Lemhi County Extension Office

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SPECIFIC RESULTS
Based on the work at the University of Idaho and the U.S. Sheep Experiment Station at Dubois, Jensen and Williams, Lemhi County extension educator, designed a demonstration area to answer three questions: 1) At what stage of growth can spotted knapweed be grazed to have the greatest reduction in seed set? 2) A what stage of plant growth will grazing have the greatest reduction in canopy cover and plant count? 3) At what stage(s) of growth will goats consume spotted knapweed?

The demonstration, established on the University of Idaho’s Nancy M. Cummings Research & Extension Center in Carmen, Idaho, just north of Salmon, evaluated four treatments: 1) grazing the knapweed at the rosette to bolt state; 2) grazing at the bud to bloom stage; 3) grazing twice, once at the rosette to bolt stage and again at the fall rosette stage; 4) ungrazed control. Each treatment cell was replicated three times, each treatment being 0.9 acres with five goats per cell. The goats, restrained by electric netting, grazed until 80-90% of the knapweed had been grazed and they started grazing other species.

 

Results indicate that grazing at the bud-to-bloom stage has the greatest potential as a control tool. Grazing at the rosette-to-bolt stage does reduce seed count, plant count and canopy cover, but not at the levels of bud to bloom. Grazing twice reduces seed heads the most but results in increased plant count, perhaps because grazing disturbs the seed bank causing quicker germination or because the goats don’t eat the dry seed heads, instead knocking them to the ground.

 

In August 2002, the project team moved 250 goats to Salmon City Water Works, the city’s water source that is infested with spotted knapweed but cannot be sprayed with herbicides. In four days, the goats consumed all of the buds and blooms and striped the leaves from the knapweed. Six weeks later, fewer than 5% of the plants bloomed again. Further, the goats selected for the spotted knapweed, consuming minimal grass and sagebrush.

 

Leafy Spurge. The Lemhi County Cooperative Weed Management Area adopted grazing to control leafy spurge with the objectives of determining the best way to manage goats on leafy spurge and to see if the goats will utilize grasses and desirable forbs.

   On June 1, 2002, the project turned 250 goats into a 40-acre test area for four days. Six weeks later, the spurge had regrown but not flowered. In 2002, the Bureau of Land Management increased the area to be grazed, using two herds of 250 goats each. Spurge grazed in June had regrown to an average of 10 inches, and spurge grazed in July had regrown only six inches.

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In 2001, the kids and does grazed together, with the kids tackling the spurge at eight weeks of age. However, as the distance to the spurge increased, both kids and does lost body condition. In 2002, the kids were weaned and returned to the ranch July 1, allowing the does to travel farther without losing body condition. Areas grazed the second year required one-third the time with the same number of goats, because the old growth had been removed and only young plants remained.

The goats don’t need water when grazing young leafy spurge, which has ample moisture, but they do need water at night.

 

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