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

Modern weed management strategies continue to be focused on controlling weed seedlings, most often using herbicides or cultivation. This focus on early season weed control effectively minimizes yield loss due to weeds, but weed seed rain from individuals that survive may be extraordinary, resulting in a recurring weed problem. Furthermore, growers are frequently encouraged to plant fall cover crops, an operation that usually involves tillage and, consequently, burial of the new seed rain. Burial effectively places the weed seed in “cold storage,” offering protection from predators, and fewer environmental cues to encourage germination, a fatal process when it occurs late in the year.

The proposed project will use our current knowledge of weed seedbank ecology to evaluate the efficacy and economics of fall weed management tactics designed to maximize weed seed mortality. Specifically, we will engage ten Maine farmers in related on-farm trials focused on fall weed management strategies including: preemption of seed rain by hand hoeing/pulling, tillage, or mowing followed by fall cover cropping; managing seed rain by flail mowing for maximal presentation to post-dispersal seed predators (exclosures will demonstrate the contribution of various weed seed predator guilds); and no-till seeding of fall cover crops to maintain weed seed on the soil surface where they are exposed to weed seed predators. A replicated comparison of these treatments will be conducted at the University of Maine Rogers Farm. Case studies detailing the on-farm fall weed management trials will demonstrate to farmers throughout the region the longer-term benefits of weed control strategies that focus on the seedbank instead of solely on early season competition.

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

1. Ten grower participants are selected based on individual meetings with 15 or more candidate growers.

2. On-farm tactics are selected by growers and researchers for implementation in the fall of two consecutive field seasons; tactics are implemented and maintained by project team.

3. Field days and talks at winter meetings present tactics to target audience.

4. Experiences are summarized in case studies to be published jointly by UM Cooperative Extension and the Maine Organic Farmers and Gardeners Association.

5. Surveys are conducted to document outcome. 


Each of the on-farm sites in 2006 and 2007 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.

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.

Impacts and Contributions/Outcomes

All of the 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 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.

This was the first year of data collection in this trial and preliminary results from the on-farm plots indicate a variable response to the treatments. As to be expected, each site varied considerably in the weed species present, their overall densities and the timing of establishment of the treatments.

When looking at the main treatment comparison of tilling in the fall, thus burying weed seeds versus flail mowing and not doing any tillage, three of the on-farm sites did see a reduction in the germinable portion of the seedbank, however only one of the sites had a reduction that was dramatic enough to be somewhat conclusive. Two sites ended up having a higher seedbank in the tilled treatment and at one site the two treatments were very similar.

The samples collected from within and next to the seed predator exclosures also varied considerably between sites. Two sites did have dramatically lower seedbank numbers outside of the caged areas, perhaps indicating seed predator activity, however at two other sites the opposite was true with more seeds found outside of the caged area. Because of these variable results the number of seed predator exclosures at each site were increased from two to ten in 2007.

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

On average, over all of the treatments, hairy galinsoga, Galinsoga ciliata, and smooth crabgrass, Digitaria ischaemum dominated the germinable portion of the seedbank comprising 41 and 43 percent of the total number of germinable seeds respectively. Common lambsquarters, Chenopodium album was the third most common weed found but when averaged over all of the treatments it comprised only 5 percent of the seedbank. When looking at these three most abundant species and the total number of all species combined there were no detectable differences found between the four treatments for the germinable portion of the seedbank.
Sampling of weed densities occurred three times in the plots during the growing season, once before primary tillage and before each of the two weed control cultivation events. At time of sampling before primary tillage, the tilled/cover crop treatment had significantly more total number of weed seedlings compared to the other three treatments. Annual grasses (most likely smooth crabgrass) and common chickweed, Stellaria media were the most abundant weeds at this sampling date and were also found in significantly higher numbers in the Tilled/cover crop treatment. At the sampling before the first weed control cultivation event, when the corn was in the 2 to 3 leaf stage, the no-seed rain treatment, not surprisingly was found to have significantly fewer total number of weed seedlings then the other three treatments, however hairy galinsoga, the most abundant weed at this sampling date was found in significantly higher numbers in the Tilled/cover crop treatment. Annual grasses were the second most common weed at this sampling and were found at similar levels in all of the treatments. At the second weed control cultivation event, when the corn was in the 6 to 8 leaf stage, the No-seed rain treatment had significantly lower total number of weeds then the other three treatments. The most abundant weed species were again, hairy galinsoga and annual grasses combined. Hairy galinsoga was more abundant in the tilled/cover crop and the flail/no-till/no cover crop treatments and the annual grasses were lower in the no-seed rain treatment.

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