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 extremely 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 with herbicides and environmental concerns surrounding herbicides. However, research focused on specific details of grazing prescription protocols for spotted knapweed control at a landscape scale is in its infancy. The purpose of this research is to determine the ideal timing(s) or combination(s) of timings of defoliation on spotted knapweed to maximally reduce flowerhead and viable seed production, and minimize seed incorporation into the seedbank.

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 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 can adversely affect 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 Goals:

Objective 1: Treatments will be applied to spotted knapweed plants in June, July, and August of Year 2 and data will be collected by mid fall. Laboratory analyses will be completed by early winter and data analyses will be completed by mid-winter.

Objective 2: We will continue to work with Montana livestock owners using sheep to graze spotted knapweed and landowners with spotted knapweed infestations, as well as with the Montana Sheep Institute, to implement effective prescribed grazing strategies to control spotted knapweed and to minimize spotted knapweed seed input into the seedbank annually.

Objective 3: Final results will be presented where opportunities arise both locally and regionally to share important findings of this research. Results will be 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, and at a field day held in conjunction with the Powell County Weed District and/or the Blackfoot Challenge.

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.

This information is very important for livestock owners and landowners who are using applied prescribed sheep grazing to control spotted knapweed. 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: