Rotational crops (including cover crops) often entail the application of practices which stimulate germination (e.g., tillage and irrigation) as well as those that kill emerged seedlings (e.g., cultivation and herbicides). Rotational crops may result in the germination and death of rice weeds in a manner analogous to the stale seedbed approach described earlier.
Two bioeconomic weed control models have been developed through cooperative work between USDA's Agricultural Research Service (ARS) and university experts: WEEDCAM (Colorado) and WEEDSIM (Minnesota). WEEDSIM was field tested for 1991–1994. After 4 years of applying WEEDSIM recommendations to the same plots, there were no increases in annual weed densities or decreases in weed control or crop yields as compared to standard herbicide management systems for the region ( Forcella et al., 1996) . In most cases, the model-generated treatments controlled weeds as well as the standard herbicide treatment. The quantity of herbicide active ingredient applied decreased 27% with the seedbank model and 68% with the seedling model, relative to the standard herbicide treatment ( Buhler et al., 1996) .
There is a recognized need to develop multiyear data to relate the impact of uncontrolled weeds in a given year on weed densities and potential crop losses in future years ( Lybecker et al., 1991) . Individual weed seedlings can produce a tremendous number of seeds per plant, which if left uncontrolled in any year, can quickly replenish the seedbank ( Chapter 6 ).
Weeds of farm crops
Changes in weed species and populations also cause changes in plant diseases and insect pests since certain weeds serve as their hosts ( Bendixen et al., 1981 ; Manuel et al., 1982 ; Weidemann and TeBeese, 1990 ; Norris and Kogan, 2000) . Herbicide-resistant weed biotypes are present in our weed populations, although often at very low frequencies, even when herbicides are not used. Weed species have acquired built-in genetic adaptability to survive most control methods used against them. For example, dandelions usually develop a vertical growth habit when growing wild, but when growing in a frequently mowed lawn, more prostrate or flat-growing biotypes evolve. We should continually add to our weed control technology and keep tools available in order to address the adaptability of weeds to different control methods. For further information on the biological characteristics of weeds, including growth strategies, mimicry with crops, plasticity of weed growth, photosynthetic pathways, weed seed reservoir, and vegetative reproduction see Cousens and Mortimer (1995) and Buhler et al. (1998) .
Jones and Medd (2000) proposed that a longer-term management approach is needed to manage weed seedbanks and to determine the optimal level of intervention required for a specific weed situation. Managing seedbanks is complex because of the difficulty in preventing seed production and introduction, as well as the persistence of certain seeds in the seedbank and the high seed production potential of many weed species ( Buhler et al., 1998) . 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. Management systems can be devised that minimize the impact of the resultant weeds.
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) .
Researchers have investigated methods for reducing scouting time requirements. In a 1994 study, visual assessments of weed seedling emergence resulted in the same recommendation from WEEDCAM as a detailed assessment ( Vangessel et al., 1996) . As farm size continues to increase, weed management methods that conserve grower time requirements tend to be used more extensively.
For example, some but not all weed species have light-responsive seeds, and dark cultivation reduces emergence only in the light responders. Similarly, careful nitrogen (N) management can reduce problems with nitrate responders but have no effect on nonresponders and could even favor a weed that is well adapted to low levels of soluble N. The best approach to weed seed bank management is to design your strategy around the four or five most serious weeds present, then monitor changes in the weed flora over time, noting what new weed species emerge as the original target weed species decline. Then change your seed bank management strategy accordingly. Plan on making such adjustments every few years, and if possible, keep a sense of curiosity and humor about the weeds!
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:
Although seed longevity of agricultural weeds is a cause for notoriety, and a proportion of the population may remain viable for several years or decades, most of the seeds of many weed species will either germinate or die shortly after being dispersed from the parent plant. The seeds of many grasses are particularly short lived. For example, in a field study conducted near Bozeman, MT, wild oat seeds were incorporated into the top four inches of a wheat–fallow field, and approximately 80 percent of them died during the first winter (Harbuck, 2007). It is important to note, however, that postdispersal survival varies widely among weed species.
Challenge of Weed Seed Bank Diversity
The first type of withdrawal—germination leading to emergence—is, of course, how weeds begin to compete with and harm crops each season. It is also the foremost mechanism for debiting the seed bank, an effective strategy if emerged seedlings are easily killed by subsequent cultivation or flaming (the stale seedbed technique, for example). Even in species with relatively long-lived seeds such as pigweeds, velvetleaf, and morning glory, the vast majority of weed emergence from a given season’s seed rain takes place within two years after the seeds are shed (Egley and Williams, 1990). Thus, timely germination (when emerging weeds can be readily killed) can go far toward minimizing net deposits into the seed bank from recent weed seed shed. Knowing when to promote or deter weed seed germination, and how to do so for the major weeds present, are important skills in seed bank management.
Figure 2. Vertical distribution of weed seeds in a loamy sand soil (top) and a silty loam soil (bottom). Figure credit: adapted from Clements et al. (1996) by Fabian Menalled, MSU Extension, Montana State University.
This is an eOrganic article and was reviewed for compliance with National Organic Program regulations by members of the eOrganic community. Always check with your organic certification agency before adopting new practices or using new materials. For more information, refer to eOrganic’s articles on organic certification.
Use these strategies to maximize losses (withdrawals) from the weed seed bank: