Interest in integrated approaches to weed management in western Canada is being driven by declining crop prices coupled with increased input costs, consumer concerns about the environmental and health effects of herbicides, and increasing incidences of weeds becoming resistant to herbicides. Scouting fields and assessing the nature and extent of a weed problem to determine if herbicides are necessary every year is an important component of integrated weed management. Applying the "economic threshold" concept to weeds is not an easy undertaking. On the other hand, applying herbicides when they are unnecessary can be a waste of time and revenue, and can lead to the selection of herbicide resistant weeds.
There are various philosophies for how pests should be managed. Pest management varies from no management, to niche resource and natural systems management (organic systems), to the intensive application of inputs to obtain high levels of pest control. Diversity of practices should be a high priority to keep pests off-balance and unprepared.
Duane Beck (Dakota Lakes Farm, South Dakota) reminds us that "successful crop production, regardless of the methods used, is a careful piecing together of numerous components into a system. Simply replacing one component with another is seldom successful". Focusing on crop competitiveness and health will lead producers to rely on packages of tools which include such things as sanitation (prevention of weed seed spread), low disturbance seeding systems, higher seeding rates, narrow crop rows, optimum fertilizer placement, and diverse crop rotations. A healthy, competitive crop is the key to ICM in any cropping system.
Why is it important to optimize crop health?
It is important to optimize crop health so that healthy roots and shoots can compete against weeds, avoid or tolerate diseases and insects, utilize nutrient and water resources efficiently and capture the sun's energy. Optimizing crop health unlocks the potential for harmonizing cropping systems with nature's biological processes and manmade inputs that can reverse the depletion of natural resources. The concept focuses on integrating all approaches to crop health, which are driven by the economic and ecological limits of the cropping system.
How can crop health be optimized?
In manipulated agriculture, natural processes are not likely to include all of the pest management and soil fertility tools that may be required in a cropping system. Conversely, intensive crop management should not be used as a substitute for effective and adequate processes such as natural soil sanitation with crop rotation, nitrogen fixation and cycling, and biological pest control. Agriculture systems that utilize biological cycles integrated with manmade inputs maximize crop health and resource conservation.
Is soil the ecological starting point for crop health?
The ecological starting point for crop health requires knowledge of what is in the soil (microbes, earthworms, pathogens, insects, weed seeds, etc.), methods to prevent pest buildup and infection of the crop, and knowledge to maximize the plants ability to defend itself. The focus of most current crop production practices is pest control with little effort applied to why pests are abundant. Cropping practices that degrade soil and limit genetic diversity contribute to increasing pest populations.
Why Integrated Approaches?
Integrated Research Studies incorporate a number of factors into an experiment that increases the complexity and the difficulty of interpreting and understanding results. Experiments with one or two variables have been common and provide valuable information on technologies and inputs. However, these experiments are conducted in isolation of any interactive effects that may be occurring in the plot. Farmers are constantly dealing with many factors that occur in their fields simultaneously. These types of studies will help determine the impact of one technology on other technologies incorporated into the cropping system.
For example, "pyramiding" technologies may be required to realize the benefits of all technologies. Purchasing side-banding equipment to place fertilizer for optimum crop utilization may be rendered ineffective if weeds are sprayed at the end of the spray window as opposed to the beginning of the labeled spray window.
Another example includes the selection of standard or semi-dwarf varieties. Wild oat is still one of the most ubiquitous weeds of cropland in Western Canada in spite of extensive herbicide application over the last 30 years, and complete elimination of wild oat seed from the soil seed bank is probably an unrealistic goal. The risk associated with seed production by uncontrolled wild oat should be weighed against the risk of selecting for herbicide resistant wild oat in future years. Seeding AC Lacombe or Seebe barley at relatively high rates may minimize the need for wild oat control with herbicides. Conversely, consistent establishment of semi-dwarf varieties at densities appropriate for optimum weed suppression may be difficult unless seeding rates are increased above those suggested by seed size and germination tests.
Consequently, combining good agronomic practices with herbicides can improve crop yield stability and herbicide performance. The more optimal agronomic practices that are stacked together in a particular field, the more effective and profitable growers will be. In experiments at Lacombe, Beaverlodge, and Lethbridge, we have confirmed that the following agronomic factors increase crop competitiveness and increase the effectiveness and reliability of herbicides:
These agronomic practices are far from new, but they obviously improve crop competitiveness. The only factor that perhaps is not intuitively obvious for its impact on crop competitiveness and weed management is fall or early-spring seeding. Fall or early-spring seeding results in earlier crop emergence relative to weeds. Some weeds will not emerge early enough to be competitive in a fall-seeded crop; whereas, the same weed species may be much more harmful in the same crop seeded in mid-May. When weeds adapted to competition with normal-seeded crops do emerge, they are at a competitive disadvantage with the earlier-emerging crop. Fall- and early spring seeding also subjects weeds to operational diversity. In other words, most weeds have been selected for emergence and seed shattering with the crop at a more normal planting date; the "confusion" can leave weeds unprepared. After years of normal seeding dates, few of the seeds in the soil seed-bank are from weeds that have emerged early, and earlier harvest dates in crops with weeds such as wild oat may cut the weed seeds off before they drop to the ground and increase the weed seed bank. Mixing a variety of seeding dates on a given piece of land will increase the confusion for weeds and the success of the crops.
Crop producers, plant breeders, and other researchers should pursue similar goals for more reliable herbicide performance, and higher, more stable yields. The seeding practices discussed herein can improve crop competition, and therefore, augment herbicide performance.
Readings
Clayton, G.W, K.N. Harker, J.T. O'Donovan, M.N., Baig, and M.J. Kidnie. 2002. Time of glyphosate application and tillage system effects on glyphosate tolerant canola (Brassica napus L). Weed Technol. (In press).
Clayton, G. W. and K. N. Harker. 2001. Plant Management Systems. In R.J. Hudson ed. Encyclopedia of Life Support Systems (EOLSS). Web and CD-ROM format pp. __-__ (36 total, In Press).
Harker, K. N., R. E. Blackshaw, and G. W. Clayton. 2001. Timing weed removal in field peas (Pisum sativum). Weed Technol. 15: 277-283.
Harker, K. N., G. W. Clayton, T. K. Turkington, J. T. O'Donovan, R. E. Blackshaw, and P. Thomas. 2001. How to implement IWM in canola. In R. E. Blackshaw and L. M. Hall eds. Integrated Weed Management: Explore the Potential. Review paper from opening plenary session on Integrated Weed Management, Annual Meeting of the Expert Committee on Weeds, Nov. 26-29, 2000, Banff, Alberta. pp. 91-98.
O'Donovan, J. T., K. N. Harker, G. W. Clayton, D. Robinson, R. E. Blackshaw, L. M. Hall. 2001. How to implement IWM in barley. In R. E. Blackshaw and L. M. Hall eds. Integrated Weed Management: Explore the Potential. Review paper from opening plenary session on Integrated Weed Management, Annual Meeting of the Expert Committee on Weeds, Nov. 26-29, 2000, Banff, Alberta. pp. 75-89.
O'Donovan J. T. 1996. Weed economic thresholds: useful agronomic tool or pipe dream? Phytoprotection 77:13-28.
O'Donovan, J.T., K.N. Harker, G.W. Clayton, D. Robinson, J.C. Newman, and L. M. Hall. 2001. Barley seeding rate influences the effects of variable herbicide rates on wild oat (Avena fatua). Weed Sci. (In press).
Thill D C; O'Donovan J T; Mallory-Smith C A (1994). Integrated weed management strategies for delaying herbicide resistance. Phytoprotection 75 (Supplement): 61-70.