D. A. Derksen

Agriculture and Agri-Food Canada, Box 1000A, RR#3, Brandon, MB. R7A 5Y3

Crop rotations form an integral component of integrated weed management systems in zero-tillage. Rotating crops has been a tool used to manage weeds prior to the advent of herbicides; however, since herbicides have come into wide usage, rotating crops has occurred less frequently. Potential exists to enhance the economic and environmental sustainability of zero tillage if crop rotation is use in conjunction with herbicide usage to manage weeds. Furthermore, the use of these tools may actually reduce herbicide usage in zero-tillage systems.

A SYSTEMS APPROACH TO WEED MANAGEMENT USING CROP ROTATIONS IN ZERO-TILLAGE SYSTEMS

In conventional tillage, herbicide usage has been focused on the in-crop control of weeds using pre- or post-emergence herbicides and more recently on pre-harvest herbicide application (Figure 1). Consequently, most of our weed management strategies in conventional-tillage systems focus on broad spectrum herbicide mixtures that attempt to control as many weed species at as wide a range of leaf stages as possible. This has served agriculture well until the problem of weed resistance to herbicides occurred due to over usage.

The approach of using broad ranging "one-shoot" herbicide mixtures for weed management was transplanted to zero-tillage with the addition of herbicide application preand post-seeding (Figure 2). Consequently, herbicide usage has gone up in zero tillage To reduce herbicide usage and costs, the emphasis on in-crop treatments will have to decrease. For example, by using IWM strategies, such as the threshold concept, the broad ranging "one-shoot" herbicide mixture may not be required every year.

To reduce costs, the typical approach being used by zero Tillage farmers today is to apply a pre-seeding burn-off herbicide treatment, usually glyphosate (Roundup) or glyphosate mixtures followed by the same in-crop herbicide mixture used in conventional-tillage systems (Figure 3). In this situation, all of the weed control for one year occurs within four to six weeks. Therefore, weeds that germinate and establish after in-crop herbicide usage have become increasing problems. This is analogous to the development of herbicide resistance, in that, applying herbicides in the same pattern in zero tillage year after year has selected for late emerging weeds. Furthermore, these weeds are difficult to control with next spring's burn-off treatment. Canada thistle, dandelion, and night-flowering catchfly are examples. In order to stop the build up of these weeds, the occasional use of a pre- or post-harvest herbicide will be necessary.

Rotating crops also changes the date of seeding within a given field. Seeding date and soil disturbance level can have an impact on the herbicides required to control weeds. In a three-year study at Indian Head, it was found that soil disturbance (tillage) increased weed densities and that early in the season weed densities were lower than at the end of the seeding season (Figure 4). Therefore, if soil disturbance is minimized by the use of zero-tillage, the pre-seeding burn-off treatment may be all that is required to control weeds at later dates of seeding. This is possible because a tillage operation that encourages a new flush of weed growth within the crop is eliminated. Furthermore, zero-tillage crops seeded early in the season may not require a preseeding burn-off or may require lower rates of burn-off treatments The success of these approaches is dependent on environmental conditions, but illustrates the potential to reduced herbicide usage in zero-tillage systems (Figure 5).

To optimize the benefits of cropping practices on weed control, factors such as crop rotation, crop competitive ability, crop life cycle, seeding dates, and herbicide usage, must come together into a multi-year plan. Figure 6 illustrates a cereal-oilseed-cereal-pulse rotation that varies seeding dates and herbicide usage in zero-tillage. The early seeded pea crop does not receive a pre-seeding burn-off based on pre-harvest or fall weed control the year before. If a pre-seeding burn-off were required low rates of glyphosate may be all that was necessary. There is an option for either pre- or post-harvest herbicide application in the pea year depending on the weed species present.

In this illustration, the seeding of the canola crop is delayed long enough for the burn-off treatment to control most of the weeds. Because weed emergence from a tillage operation is eliminated, a reduced in-crop herbicide application may be sufficient to suppress weed growth to the extent required to achieve optimum crop yield. Since the wheat crops are sown mid-season, some pre-seeding burn-off treatment and some in-crop treatments will likely be necessary. The figure illustrates several options for preand post-harvest weed control. Depending on the severity of perennial and winter annual weeds, these options may have to be exercised more frequently. Of greatest importance is the potential cumulative effect of managing weeds at different times of the season throughout the four years. This is a means of suppressing weed community changes.

The previous figures illustrate that weed communities are the result of the selection pressure imposed on them. Varying the selection pressure is the primary tool at the disposal of the zero-tillage farmer to reduce herbicide usage and to reduce the likelihood of adverse weed community changes. Crop rotations could be expanded to include other cropping options that vary selection pressure on weeds, such as the production of winter cereals and forage crops. The use of surface applied granular herbicides in zero-tillage could also be added to a multi-year weed management program to control certain broadleaf weeds and to reduce the potential for weed resistance. Furthermore, using the threshold concept to potentially eliminate one application of a wild oat herbicide in four years would reduce herbicide usage, vary selection pressure, and reduce the changes of herbicide resistance occurring.

Figure 6. A four year crop sequence illustrating different weed management options in zero tillage.

SPECIFIC EXAMPLES OF WEED MANAGEMENT OPTIONS FOR EACH HERBICIDE TIMING IN ZERO-TILLAGE SYSTEMS

Pre-seeding Burn-off treatments:

Tillage encourages weed germination throughout the spring. In zero tillage, weed emergence is slow in early spring, but the majority of weeds generally emerge by mid-season and a "second flush" of weeds does not occur because of the absence of tillage For early seeded crops, the in-crop treatment is most important while for late seeded crops the burn-off treatment is the most important (Figure 7). This principle must be applied with flexibility. For example, if winter annual weeds were not controlled during the previous fall then a preseeding burn-off treatment may be necessary early in the season. Furthermore, if the season is initially dry followed by extensive rainfall, then weeds may germinate within the crop at later dates of seeding.

In-crop herbicide usage: Several options exist for reducing in-crop herbicide usage including: reduced herbicide rates, using thresholds to avoid herbicide application where possible, or using cropping practices, such as delayed seeding, to avoid in-crop herbicide usage. The following example illustrates the use of delayed seeding and crop rotation to avoid in-crop herbicide usage. The Special Crop Management Study (SGMS) is a new study at the Indian Head Experimental Farm. There are six rotations in both zero and conventional tillage that 10110w a cereal-oilseed-cereal-pulse sequence similar to Figure 6. In this paper, only rotations one to three will be presented. All three rotations follow a spring wheat-canola-spring wheat-lentil sequence. Rotations one and two are high-input approaches where rotation one uses only post-emergence herbicides and rotation two uses pre-emergence herbicides in the broadleaf crop years and post-emergence herbicides in wheat. Rotation three is a low-input rotation where the wheat seeding is delayed by 7-14 days and no grassy weed herbicides are applied in-crop. A reduced broadleaf weed herbicide approach is used in all crops.

In 1994, wheat yields were not reduced in the low-input rotation despite the lack of wild oat control (Figures 8 and 9). This translated into greater net returns. Yields were similar in both tillage systems and were greater following lentil than canola The 1994 results are typical of those occurring in 1992 and 1993. Due to the rotational effect of controlling wild oats in the non-wheat years within canola and lentil crops and the effect of controlling wild oats with Roundup or tillage prior to seeding, weed densities were the same in the high and low input rotations (Figures 10 and 11).

Pre-harvest herbicide usage: Preharvest herbicide usage is a relatively new option for farmers in zero- and conventional-tillage systems. Excellent control of Canada thistle, quackgrass, and sow-thistle has been noted using Roundup. There may also be the potential to control or suppress many other weeds that are difficult to control even with Roundup For example, in the study described above, preharvest Roundup applications were made to the lentil phase of the crop rotations. Seventy-five to eighty percent control of dandelion has occurred. Further research is required, but pre-harvest Roundup has also suppressed other difficult to control weeds, such as toad flax

Fall post-harvest herbicide usage: The use of phenoxy type herbicides has long been recommended to control winter annual weeds, such as stinkweed and flixweed. Due to the reduced cost of Roundup and the availability of Rustler (glyphosate plus dicamba), fall control of winter annual weeds in zero Tillage Systems is no longer practised on a regular basis. Some weeds that occur as summer annuals in conventional tillage can over winter in zero-tillage stubble and are difficult to control the following spring. Weeds such as night-flowering catchfly and blue bur have been confirmed to have a winter annual habit in zero tillage and cleavers has been reported to do so. Since these weeds are not controlled with the low rates of phenoxy herbicides used for winter annual weed control, further research is required with different herbicides or herbicide combinations. Moreover, some winter annuals and biennial weeds that generally occur in low densities may increase in zero tillage Of particular concern are biennial wormwood, scentless chamomile, fleabane, pygmy flower, and American dragon's head.

CONCLUSIONS

Rotating crops, seeding dates, and varying herbicide usage can suppress weed community changes and potentially reduce herbicide usage. The continued development of IWM strategies for zero tillage systems is essential to enhance the sustainability of the system.

ACKNOWLEDGEMENTS

The financial support for this research from Agriculture and Agri-Food Canada and the Parkland Agricultural Research Initiative are gratefully acknowledged.

REFERENCES

Blackshaw, R. E., Larney, F. J., Lindwall, C. W. and Kozub, G. C. 1994. Crop rotation and tillage effects on weed populations on the semi-arid Canadian prairies. Weed Technology. 8:231-237.

Derksen, D. A., A. G. Thomas, G. P. Lafond, H. A. Loeppky, and C. J. Swanton. 1994. The impact of agronomic practices on weed communities: fallow within tillage Systems. Weed Science. 42:184-194.

Derksen, D. A., Lafond, G. P., Thomas, A. G., Loeppky, H. A. and Swanton, C. L. 1993. Impact of agronomic practices on weed communities: tillage Systems. Weed Science. 41 :409A1 7.

Derksen, D. A., A. G. Thomas, G. P. Lafond, H. A. Loeppky, and C. J. Swanton. 1995. The impact of herbicides on weed community diversity within conservation-tillage systems. Weed Research. (accepted)

Lafond, G. P. and D. A. Derksen. 1995 Long-term potential of conservation tillage Systems on the Canadian prairies. Canadian Journal of Plant Pathalog). (in press).

Moyer, J. R., E. S. Roman, C. W. Lindwall, and R. E. Blackshaw. 1994. Weed management in conservation tillage Systems for wheat production in north and south America. Crop Protection 13:243-259.

Swanton, C. J. and S. F. Weise. 1991. Integrated Weed Management: The Rationale and Approach. Weed Technology. 5:657-663.

Walker, R. H. and G. A. Buchanan. 1982. Crop manipulation in integrated weed management systems. Weed Science 30(Supplement): 17-24.