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.

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