Weed Prevention: Priority Research for Alternative Weed Management

Managers must make effective use of cultural and biological methods of weed control in order to respond to increasing calls for reductions in herbicide and tillage use. Unfortunately, these methods are often of low and unreliable efficacy, only effective against low‐density weed infestations and frequently permit considerable weed seed production. Therefore, use of cultural and biological methods must be supported by a concerted effort to reduce soil weed seed populations and prevent their long‐term growth. Preventive weed management serves this purpose. However, scientific understanding of preventive management is limited and largely descriptive. Increased understanding of two major issues is needed: costs and risks associated with preventive practices must be quantified, and ecological mechanisms underlying preventive practices must be elucidated to determine how and why preventive practices affect weed populations. Promising avenues for basic and applied research in preventive weed management include: managing to increase mortality and limit dispersal of weed seeds and propagules, structuring farm operations in time and space to maximize preventive impact, and analyzing the functioning of existing preventive weed management systems. Production agriculture needs the support of a major research effort to improve preventive weed management. Research Question Compared with herbicides and tillage, alternative weed control methods (biological and other cultural controls) are usually only moderately effective. For this reason, alternative methods will effectively protect crops only when weed densities are relatively low. Therefore, preventive measures to reduce future weed densities play an essential supporting role in the implementation of alternative weed management. Effective preventive measures must be in place if alternative methods are to be used to reduce reliance on herbicides and tillage. Unfortunately, we do not know enough about the effects of most preventive measures, nor how to combine these into integrated, cost‐effective systems. The objective of this study was to identify research needed to improve preventive weed management. Literature Summary Preventive measures against weed populations include a wide range of practices aimed at weed plants and weed seedbanks. Examples include crop rotation, delayed planting (e.g., stale seedbank technique), mowing, interseeding, and managing to maintain populations of weed seed consumers. Information is available concerning the effects of some of these practices on weed populations. However, the ability of most preventive measures to reduce future weed populations has not been quantified, and in many cases it is unclear how and why a preventive practice affects weed populations. To develop effective preventive weed management systems, research is needed to answer two major questions. What are the weed control benefits, costs, and risks associated with preventive measures? What ecological mechanisms underlie preventive measures? That is, how does a preventive measure affect the life cycle of a weed species, and why does a preventive measure affect a particular stage of a weed life cycle? Study Description The weed science literature was surveyed to evaluate the current understanding of preventive weed control practices. Computer simulation models that forecast long‐term effects of preventive practices on weed populations were used to demonstrate the value of understanding how and why preventive methods work. A list of questions that should be answered to develop preventive weed management systems was developed. This list is followed by a list of specific preventive practices that should receive research attention. Applied Questions What stage of the weed life cycle is most vulnerable for effective reduction of future weed populations? A variety of computer forecasts of future weed populations have concluded that efforts to kill newly produced weed seed have a larger impact on future populations than comparable efforts aimed at other portions of weed life cycles. For example, efforts to increase weed seed death rate in a crop rotation were predicted to reduce fbture weed populations by twice as much as comparable efforts to increase the death rate of weed seedlings. Of course, these different weed control benefits must be weighed against the costs of various measures to increase weed death rates. Many cost‐effective management measures that kill weed seeds may exist (e.g., use of short rotations instead of monocultures may have ecological effects that increase seed death rates). Long‐term field studies are needed to test the computer forecasts. Conclusions Can preventive measures increase the profitability of threshold weed management? Threshold management could reduce monetary and environmental costs of weed control by restricting use of herbicides and tillage to fields where weed densities are high enough that the benefits of herbicide and tillage use are greater than their costs. Unfortunately, when the future cost of controlling weeds resulting from weed seed production is taken into account, only low weed densities can be left uncontrolled without incurring high hture weed control costs. Computer forecasts show, however, that if more effective preventive measures are in place, then much higher weed densities can be tolerated without leading to high future control costs. Then threshold weed management could become a more useful tool in efforts to reduce herbicide and tillage use. Again, long‐term field studies are needed to test the computer forecasts. Can preventive measures be reliable and cost‐effective? Non‐chemical preventive meaures are often perceived as unreliable and ineffective. This perception is accurate when these measures are considered individually. When multiple preventive measures are integrated into a system, however, as in a crop rotation, field studies and computer forecasts indicate that an integrated preventive system can rapidly reduce weed population, even when the system consists of practices that are of low to moderate effectiveness individually. How can weed managers respond to demands to reduce reliance on herbicides and tillage? Effective systems that prevent most weed‐crop interactions are essential to the success of efforts to use alternative weed control methods for reducing herbicide and tillage use. Development of preventive systems deserves a research effort similar to that needed to develop herbicide technology. This research effort is critical if production agriculture is to respond to demands to reduce cost and environmental impact of weed control. Practicing agricultural professionals can play a major role in this research effort by active partnership with university scientists and by forcefully voicing the need for basic and applied research in preventive weed management. Funding mechanisms that will support preventive weed management research require consideration; check‐off systems or other means of producer support are possible. Public research universities will need to play a leadership role.