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Block Reference: #e1ccc7e0-f9d0-11eb-8066-334027aa517d
Date and time: Tue, 10 Aug 2021 11:48:30 GMT
Using clean seed and cleaning equipment between fields can prevent new weed problems.
Preventing weed seeds from returning to the soil seedbank can help to avoid weed problems in future years. In one study modeling seedbank depletion (Jones and Medd, 2000), very low tolerance for weeds in the first few years brought the wild oat seedbank almost to zero after 15 years of chemical weed control and after 7 years of integrated weed management. In this study, herbicide use was increased for several years, but was the total herbicide use over 20 years was greatly reduced, especially when integrated weed management practices were used.
Spring N fertilizer
Table 2. Total density (+/- SE) of volunteer spring wheat in the field (plants/m 2 ) and in the soil seedbank (SB)(seeds/m 2 ) in canola stubble 2002-2004, in a Pesticide Free Production (PFP) rotation study (BRC).
Agronomic practices that improve the competitive ability of the crop compared to weeds can result in lower seed production by weeds that survive. Other practices such as clipping tall weeds above the crop canopy (see Other Mechanical Weed Control Practices ) or terminating crops early, either as a silage crop or green manure, can prevent weeds from successfully producing seed and returning it to the weed seedbank.
Chaff collection has the potential to reduce weed seed return and possible reduce the need for weed control. In a survey of Saskatchewan farms where chaff was collected, two-thirds of farmers reported being able to reduce herbicide use or eliminate at least one tillage operation per year, because of reduced weed pressure (Olfert et al., 1991). On the other hand, in a Brandon field study, chaff collection significantly reduced the amount of volunteer wheat in the subsequent years’ seedbank but had limited impact on the numbers of wheat plants emerging in the crop (Table 2).
Weed seedbank depletion is generally accomplished by stimulating weed germination and emergence and then destroying weeds that have emerged. Preventing weed seeds from entering the seedbank can also be effective.
Thus, one strategy for managing the weed seed bank, especially for smaller-seeded weeds, is to maintain seeds at or near the soil surface. It is here that seeds experience the greatest exposure to environmental cues that will encourage germination—the most effective means of debiting the seed bank—as well as greater exposure to seed predators (see Encouraging Weed Seed Predation and Decay). Studies have confirmed that some weed seeds, including velvetleaf, morning glory, and pigweed, germinate in larger numbers in untilled than in tilled soil during the first year after seed shed (Egley and Williams, 1990). It may be tempting to use inversion tillage to place seeds below the depth from which they can emerge. This may be an effective strategy for species with short-lived seeds (see below), but it may simply protect longer-lived seeds from mortality factors like seed feeding animals and decomposer fungi, only to be returned to the soil surface by the next deep plowing event.
While it is sometimes advantageous to cause weed seeds to germinate, it is important at other times to keep them quiescent long enough for the crop to get well established. Several practices can help reduce the number of weeds emerging in the crop.
Weed seeds can have numerous fates after they are dispersed into a field (Fig. 1). Some seeds germinate, emerge, grow, and produce more seeds; others germinate and die, decay in the soil, or fall to predation. The seeds and other propagules of most weeds have evolved mechanisms that render a portion (a large majority in some species) of propagules dormant (alive but not able to germinate) or conditionally dormant (will not germinate unless they receive specific stimuli such as light) for varying periods of time after they are shed. This helps the weed survive in a periodically disturbed, inhospitable, and unpredictable environment. Weed seeds can change from a state of dormancy to nondormancy, in which they can then germinate over a wide range of environmental conditions. Because dormant weed seeds can create future weed problems, weed scientists think of dormancy as a dispersal mechanism through time.
Factors Affecting Weed Seed Longevity
The Iowa State University Cooperative Extension Service has evaluated seed germination response of common weeds of field corn in relation to GDD calculated on a base temperature of 48°F beginning in early spring, and categorized the weeds into germination groups (cited in Davis, 2004). For example, winter annuals like field horsetail and shepherd’s purse germinate before any GDD accumulate in the spring; giant ragweed and common lambsquarters require fewer than 150 GDD and therefore emerge several weeks before corn planting; redroot pigweed, giant foxtail, and velvetleaf germinate at 150–300 GDD, close to corn planting time; whereas large crabgrass and fall panicum require over 350 GDD and usually emerge after the corn is up. A few species, such as giant ragweed, emerge only during a short (<3 week) interval, whereas others, such as pigweed and velvetleaf, continue to emerge for an extended period (>8 weeks). Knowing when the most abundant species in a particular field are likely to emerge can allow the farmer to adjust planting dates and cultivation schedules to the crop’s advantage.
Maintaining excellent weed control for several consecutive seasons can eliminate a large majority of the weed seed bank, but a small percentage of viable, highly dormant seeds persist, which can be difficult to eliminate (Egley, 1986). Researchers are seeking more effective means to flush out these dormant seeds through multiple stimuli (Egley, 1986).
Use these strategies to minimize annual inputs (deposits) to the weed seed bank:
This article is part of a series on Twelve Steps Toward Ecological Weed Management in Organic Vegetables. For more on managing the weed seed bank, see: