NE1047: Ecological Bases for Weed Management in Sustainable Cropping Systems
Statement of Issues and Justification
The need as indicated by stakeholdersRecent surveys, farmer focus groups, discussions at grower meeting, and individual interactions with farmers indicate that weeds are the most critical management problem facing organic or sustainable farmers who limit their use of herbicides. Weeds reduce crop yields and quality, increase harvest difficulty, and add to the seed bank contributing to future management problems. Our previous research developed cover crop systems, cultivation methods, and natural product herbicides to manage weeds early during the growing season. Sustainable weed management, however, is based on the integration of multiple tactics that attack weed populations throughout the weed life cycle. This requires an understanding of how ecological processes interact with control measures at various life stages. First, cultivation is a major method of weed management in organic and reduced herbicide systems, but little is known about how soil texture, tilth, and moisture affect the efficacy and selectivity of cultivation implements. Substantial gains in weed control are possible through better matching of the type and timing of implement use to soil conditions. Second, some weeds are missed by cultivation, or are resistant to herbicides or emerge through gaps in the crop canopy. Controlling these weeds by rogueing or post-harvest operations can greatly affect seed input to the weed seed bank. Weed seeds sometimes continue to develop on plants that have been cut, pulled or treated with herbicide, but information is sorely lacking on how the method of termination and the developmental stage at death affect the number of viable seeds that various weed species ultimately produce. We regularly get questions from growers about the potential for seed production on weeds that have been killed but not removed from the field. Third, once weed seeds have entered the seed bank, they may die, produce emerged seedlings, or persist in the soil and germinate in subsequent years. Information is available on how tillage and seed position in the soil affect the relative probabilities of these three outcomes. Much less is known, however, about how organic matter sources like cover crops and compost affect seed survival and seedling emergence. Some types of organic matter inputs may reduce seed persistence by promoting germination in circumstances where emergence is unlikely or by increasing microbial populations that cause seed decay. Reducing the persistence of weed seeds would reduce weed infestations in subsequent crops. Organic farmers in particular are interested in multi-tactic weed management strategies that involve attack on several stages of the weed life cycle. With the growing prevalence of herbicide resistant weeds, conventional growers too are realizing the usefulness of integrating a variety of practices into their weed management programs. The applied research proposed here will provide a knowledge base for improving several important management tactics.
The importance of the work, and what the consequences are if it is not done
Multiple weed management strategies directed at seeds and weed seedlings improve weed control in organic and sustainable systems as compared to a single tactic, thereby improving food production, quality and net return to farmers.
Objective 1. Determine how soil conditions affect efficacy and selectivity of cultivation implements for the control of various weed species.
Physical weed control implements kill only a proportion of weed seedlings. Improving the mean efficacy and reducing within field variability in efficacy should be possible by choosing implements and making adjustments that are most appropriate for soil conditions in a field. This requires an understanding of how site conditions affect the efficacy of particular implements against various weed species. Highly experienced operators are often able to achieve reasonable performance of their cultivators. But in the absence of the information provided by the proposed work, they may have no way of knowing that a delay in cultivation to reduce soil moisture, a change in setting based on moisture conditions, or a different cultivator may improve weed control. Moreover, the recent increase in the number of organic growers and the growing need for cultivation to control herbicide resistant weeds means that many growers are relatively new to cultivation. They could be helped by the availability of information on how to choose implements and settings and time cultivation based on their soil conditions.
Objective 2. Determine the reproductive growth stage at which summer annual weeds can be terminated and still produce viable seeds and quantify the effect of method of life-termination on seed production.
At present, the point in plant development at which seeds become physiologically independent of the parent plant is known for only a few weed species. Consequently, growers cannot assess whether the weeds are about to produce viable seeds, and hence cannot assess the urgency of control measures. Moreover, knowledge about how the method of killing a weed influences seed production is meager. Can a plant that is merely in flower produce seeds if it is cut or uprooted or sprayed with a slow acting herbicide like glyphosate? Understanding how production of viable seeds relates to the point in the life-cycle at which a weed is killed and the method by which it is killed will improve preventive weed management. The findings of this effort will directly benefit agricultural producers who rely on hand labor, mowing, or glyphosate applications for late-season removal of summer annual weeds. If weeds continue to produce viable weed seeds after being mowed, pulled, or following a herbicide application, growers will need to know how early in the life history escaped weeds must be removed to prevent viable weed seed from forming and entering the soil seed bank. If this work is not done, growers will continue to allow weed seed set, even when they are trying to avoid this.
Objective 3. Determine the extent to which soil amendments such as green manures and compost affect seed mortality of various weed species.
At present, the principal method growers have for decreasing the weed seed bank is tilled fallow in which the soil is disturbed and left bare. Weeds are stimulated to germinate by the soil disturbance and then killed by further tillage or by herbicides. Improved understanding of how organic matter amendments, including green manures and compost, affect seed survival following initial seed rain will allow growers to implement cropping systems that reduce weed seed banks and thus weed pressure in subsequent crops. Without this work, growers will either have to suffer the consequences of high density seed banks or risk the erosion potential of bare fallows.
The technical feasibility of the research
Objective 1. Pilot studies at Maine and New York have examined effects of various measures of soil moisture and soil tilth on the performance of cultivators. The development of hand held soil moisture meters allows rapid objective measurements of soil moisture that will be correlated with visual and tactile information that could be obtained by a farmer. Soil texture can be read from county soil surveys and, for research, measured more precisely by the hydrometer method. Members of the research group are experimenting with photographic and dry screening methods of measuring soil roughness. Cultivation efficacy will be assessed by weed counts before and after cultivation.
Objective 2. A preliminary study in Michigan examined the reproductive growth stage at which three summer annual weeds produced viable seeds. Termination methods that farmers use include pulling, cutting, chopping, and applying herbicides. Grass and broadleaf weeds will be terminated by these methods during flowering and seed development and the plants confined in fine mesh to retain any seeds that subsequently mature. In late autumn, seeds will be removed from the plants, counted and tested for viability to determine how mode of death and phenology at termination affect seed production. Some seeds will be returned to the soil and tested again in the spring to determine whether these factors affect the short term persistence of seeds in the soil.
Objective 3. Several of the investigators have previous experience with buried seed experiments. Seeds will be buried in soil in mesh packets in the fall and exhumed annually. At each exhumation, the soil will be stirred and returned to the packet, or an organic matter amendment (e.g., chopped clover, compost) mixed into the soil and then returned to the packet. Packets will be buried in plots receiving the same type and rate of the soil amendment. A subsample of the packets will be assayed for surviving seeds at each exhumation and the seed mortality trajectories of various treatments compared.
The advantages for doing the work as a multistate effort
Objectives 1-3 are all better accomplished through a Multi-State project as opposed to an intensive effort at one or two sites.
For Objective 1, a Multi-State initiative will allow assessment of many more types of machines in a much broader range of soil conditions than could be obtained by an investigator in one state. The utility of the study is directly proportional to the number of observations, and since most cultivation is concentrated in early summer, the number of observations that can be made by any one research team is limited. Cross comparison between states will also help in assessing how the optimal equipment and settings may shift with climate change.
For Objective 2, if each participating lab investigates three weed species at three phenological stages of flower development and includes three termination methods there will be over 150 samples to assess. A Multi-State effort, will allow (i) the assessment of many more species, which will reveal taxonomic patterns in the response of seed production to time and mode of death, (ii) replication in different environments to assess the robustness of results, (iii) processing of seed samples in a timely manner, and (iv) allow a comparison across latitudes to provide an indication of how climatic shifts may affect seed production on terminated plants of various species.
For Objective 3, a Multi-State project will allow assessment of the effects of more types of amendments on more species of weeds than could be accomplished in any one state. We believe that legume green manures may be particularly useful for promoting seed mortality, but the types of legume cover crops used across the U.S. varies widely. A Multi-State initiative will allow us to determine whether effects are general to broad classes of organic amendments or highly specific to particular inputs and whether they relate to measurable characteristics such as carbon/nitrogen ratio.
What the likely impacts will be from successfully completing the work This research will be coupled with extension efforts in each state that will disseminate results to growers through field days, presentations, and publications. Results will be widely disseminated through synthesis articles in the e-Organic/eXtension web site, New Ag Network, Organic Broadcaster, and MOSES. We expect this work to reduce weed problems in organic and low herbicide cropping systems with consequent benefits to growers.
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