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what increases or decreases the weed seed bank

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Pre-seed wild oats (plants/m 2 )

Guldan, R.H., S.J. Shirtliffe and A.G. Thomas. 2003. Harvest losses of canola ( Brassica napus ) cause large seedbank inputs. Weed Sci. 51:83-86.

Stimulating weed germination

After allowing weeds to grow for a short time, these weeds must be destroyed, usually with a tillage operation or application of a herbicide. (See section on Stale Seedbed in Spring Tillage)

Prevention of seed set by application of herbicides, in-crop tillage and the use of perennial or annual forages which are harvested prior to seed maturation are effective strategies but care must be taken to alternate timing of harvest or one weed species may be replaced with another. Early harvest of barley for silage can be an effective means of reducing wild oat numbers (Harker et al., 2003). The use of winter cereals cut for silage has been even more effective. However, care must be taken that strategies which deplete the wild oat seedbank do not increase the seed production of other weeds. Weeds such as dandelion can establish late in the growing season and numerous species use as wild buckwheat can grow and produce significant amounts of seed after early harvests. Integration of herbicides or cover crops will be required to ensure success in this strategy.

Weed seeds can be moved from field to field unwittingly, allowing weed species to be spread to new areas, causing new weed problems. Weed seed spread can be minimized by using clean seed and cleaning equipment between fields.

These initial results show the potential usefulness of these models, however, the major limitation to widespread adoption is the amount of information needed on biology and ecology of common weeds within any particular region. Such information is not readily available for most weed species ( Forcella et al., 1996) . Furthermore, growers would be required to sample their fields to measure weed seedbanks and to scout for emerged weeds. The increased time and management requirements for growers to perform the detailed weed assessments are substantial ( Vangessel et al., 1996) .

As with a stale seedbed, the success of rotational crops in reducing the rice weed seedbank depends on an appropriate stimuli being applied at the right time (when seeds are relatively nondormant), as well as on the use of effective postemergence termination methods. Likewise, the efficacy of rotational crops in promoting fatal germination is likely to be the greatest for weeds with limited dormancy and in rotational crops for which weed management is relatively easy and inexpensive. Indeed, if rice weed seeds are stimulated to germinate in rotational crops and are not effectively terminated, they may exacerbate weed problems through reproduction.

Weed seedbanks are an ever-present component of agricultural land, and resources directed to understanding, interpreting, and predicting seed germination potential can improve agricultural production.

Preventive Weed Management in Direct-Seeded Rice

The rate of evolution of resistant weeds is based on several factors, including characteristics of the weed and herbicide, gene frequency, size and viability of the soil seedbank, weed fitness, herbicide potency, frequency and rate of application, and persistence in soil. Various attempts have been made to use modeling to determine the relative importance of these factors and to predict the probability of resistance, as well as to evaluate how to avoid, delay, or solve the problem ( Gressel and Segel, 1990 ).

Using a model to maximize strategies for herbicide-resistant blackgrass, Cavan et al. (2000) gave estimates on the effectiveness of various strategy options. Based on research with a long-term model for control of blackgrass and annual bluegrass, Munier-Jolain et al. (2002) concluded that threshold-based weed management strategies can be more cost-effective than spraying every year and may enable important reductions in herbicide use. However, the highest long-term profitability was obtained for the lowest weed level threshold tested.

Figure 5.10 . Decline in seed numbers of four weed species at the Ruakura Research Centre site following monthly tillage over a period of 4 years. Note y-axis is a logarithmic scale.

Fig. 6.8 . There is no difference between years in the mixed-species TPL (NQ = 254, NB = 68) of a community of 30 weed species in the seedbank of a field in Catalonia. Each point in this graph is a different weed species recovered in sampling the seedbank by soil core.