Managing weed seed rain: A new paradigm for organic and low-input farmers

Managing weed seed rain: A new paradigm for organic and low-input farmers

Summary

Intensified sampling efforts overcame within-field heterogeneity which had possibly obscured detection of the effects of weed seed predators. In both on-farm, and in our on-station replicated experiment, the effects of weed seed predators on the subsequent year germinable weed seedbank was dramatic, averaging nearly a 40% reduction over the fall, winter, and early spring period of predation. The fall seedbank management practices, however, generally resulted in similar levels of predation, despite our expectation that no-till cover cropping would result in greater levels of predation and thus a lower spring germinable seedbank. The “zero-seed” treatment consistently resulted in the lowest seebank levels, as expected. Notable, however, is the fact that a single season of seed rain prevention resulted in large reductions in the germinable seedbanks of both long-lived (e.g., common lambsquarters) and short-lived seeds (e.g., hairy galinsoga and annual grasses), suggesting that the benefits of such a practice may be realized within a shorter time frame that is generally expected.

Objectives/Performance Targets

Through field days, meeting presentations, and published case studies, 150 New England vegetable and organic dairy farmers will learn about the Fall Weed Management Project; direct-mail and follow up telephone surveys will demonstrate that one third of this target audience implemented a new strategy focused on fall weed management, with 10% of the group adopting multiple tactics to preempt seed rain and maximize weed seed predation and mortality.

Evidence Performance Targets Have Been Reached

1. Attendance at on-farm site field days and grower meetings will reach an audience of over 300 mixed vegetable growers.

2. Surveys of growers in cooperation with the Maine Organic Farmers and Gardeners Association (MOFGA) and the Northeast Organic Farming Association (NOFA) will indicate the number of growers implementing strategies for weed seed rain management, and the source of their information.

3. Requests for information and on-farm visits will be used as an indicator that performance targets are being met.

4. Lastly, we will anonymously review MOFGA applications for organic certification (for growers who agree to participate in the survey) from a one- to three-year period prior to implementation of our project, and applications submitted in the fall and winter of 2008, recording the proportion of applicants implementing weed seed rain management practices.

Accomplishments/Milestones

Demonstrations were conducted at three on-farm sites in 2007 and 2008, and each had at least 3 large, single-block treatments, a weed free area, a flail mowed no tillage area and a flail mowed tilled area. When possible, fall cover crops were planted at the sites using a no-till drill. Weed seed predator exclosures, which are cages made of fine mesh screen were put out in the flail mowed no tilled treatment. To estimate the germinable weed seedbank, soil cores were collected on 14 May 2008 at Goranson, Peacemeal, and Rainbow Valley farms (the former, organic diversified vegetable farms; the latter an organic diary).

The weed-free control plots had a particularly dramatic impact on the seedbank at the Peacemeal Farm, and on the participating grower as well. Total germinable weed seeds were 6,000 per square meter (10 cm deep) in weed-free plots, compared to 21,000 per square meter in the bulk area of the field. This large effect of “complete weed control” was evident in both hairy galinsoga, which has a relatively short-lived seedbank (1,500 vs. 9,000 germinable seeds per sq. m, weed-free vs. weedy, respectively), and common lambsquarters, which is known to have a high degree of complex seed dormancy (600 vs. 4,700 germinable seeds per sq. m, weed-free vs. weedy, respectively).

In addition to the on-farm sites, replicated trials were established at the University of Maine’s Rogers Farm in the fall of 2006 and repeated in 2007. The four treatments in each of the trials were; standard fall tillage with cover crop (tilled/cover crop), No-seed rain, No-till planting with cover crop (no-till/cover crop) and Flail mow with no tillage and no cover crop (flail/no-till/no cover crop). The establishment crop for the 2006 phase was spring triticale and the test crop, grown in 2007 was sweet corn. The establishment crop for the 2007 phase was winter squash and the test crop to be grown in 2008 will again be sweet corn.

There has been an increasing interest in the topic of weed seedbank management at local and regional grower meetings. The new datasets from Managing Weed Seed Rain were featured at three invited presentations during the 2008 calendar year:

• Managing the weed seedbank with many little hammers. Growing Organic Grains, University of Vermont Cooperative Extension Spring Grain Growers’ Meeting, Berlin, VT (April 3, 2008).
• Innovative strategies for managing weeds in cereals. Growing Organic Grains, University of Vermont Cooperative Extension Spring Grain Growers’ Meeting, Berlin, VT (April 3, 2008).
• A multiple stress approach to managing the weed seedbank. Northeast Organic Farming Association-New York 26th Annual Winter Farming & Gardening Conference, Saratoga Springs, NY (January 25, 2008).

Impacts and Contributions/Outcomes

The Rogers Farm and on-farm sites were visited before primary tillage to collect soil samples to measure the effect of the treatments on the germinable portion of the weed seed bank. Ten soil cores (5 at selected on-farm sites) using probes that were 6.5 cm in diameter were collected from each of the large treatment blocks. The seed predator exclosure areas were sampled separately with ten cores collected from within the caged area and a corresponding 10 core sample collected immediately outside the caged area. The samples were placed in greenhouse flats and over the course of the summer germinating weeds were identified and counted.

Data collection from the Rogers Farm replicated trial included both soil seedbank work and infield weed density counts prior to tillage.

Evidence from both on-farm sites, and the replicated on-station support our original hypothesis that seed predators may contribute to significant fall/overwinter/spring seed removal. Using an increased number of predation exclosure in the 2008 field season (compared to prior years), we no change in the spring germinable seedbank at one on-farm location (an average of 5,000 germinable weed seeds within predation exclosures, and 5,000 germinable seeds outside the exclosures). However, at two other on-farm sites, and at the the replicated on-station trial, predator exclosures resulted in an average of 60% more germinable weed seeds, suggesting that predators were responsible for a 40% reduction in the seedbank. For example, at the Goranson Farm, the germinable weed seedbank averaged 10,000 seeds within exclosures, but 6,000 outside the exclosures. In the replicated trial the germinable weed seedbank averaged 62,000 seeds within exclosures, but 36,000 outside the exclosures.

Inexplicably, however, we failed to detect a significant difference in the germinable seedbank between fall no-till and tilled cover crop treatments. For example, the germinable seedbank of common lambsquarters, measured in the spring following either no-till or tilled cover cropping, was 11,000 and 13,000 germinable seeds per square meter, respectively. The impact of the weed-free control was dramatic: 600 germinable common lambsquarters seeds per square meter. While it may not be surprising that eliminating weed seed rain causes a reduction in the germinable seedbank, the magnitude of this single-season effect is noteworthy. It demonstrates that, even for a species with complex seed dormancy such as common lambsquarters, benefits of seebank management practices may be realized within a short time frame.

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