Does Timing of Defoliation Affect Spotted Knapweed Seed Viability and Germination?

Does Timing of Defoliation Affect Spotted Knapweed Seed Viability and Germination?

Summary

Spotted knapweed, an aggressive competitor and ecologic threat, produces 25-35 flowers/head, 60 heads/plant, and 5,000-40,000 seeds/meter squared annually. Prescribed sheep grazing is a tool used to control spotted knapweed that offsets high costs of control and environmental concerns surrounding herbicides. However, spotted knapweed forms new flowers after prescribed sheep grazing is applied during the bolting or flowering stage. It is unknown if these new flowers produce viable seeds by the end of the growing season. The purpose of this 2-year study was to determine the appropriate timing(s) or combination(s) of timings of defoliation on spotted knapweed to reduce its viable seed production.

Objectives/Performance Targets

Objective 1: Evaluate the effects of timing of defoliation of spotted knapweed on: a) the number of buds/flowers present, b) the percent viability of filled seeds, and c) the total number of viable seeds produced to determine the best timing of defoliation of spotted knapweed to minimize viable seed incorporation into the seedbank annually.

Objective 2:  Combine results of this study with past research and existing grazing prescription protocols to refine the technique of using sheep grazing to control spotted knapweed.

Objective 3:  Educate ranchers, other landowners, livestock operators, county Extension personnel, county weed district personnel, state and federal agency personnel, Montana State University researchers, and concerned citizens about how properly timed sheep grazing to control spotted knapweed can reduce the amount of viable seed applied to the seedbank in a single growing season and can enhance the ecological integrity of the land.

Accomplishments/Milestones

Year 1 Accomplishments:

Objective 1:  Clipping treatments were applied to spotted knapweed plants in June, July, and August and first-year data were successfully collected by mid-fall. Laboratory analyses of the spotted knapweed seeds was also successfully completed by winter. During the laboratory analysis process, we made a decision to eliminate the germination test on the spotted knapweed seeds. The reasons for this were two-fold. First, in many treatments, plants did not produce enough seeds to complete a sound test for both viability and germination, and secondly, from previous experience we know that there is an extremely high potential for seeds to mold during the germination process, which adversely affects germination. For these reasons, we restricted our laboratory analyses to spotted knapweed seed viability. Data for Year 1 were analyzed for presentation of preliminary results.

Objective 2:  We have been working with Montana livestock owners and landowners with spotted knapweed infestations, as well as the Montana Sheep Institute, to apply what we have learned in Year 1 to grazing management situations on the landscape.  

Objective 3:  Preliminary results of this research have been presented to a variety of groups. Results were presented to the Multi-Species Grazing Roundtable and Research Group on two occasions (fall, spring), resulting in the information being conveyed to approximately 40 participants. Results were also presented in poster format at the international Society for Range Management annual meeting in Reno, NV, to approximately 75 participants at a Weed Management Field Day hosted by the Blackfoot Challenge, and to 35 participants at the 5th Annual Joe Skeen Institute for Rangeland Restoration Annual Meeting and Field Tour.

Year 2 Accomplishments:

Objective 1:  Clipping treatments for Year 2 were applied to spotted knapweed plants in June, July, and August and second-year data were collected by mid-fall. Laboratory analyses for Year 2 was also completed by early winter. Data analysis and synthesis of combined data from Years 1 and 2 is in progress.

Objective 2:  Information gleaned from this project in collaboration with the Multi-Species Grazing Roundtable and Research Group has been incorporated into a Montana State University bulletin. This bulletin is a defined livestock grazing prescription for controlling spotted knapweed that clearly describes the appropriate timing, intensity, frequency, and duration of grazing, as well as appropriate stocking rates and monitoring indicators for sustainable grazing. This publication is currently in draft form and is expected to be published in 2009. Additionally, we have continued to work with livestock producers and landowners who are utilizing prescribed livestock grazing or mowing to manage spotted knapweed. We have also continued to collaborate with the Montana Sheep Institute (MSI) to provide results for use on MSI projects.

Objective 3:  Final results will be presented where opportunities arise to share important findings of this research. Results have been presented at the international Society for Range Management Annual meeting in Louisvile, KY, at a fall meeting of the Multi-Species Grazing Roundtable and Research Group, at the Montana Weed Control Association annual meeting, and at Joe Skeen Institute for Rangeland Restoration field day. Additional presentations of results will occur when the opportunity arises. Additionally, we will publish a Master’s thesis, a popular press article, and a refereed journal manuscript in Rangeland Ecology and Management.

Impacts and Contributions/Outcomes

First-year results indicate that the number of buds/flowers per plant, the number of seeds per plant, and the total number of viable seeds per plant can be reduced by defoliation in summer. Clipping in June, July, or June+July reduced the number of buds by 72% and clipping in August, June+July, June+August, July+August, or June+July+August reduced the number of buds by 92% compared with no clipping. Clipping in June reduced the number of seeds per plant by 75%, and clipping at all other times or combinations of timings reduced the total number of seeds by 98% compared with no clipping. Clipping at any time or combination of timings reduced the total number of viable seeds by 98% compared with no clipping. Spotted knapweed plants appeared to have responded similarly to defoliation in Year 2 as they did in Year 1, which is very encouraging. However, Year 2 was a much hotter, drier year, which may have some impact on results.

Defining the most effective timing of defoliation of spotted knapweed to reduce viable seed production is critical for suppressing spotted knapweed. Livestock owners and landowners who are using applied prescribed sheep grazing to control spotted knapweed can benefit greatly from knowing when the most effective timing of grazing is, so they can fine-tune their grazing strategies to most effectively treat their spotted knapweed-infested rangeland. Preliminary results suggest that because spotted knapweed is a short-lived perennial that reproduces solely by seed, prescribed livestock grazing in summer should effectively suppress this weed, especially when seedheads are removed by grazing in July or August.

Collaborators: